anode is a JavaScript application packed into an EXE with NEXE. The challenge would be straight forward, but the instance of Node that’s packed in the executable with the JavaScript is dorked such that BigInts don’t evaluate as booleans correctly and the random numbers are seeded the same way each time. I’ll patch the JavaScript in the executable (carefully to maintain the length) to print out all 1024 steps that it takes changing the input into an encoded value, and write a Python script to reverse that and recover the flag.

## Challenge

You’ve made it so far! I can’t believe it! And so many people are ahead of you!

The download contains a 64-bit Windows executable:

oxdf@hacky$file anode.exe anode.exe: PE32+ executable (console) x86-64, for MS Windows  ## Run It Double-clicking the application pops a console with a prompt: If I enter something, it prints “Try again” and disappears. That’s more easily seen running it from a terminal: Hovering over the exe shows some additional details: ## JS Source ### Identify Framework Looking at the strings in the binary, there are a ton that include nexe: oxdf@hacky$ strings anode.exe | grep nexe | wc -l
2047
oxdf@hacky$strings anode.exe | grep nexe c:\users\vssadministrator\.nexe\14.15.3\src\inspector\node_string.h:98 c:\users\vssadministrator\.nexe\14.15.3\out\release\obj\global_intermediate\src\node\inspector\pro tocol\protocol.cpp:664 const __nexe_patches = (process.nexe = { patches: {} }).patches const __nexe_noop_patch = function (original) { const __nexe_patch = function (obj, method, patch) { __nexe_patches[method] = patch return __nexe_patches[method].apply(this, args) __nexe_patch((process).binding('fs'), 'internalModuleReadFile', __nexe_noop_patch) __nexe_patch((process).binding('fs'), 'internalModuleReadJSON', __nexe_noop_patch) __nexe_patch((process).binding('fs'), 'internalModuleStat', __nexe_noop_patch) const footerPosition = tailWindow.indexOf('<nexe~~sentinel>'); Object.defineProperty(process, '__nexe', (function () { let nexeHeader = null; return nexeHeader; if (nexeHeader) { nexeHeader = Object.assign({}, value, { Object.freeze(nexeHeader); c:\users\vssadministrator\.nexe\14.15.3\src\tls_wrap.cc c:\users\vssadministrator\.nexe\14.15.3\src\env-inl.h:250 c:\users\vssadministrator\.nexe\14.15.3\src\base_object-inl.h:77 ...[snip]... c:\users\vssadministrator\.nexe\14.15.3\deps\v8\src\compiler\verifier.cc:1928 c:\users\vssadministrator\.nexe\14.15.3\out\release\obj\global_intermediate\node_code_cache.cc:742 C:\Users\VssAdministrator\.nexe\14.15.3\out\Release\node.pdb !(function () {process.__nexe = {"resources":{"./anode.js":[0,321847]}}; if ((process.env.DEBUG || '').toLowerCase().includes('nexe:require')) { process.stderr.write('[nexe] - MANIFEST' + JSON.stringify(manifest, null, 4) + '\n'); process.stderr.write('[nexe] - DIRECTORIES' + JSON.stringify(directories, null, 4) + '\n'); return process.stderr.write('[nexe] - ' + text + '\n'); const patches = process.nexe.patches || {}; delete process.nexe; shimFs(process.__nexe) <nexe~~sentinel>  Nexe is a tool for packaging a NodeJS application into an executable file. It actually brings along a full node interpreter and any used JS libraries, and uses that to run JavaScript that’s also embedded in the application. ### Extract #### Manual Unpack Looking at the end of the binary, there are about 10,000 lines of JavaScript code: oxdf@hacky$ tail -n 10 anode.exe
}

var target = [106, 196, 106, 178, 174, 102, 31, 91, 66, 255, 86, 196, 74, 139, 219, 166, 106, 4, 211, 68, 227, 72, 156, 38, 239, 153, 223, 225, 73, 171, 51, 4, 234, 50, 207, 82, 18, 111, 180, 212, 81, 189, 73, 76];
if (b.every((x,i) => x === target[i])) {
console.log('Congrats!');
} else {
console.log('Try again.');
}
});
<nexe~~sentinel>@ܤA


