For Ethereal, I found a DOS application, pbox.exe, and a pbox.dat file. These were associated with a program called PasswordBox, which was an early password manager program. To solve this box, most people likely just guessed the password, “password”. But what if I had needed to brute force it? The program was not friendly to taking input from stdin, or from running inside python. So I downloaded the source code, installed the FreeBasic compiler, and started hacking at the source until it ran in a way that I could brute force test 1000 passwords in 5 seconds. I’ll walk through my steps and thought process in this post.

## Prep

In order to create a modified pbox binary, I needed to figure out how to compile Basic. I’ll be working from my Kali box.

### Get Source

I got the source code from here, and unzip it:

root@kali# unzip pbox011-src.zip
Archive:  pbox011-src.zip
inflating: changes.txt
inflating: pbox.bas
inflating: pbox.ico
extracting: pbox.png
inflating: pbox.rc
inflating: pbox.svg
inflating: pbox.txt
inflating: reprint.bi
inflating: rijndael.bi
inflating: throwmsg.bi
inflating: wordwrap.bi


### Install FreeBasic

First, I’ll make sure I have all the prerequisites for the compiler:

apt install binutils gcc make lib{ncurses5,gpm,x11,xext,xpm,xrandr,xrender,gl1-mesa,ffi}-dev


Next, I’ll get the software from the SourceForge page. Decompress, and run the installer:

root@kali# tar xzf FreeBASIC-1.06.0-linux-x86_64.tar.gz

root@kali# ./install.sh
usage:
./install.sh -i [prefix]    install FB into prefix directory
./install.sh -u [prefix]    uninstall FB from prefix directory
(default prefix: /usr/local)

root@kali# ./install.sh -i
FreeBASIC compiler successfully installed in /usr/local


### Verify Compile

I’ll try to compile the software as is, before I start messing with it. Unfortunately, I get some errors:

root@kali# fbc pbox.bas
/media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(209) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'T(x,y) = Asc(Mid$(text,(((y-1) Shl 2) + x),1))' /media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(215) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'T(y,x) = Asc(Mid$(text,(((y-1) Shl 2) + x),1))'
/media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(235) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 's += Chr$(T(x,y))' /media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(247) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'ftext += Hex$(Asc(Mid$(convstr,i,1)),2)' /media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(259) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'f += Chr$(Val("&H"+Mid$(convstr,i,2)))' /media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(320) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'ptext += Chr$(0)'
/media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(331) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'mtext = Mid$(ptext, ((i-1) Shl 4) + 1, 16)' /media/sf_CTFs/hackthebox/ethereal-10.10.10.106/temp/rijndael.bi(339) error 148: Suffixes are only valid in -lang fb or deprecated or fblite in 'mtext = Mid$(ptext, ((i-1) Shl 4) + 1, 16)'


Looking at the man page for fbc, I see:

  -lang name
Select FB dialect: fb (default), deprecated, qb


I’ll try with -lang deprecated, and it works:

root@kali# fbc pbox.bas -lang deprecated
root@kali# ./pbox


## pbox Overview

The pbox binary (in my case, elf) is pretty simple. I covered a bit of the overview in the Ethereal walkthrough. After putting the pbox.dat file into /root/.pbox.dat, I can start the program, and it will prompt for the password:

root@kali# ./pbox


I can also run it with the --dump option, which will change some behavior once I know the password.

## Source Code Analysis

### Main Program

Other than analysis of VBA in Word document macros, I have no experience with Basic. I’m going to look through the source and see what I can figure out. I know there’s a prompt “Enter your master password:”, so I’ll search for that, and look around it. A bit up from that prompt, I see a comment (which apparently is REM and also ' in Basic) that says REM * * * Here begins the main program * * *. That section looks like:

REM  * * *  Here begins the main program  * * *

DIM AS INTEGER x, SelectedEntry = 1, FirstDisplayedEntry = 1
DIM AS STRING LastKey

IF LEN(COMMAND(2)) > 0 THEN About()
IF LEN(COMMAND(1)) > 0 AND LCASE(COMMAND(1)) <> "--dump" THEN About()

ConfigFile = GetConfFile()
DebugOut("ConfigFile = " + ConfigFile)

SELECT CASE CheckDataFile()
CASE 0
PRINT "Invalid datafile. Program aborted."
END(2)
CASE -1
PRINT "No database have been found. Your encrypted database will be initialised now."
PRINT "The database will be stored at the following location:"
PRINT GetConfFile()
PRINT
PRINT "Choose a master password: ";
PassPhrase = GetText("", 30, 0)
IF LEN(PassPhrase) = 0 THEN PRINT "Invalid master password! (can't be empty)." : END(1)
PassPhrase = LEFT((PassPhrase & CHR(3,141,59,26,53,58,97,93,238,46,26,43,38,32,79,50,28,8)), 16)
ModFlag = 1
CASE 1
PassPhrase = GetText("*", 30, 0)  ' Hash with "*", max 30 chars, EscapeChar not allowed
PRINT ' Carriage return
IF LEN(PassPhrase) = 0 THEN SLEEP 2000, 1 : PRINT "Password rejected." : END(1)
PassPhrase = LEFT((PassPhrase & CHR(3,141,59,26,53,58,97,93,238,46,26,43,38,32,79,50,28,8)), 16)
IF CheckPassPhrase() <> 1 THEN SLEEP 2000, 1 : PRINT "Password rejected." : END(1)
END SELECT


Here’s what I see that code doing:

1. Verifies the command line arguments, and if there are 2, or if there is 1 and it isn’t --dump then call About(). COMMAND(#) seems to refer to command line arguments.
2. Calls CheckDataFile() which presumably returns 0 for a bad file, -1 if it doesn’t exist, and 1 if it does.
3. Switches on the result of CheckDataFile(). In the case I’m interesting in, where the file exists, the return is one, and I’ll continue on that path.
5. Calls GetText("*", 30, 0) and store the result in PassPhrase. There’s a comment about hashing with “*”, max 30 characters, and no escape chars. I’ll need to check what “hash” means here.
6. If the length of PassPhrase is 0, sleep for two seconds, print rejection message, and exit.
7. Append bytes to the end of the string, and then take the first 16 characters.
8. Call CheckPassPhrase(), and if it isn’t 1, then sleep for two seconds, print rejection message, and exit.
9. Call the LoadBox() function.

### CheckPassPhrase

The CheckPassPhrase function is a good place to start looking. Here it is:

FUNCTION CheckPassPhrase() AS BYTE
REM Returns 1 if the Passphrase decrypted the file's header properly, 0 otherwise.
DIM AS BYTE Result = 0
DIM AS STRING LineBuff = SPACE(16), DecryptedHeader
OPEN ConfigFile FOR BINARY AS #1
GET #1, 13, LineBuff
CLOSE #1
IF LEFT(DecryptedHeader, 10) = "Monika <3 " THEN Result = 1
RETURN Result
END FUNCTION


I think this function:

1. Creates a 16 byte buffer of spaces.
2. Reads the first 13 bytes of the config file into that buffer.
3. Calls RIJNDAEL_Encrypt, giving it the buffer, the password, and the argument 2 (which I’ll guess means decrypt).
4. If the first ten characters of the decrypted header are “Monika <3 “, then it sets the return value to 1.

Were I better at crypto, I suspect it’d be really easy to write a brute forcer or even some kind of pbox2john like function. That’s not my best area, so I’ll leave that for now.

## Modifying the Code

### Password As Command Line Arg

I need some way to pass in the password automatically. In the original state, I can’t pass in command lines from a loop. I’m going to edit the code so that it takes the password as the argument.