There are several patches of JavaScript in the binary, first at line 37,144. At the end of line 284,993, Nexe JS starts:

!(function () {process.__nexe = {"resources":{"./anode.js":[0,321847]}};


A few hundred lines later on line 285392, the custom code for this application starts (marked with a comment added by me):

...[snip]...
shimFs(process.__nexe)
})();!(function () {
if (process.argv[1] && process.env.NODE_UNIQUE_ID) {
const cluster = require('cluster')
cluster._setupWorker()
delete process.env.NODE_UNIQUE_ID
}
})();!(function () {
if (!process.send) {
const path = require('path')
const entry = path.resolve(path.dirname(process.execPath),"./anode.js")
process.argv.splice(1,0, entry)
}
})();;const readline = require('readline').createInterface({  // <-- here
input: process.stdin,
output: process.stdout,
});

readline.question(Enter flag: , flag => {
readline.close();
if (flag.length !== 44) {
console.log("Try again.");
process.exit(0);
}
var b = [];
for (var i = 0; i < flag.length; i++) {
b.push(flag.charCodeAt(i));
}
...[snip]...


I can cut out the interesting JavaScript into a file and proceed to analysis from there.

#### NEXE UNPACKER

NEXE UNPACKER is a NodeJS script to unpack NExe applications. It’s as simple as running:

oxdf@hacky$nexe_unpacker anode.exe --out .  This command creates a anode.js: const readline = require('readline').createInterface({ input: process.stdin, output: process.stdout, }); readline.question(Enter flag: , flag => { readline.close(); if (flag.length !== 44) { console.log("Try again."); process.exit(0); ...[snip]...  ### Source Analysis #### Read Flag The script starts out by reading the flag, making sure it’s exactly 44 characters long, and then creating an array of ints named b which initializes to the numerical value of each character in the input flag. const readline = require('readline').createInterface({ input: process.stdin, output: process.stdout, }); readline.question(Enter flag: , flag => { readline.close(); if (flag.length !== 44) { console.log("Try again."); process.exit(0); } var b = []; for (var i = 0; i < flag.length; i++) { b.push(flag.charCodeAt(i)); }  #### Oddness Next there’s an odd bit that doesn’t make much sense (as the comment the Flare team included points out):  // something strange is happening... if (1n) { console.log("uh-oh, math is too correct..."); process.exit(0); }  In theory, this should always exit. 1n is a BigInt object, with the value 1, and therefore should be true. And yet, running the binary doesn’t exit here. In fact, running the extracted source does fail this way: oxdf@hacky$ node anode.js
Enter flag: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
uh-oh, math is too correct...


I’ll come back to this.

#### State Machine

Next there is a giant loop working as a state machine:

  var state = 1337;
while (true) {
state ^= Math.floor(Math.random() * (2**30));
switch (state) {
case 306211:
if (Math.random() < 0.5) {
b[30] -= b[34] + b[23] + b[5] + b[37] + b[33] + b[12] + Math.floor(Math.random() * 256);
b[30] &= 0xFF;
} else {
b[26] -= b[24] + b[41] + b[13] + b[43] + b[6] + b[30] + 225;
b[26] &= 0xFF;
}
state = 868071080;
continue;
case 311489:
if (Math.random() < 0.5) {
b[10] -= b[32] + b[1] + b[20] + b[30] + b[23] + b[9] + 115;
b[10] &= 0xFF;
} else {
b[7] ^= (b[18] + b[14] + b[11] + b[25] + b[31] + b[21] + 19) & 0xFF;
}
state = 22167546;
continue;
case 755154:
if (93909087n) {
b[4] -= b[42] + b[6] + b[26] + b[39] + b[35] + b[16] + 80;
b[4] &= 0xFF;
} else {
b[16] += b[36] + b[2] + b[29] + b[10] + b[12] + b[18] + 202;
b[16] &= 0xFF;
}
state = 857247560;
continue;
case 832320:


There are 1025 possible cases in the switch statement:

oxdf@hacky$grep case anode.js | wc -l 1025  One of them does break the loop:  case 185078700: break;  Each of the rest seem to branch based on a random number or a BigInt, and then change one byte in b. The fact that I expect the flag to be the same every run also shows more oddness of the same type as experienced above. #### Check Flag At the bottom after the loop, the code checks b against a static array of ints, and prints either “Congrats!” or “Try again.”:  var target = [106, 196, 106, 178, 174, 102, 31, 91, 66, 255, 86, 196, 74, 139, 219, 166, 106, 4, 211, 68, 227, 72, 156, 38, 239, 153, 223, 225, 73, 171, 51, 4, 234, 50, 207, 82, 18, 111, 180, 212, 81, 189, 73, 76]; if (b.every((x,i) => x === target[i])) { console.log('Congrats!'); } else { console.log('Try again.'); }  ## Static Analysis ### Strategy At this point it’s clear that something is dorked in the JavaScript environment that’s running this code. Specifically, the random numbers generated are not random, and the BigInts are not evaluating as booleans correctly. As to how I ended up solving this challenge, I didn’t end up using this analysis at all - this section can be completely skipped. But it is interesting, so I wanted to include it. ### Random Opening anode.exe in Ghidra showed just how much there was in this binary. It took several minutes to process, and there are a ton of functions. Once it completes, I’ll go into the Symbol Tree section, and go to Classes, and find RandomNumberGenerator: Poking around in these a bit, I’ll notice that SetSeed seems interesting: To get a feel for what this should look like, I’ll open another Ghidra window and load (after a long time) my local copy of node. This is a Linux copy, vs the Windows part from the EXE, and I’m not sure if they are the same version, but it’s close enough to get an idea what’s different. There, SetSeed looks like this: Without going too deep into how random numbers work in JavaScript (this video from PwnFunction is a really neat watch for more details), it keeps two state values and each time a random number is created, it returns one, and then mixes both together to replace the one sent off. The fact that the seed function in anode.exe is just setting two static values is a good clue as to why the random numbers come back the same each time. If the seeds are always the same, the random stream resulting from those seeds will be as well. I did even go down the rabbit hole of writing a Python script to generate numbers based from these seeds: #!/usr/bin/env python3 import struct import z3 state0 = 0x60c43c4809ad2d74 state1 = 0xce6a1a53db4c5403 for _ in range(100): s0 = state1 s1 = ((state0 << 0x17) & 0xffffffffffffffff) ^ state0 state0 = s0 s1 = (s0 >> 9 ^ s1) >> 0x11 ^ s0 ^ s1 state1 = s1 u_long_long_64 = (state0 >> 12) | 0x3FF0000000000000 float_64 = struct.pack("<Q", u_long_long_64) next_seq = struct.unpack("d", float_64)[0] next_seq -= 1 print(next_seq)  Though, as mentioned above, I don’t actually use this to solve the challenge. ### BigInt I had less success identifying what was going on with BinInt. I can find it in Ghidra as well (there are two there, I’m not sure why): Clicking around on a few of these functions, one subtle thing jumps out as potentially interesting. For example, the UInt64Value function: There’s a call to FUN_14082b390. That’s a local function that’s not a part of the class (or any other class). This looks similar to what’s in node, thought it’s weird to not be using the v8::internal::BigInt class in the anode.exe version: Something is defintiely up with this class. ## Patching Binary ### POC Given that the JavaScript that’s run is all in plaintext in the binary, an interesting thought is to check and see if I can edit that JavaScript and have the changes take effect. I’ll copy anode.exe to anode-mod.exe and edit that, keeping a good copy so I can start over when I mess up the binary. I’ll use notepad++ on Windows or vim on Linux because I know they are good about keeping all the non-ascii characters the same. To start simple, I’ll try adding some text to the message that prints if the flag is not the right length: Making sure to hit Save, now I’ll run that, and it fails: Spacing is often important in binaries. I’ll try again, but this time, instead of adding text, I’ll replace it: It works: ### Observe States It would be potentially useful to know the order of the various states that are visited, and to confirm that they are the same each time. I’ll add a print statement just after the state updates to show that: The input print statement adds 22 characters to the JavaScript, so I’ll need to remove 22 characters. An easy place to take from is the messages printed to the screen, as shown above. Running this results in the states printing: PS > .\anode-mod.exe Enter flag: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 1010356043 497278214 181384715 957436102 605458814 429116243 304625349 ...[snip]... 443632296 1048216731 210975861 185078700 Try again.  I can run a couple times and verify that the states are the same each time. Saving the results to a file and counting lines confirms 1025 unique states, so each is visited once. ### Observer Actions #### POC #1 In theory, now I can visit the states in order and track their actions. The challenge is that every state starts with an if / else based on either a BigInt or a random number, and keeping track of that, while possible, would be a ton of work. Instead, I’d like to just record each of the operations that occur. I’ll start with a fresh copy of the binary, and change one of the states so that it prints the values rather than doing them: I’ll add 15 characters in two sports (in red), and remove from strings and comments to make up for it (blue). This runs and prints the operation, but then fails: PS > .\anode-mod.exe Enter flag: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA b[30] -= b[34] + b[23] + b[5] + b[37] + b[33] + b[12] + Math.floor(Math.random() * 256) uh-oh, math.random() is too random...  The failure is because I printed Math.random instead of calling it, and therefore on the next loop it’s out of sync, and ends up in an state that’s not intended. #### POC #2 I’ll edit this so that Math.random is called, and the result is printed: This time, it runs to completion: PS > .\anode-mod.exe Enter flag: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA b[30] -= b[34] + b[23] + b[5] + b[37] + b[33] + b[12] + 60 Try again.  #### Script I really want to add this printing to 1024 cases (excluding the one that just breaks), and not knowing which way the if will resolve, that’s 2048 edits to make. I’m not going to do that manually. I’ll use a Python script to create a modified binary: #!/usr/bin/env python3 import re import sys with open(sys.argv[1], 'rb') as f: exe = f.read() js_start = exe.index(b'})();;') + 6 js_end = exe.index(b'\n});\n<nexe') js = exe[js_start:js_end] print(f"[*] Extracted JS [{len(js)} bytes]") new_js = re.sub(b'(b$\d+$ .= .{10,});', b'console.log("\\1");', js) print(f"[*] Wrapped lines in console.log [{len(new_js)} bytes]") new_js = re.sub(b'Math.floor$$Math\.random\($$ \* 256\)', b'" + Math.floor(Math.random() * 256) + "', new_js) print(f"[*] Moved Math.random() calls outside the logging to get values and keep in sync [{len(new_js)} bytes]") new_js = re.sub(b'\n +', b'\n', new_js) print(f"[*] Removing leading spaces [{len(new_js)} bytes]") size_diff = len(js) - len(new_js) new_js += b" " * size_diff print(f"[*] Added {size_diff} spaces to the end to keep size [{len(new_js)} bytes]") new_exe = exe[:js_start] + new_js + exe[js_end:] fn_bits = sys.argv[1].split('.') new_fn = '.'.join(fn_bits[:-1]) + "-mod." + fn_bits[-1] with open(new_fn, 'wb') as f: f.write(new_exe) print(f"[+] Modified binary written to {new_fn}")  First I’ll read the binary into a variable named exe. I’ll use vim to get the start and end offsets to the JavaScript I’m going to mess with, and pull that code into a variable named js. Now there’s a series of substitutions using regex. The first will find b[digits] ?= {10 or more characters}, and replace it by wrapping console.log around it. I require 10 or more characters so I skip the lines that are just & 0xFF. I’ll make sure to note that the values in b are always 0-255. The next finds calls to Math.floor(Math.random() * 256) and removes them from the console.log. I am able to verify with some grep that these only occur in the lines that I’m wrapping with console.log. Finally, the third regex will find every instance of a line that starts with spaces, and remove those spaces. That is a quick way to create a ton of extra space. Now I’ll calculate the size difference between the original JS and the modified version, and add spaces at the end of make them the same. The rest is just writing the result to a new file. It generates a file: oxdf@hacky$ python patch.py anode.exe
[*] Extracted JS [321842 bytes]
[*] Wrapped lines in console.log [352562 bytes]
[*] Moved Math.random() calls outside the logging to get values and keep in sync [356090 bytes]
[*] Removing leading spaces [274810 bytes]
[*] Added 47032 spaces to the end to keep size [321842 bytes]
[+] Modified binary written to anode-mod.exe