#### GetText

Since I’m going to replace GetText, I want to make sure I know what it is returning. Since there’s a reference to hashing with “*”, I could see that being some kind of mangling of the string, or just hiding the input from the screen with *s. I’ll add a simple line immediately after it’s called to print it back to me:

    PassPhrase = GetText("*", 30, 0)  ' Hash with "*", max 30 chars, EscapeChar not allowed
PRINT ' Carriage return -- 0xdf added
PRINT PassPhrase ' -- 0xdf added
PRINT ' Carriage return


Now when I run it, I’ll see that my input, 0xdf0xdf, comes back:

root@kali# fbc -lang deprecated pbox.bas
root@kali# ./pbox
0xdf0xdf



Now that I understand the call, I’ll remove those two added lines.

#### Remove Check on Args

I’ll comment out the check that requires the first arg be nothing or --dump so that I can make that my password:

IF LEN(COMMAND(2)) > 0 THEN About()
REM IF LEN(COMMAND(1)) > 0 AND LCASE(COMMAND(1)) <> "--dump" THEN About()


#### Replace GetText with Arg

Next, I’m going to comment out the call to GetText(), and set PassPhrase to be COMMAND(1), which seems to be the first arg passed in:

    REM PRINT "Enter your master password: ";
REM PassPhrase = GetText("*", 30, 0)  ' Hash with "*", max 30 chars, EscapeChar not allowed
PassPhrase = COMMAND(1)
REM PRINT ' Carriage return


Now I can test:

root@kali# fbc -lang deprecated pbox.bas
root@kali# time ./pbox 0xdf

real    0m2.019s
user    0m0.000s
sys     0m0.007s


Success. I didn’t enter anything. It tested my input, failed, slept for two seconds, and exited.

### Remove the Sleeps

If I’m going to brute force on this code, I need to take out these sleeps. That’s easy enough. I’ll comment out the original lines with sleep and message, and add a line that just ends on bad password:

    REM IF LEN(PassPhrase) = 0 THEN SLEEP 2000, 1 : PRINT "Password rejected." : END(1)
IF LEN(PassPhrase) = 0 THEN END(1)
PassPhrase = LEFT((PassPhrase & CHR(3,141,59,26,53,58,97,93,238,46,26,43,38,32,79,50,28,8)), 16)
REM IF CheckPassPhrase() <> 1 THEN SLEEP 2000, 1 : PRINT "Password rejected." : END(1)
IF CheckPassPhrase() <> 1 THEN END(1)


Here’s my running the program, giving a blank password before and after that change:

root@kali# time ./pbox 0xdf

real    0m0.006s
user    0m0.003s
sys     0m0.000s


Perfect, it worked.

### Message On Success

Currently, I have a program that takes a password as a command line argument, and immediately and silently exits on failure. I just want to update what happens on success. Instead of loading the box, I just want it to print the password it found and exit.

    IF CheckPassPhrase() <> 1 THEN END(1)
END(0)


I’ll print the password as the argument passed in instead of PassPhrase because the one gets things appended to it. I’ll also have it exit with a 0 for success, as opposed to the END(1) exits for the failures above.

## Brute Force It

### Success

Now I’ll write a bash loop to check. Since failure prints nothing, and success prints the password, I can do this:

root@kali# time for pass in $(cat /usr/share/seclists/Passwords/darkweb2017-top1000.txt); do ./pbox$pass; done

real    0m5.808s
user    0m1.593s
sys     0m1.096s


In under 6 seconds, it checks 1000 passwords.

### Better Success

But I can do better. Since the program returns 0 on success and 1 on failure, I can use that. I can use and (&&) and/or or (||) to chain commands together based on the return values.

Bash is weird in that a return value of 0 is success. So if I have ./a && ./b, it will only run ./b if ./a returns success, which is 0.

I’ll add an && break to my loop. If the program is successful (returns 0), the it will also run break and exit the loop. That way I don’t have to keep checking passwords after I find the right one.

Now I can run in less than 0.03 seconds:

root@kali# time for pass in $(cat /usr/share/seclists/Passwords/darkweb2017-top1000.txt); do ./pbox$pass && break; done