It works:

PS > .\anode-mod.exe
Enter flag: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
b[29] -= b[37] + b[23] + b[22] + b[24] + b[26] + b[10] + 7
b[39] += b[34] + b[2] + b[1] + b[43] + b[20] + b[9] + 79
b[19] ^= (b[26] + b[0] + b[40] + b[37] + b[23] + b[32] + 255) & 0xFF
b[28] ^= (b[1] + b[23] + b[37] + b[31] + b[43] + b[42] + 245) & 0xFF
b[39] += b[42] + b[10] + b[3] + b[41] + b[14] + b[26] + 177
b[9] -= b[20] + b[19] + b[22] + b[5] + b[32] + b[35] + 151
b[14] -= b[4] + b[5] + b[31] + b[15] + b[36] + b[40] + 67
...[snip]...
b[21] += b[39] + b[6] + b[0] + b[33] + b[8] + b[40] + 179
b[34] += b[35] + b[40] + b[13] + b[41] + b[23] + b[25] + 14
b[22] += b[16] + b[18] + b[7] + b[23] + b[1] + b[27] + 50
b[39] += b[18] + b[16] + b[8] + b[19] + b[5] + b[23] + 36
Try again.


I can do a quick check against my printed states to verify that it’s following the same course.

## Solve

### Strategy

With the full list of operations performed, I can now work backwards from the correct result to get the right input.

For example, if I had a much simpler challenge where there was only one byte, x, and the operations were:

x += 5
x ^= 7
x -= 3
if (x == 5):
print ("yay")


I would start with 5, and work backwards undoing the operations:

• add 3 to get 8
• xor with 7 to get 15
• subtract 5 to get 10

### Script

I’ll write a Python script to handle this as well:

#!/usr/bin/env python3

import re

b = [106, 196, 106, 178, 174, 102, 31, 91, 66, 255, 86, 196, 74, 139, 219, 166, 106, 4, 211, 68, 227, 72, 156, 38, 239, 153, 223, 225, 73, 171, 51, 4, 234, 50, 207, 82, 18, 111, 180, 212, 81, 189, 73, 76]

with open('ops.txt', 'r') as f:
ops = f.readlines()[::-1]

for line in ops:
target, op, eq = line.split(' ', 2)
target_num = int(re.findall('\d+', target)[0])
if op == '+=':
b[target_num] = (b[target_num] - eval(eq)) & 0xff
elif op == '-=':
b[target_num] = (b[target_num] + eval(eq)) & 0xff
elif op == '^=':
b[target_num] = b[target_num] ^ eval(eq)
else:
assert(False)

print(''.join([chr(x) for x in b]))


This will create b from the target in the JS. Then it will read in the operations, reversing the order ([::-1]). Now I loop over the lines, for each splitting out the target register, the operation, and the rest of the equation.

I’ll switch based on the op. If it’s plus, I need to subtract, and vice-versa. It’s good to throw an error if I get an unexpected op . For each op, I will eval(eq) to get a vlaue, and then apply it to the number, mask it with 0xff to get a byte, and store it back in the same register.

At the end, I’ll print the flag, and it works:

oxdf@hacky\$ python3 solve.py
n0t_ju5t_A_j4vaSCriP7_ch4l1eng3@flare-on.com


Flag: n0t_ju5t_A_j4vaSCriP7_ch4l1eng3@flare-on.com