Square CTF 2022: Yet Another Reversing Activity

WriteUp 2年前 (2022) admin

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This was one of the reversing challenge during Square CTF, i found it interesting and unusual as it involves YARA.

1. Description

Supposedly this file can recognize a flag. But what could it be?

2. Introduction

The file given is a compiled YARA rule.

From their website:

YARA is a tool aimed at (but not limited to) helping malware researchers to identify and classify malware samples. With YARA you can create descriptions of malware families (or whatever you want to describe) based on textual or binary patterns. Each description, a.k.a. rule, consists of a set of strings and a boolean expression which determine its logic. Let’s see an example:

rule silent_banker : banker
{
    meta:
        description = "This is just an example"
        threat_level = 3
        in_the_wild = true

    strings:
        $a = {6A 40 68 00 30 00 00 6A 14 8D 91}
        $b = {8D 4D B0 2B C1 83 C0 27 99 6A 4E 59 F7 F9}
        $c = "UVODFRYSIHLNWPEJXQZAKCBGMT"

    condition:
        $a or $b or $c
}

The obvious goal of the challenge is to find a flag that matches the rule in flag.yarc.

3. Preparation

I tried a YARA decompiler i found somewhere on github but it was kind of old and just didn’t work for the version of YARA used for this challenge, so i started to dig in the YARA source code.

Compiled YARA rules apparently contain some kind of bytecode and fortunately for us, there’s some debug only waiting to be enabled.

The function yr_execute_code() in libyara/exec.c has all of it:

int yr_execute_code(YR_SCAN_CONTEXT* context)
{

// cut for brevity
 while (!stop)
  {
    // Read the opcode from the address indicated by the instruction pointer.
    opcode = *ip;

    // Advance the instruction pointer, which now points past the opcode.
    ip++;

    switch (opcode)
    {
    case OP_NOP:
      YR_DEBUG_FPRINTF(2, stderr, "- case OP_NOP: // %s()\n", __FUNCTION__);
      break;

    case OP_HALT:
      YR_DEBUG_FPRINTF(2, stderr, "- case OP_HALT: // %s()\n", __FUNCTION__);
      assert(stack.sp == 0);  // When HALT is reached the stack should be empty.
      stop = true;
      break;

// etc...
  }
}

After a few grep here in there, turning debug mode can be done by passing the --with-debug-verbose=8 to the configure script (8 is kind of arbitrary here).

It’s best done by editing the build.sh script:

#!/bin/sh
./bootstrap.sh
./configure --with-debug-verbose=8
make

then rebuild YARA:

yara-4.2.3% ./bootstrap.sh
yara-4.2.3% ./build.sh

4. Solution

Now that we have our debug build, we can try it on a random file.

We know the flag has a format like flag{....}, so we can use it as a test input file:

% echo "flag{test}" > test.txt
% ./yara-4.2.3/yara -C flag.yarc test.txt
0.000000 006981 + yr_initialize() {
0.001316 006981   - hash__initialize() {}
0.001332 006981 } // yr_initialize()
0.001434 006981 - yr_scanner_create() {}
0.001485 006981 + yr_scanner_scan_mem(buffer=0x7f91d236d000 buffer_size=11) {
0.001492 006981   + yr_scanner_scan_mem_blocks() {
0.001505 006981     - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001517 006981     + _yr_scanner_scan_mem_block(block_data=0x7f91d236d000 block->base=0x0 block->size=11) {
0.001532 006981     } = 0 AKA ERROR_SUCCESS 0 // _yr_scanner_scan_mem_block()
0.001542 006981     - _yr_get_next_block() {} = (nil) // default iterator; single memory block, blocking
0.001549 006981     - _yr_get_file_size() {} = 11  // default iterator; single memory block, blocking
0.001556 006981     + yr_execute_code() {
0.001576 006981       - case OP_INIT_RULE: // yr_execute_code()
0.001583 006981       - case OP_PUSH_8: r1.i=0 // yr_execute_code()
0.001590 006981       - case OP_INT8: // yr_execute_code()
0.001595 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001604 006981       - case OP_PUSH_8: r1.i=95 // yr_execute_code()
0.001612 006981       - case OP_PUSH_8: r1.i=57 // yr_execute_code()
0.001619 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001627 006981       - case OP_INT_EQ: // yr_execute_code()
0.001634 006981       - case OP_JFALSE: // yr_execute_code()
0.001642 006981       - case OP_PUSH_8: r1.i=1 // yr_execute_code()
0.001650 006981       - case OP_INT8: // yr_execute_code()
0.001657 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001664 006981       - case OP_PUSH_8: r1.i=51 // yr_execute_code()
0.001671 006981       - case OP_PUSH_8: r1.i=95 // yr_execute_code()
0.001678 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001685 006981       - case OP_INT_EQ: // yr_execute_code()
0.001693 006981       - case OP_AND: // yr_execute_code()
0.001700 006981       - case OP_JFALSE: // yr_execute_code()
0.001707 006981       - case OP_PUSH_8: r1.i=2 // yr_execute_code()
0.001714 006981       - case OP_INT8: // yr_execute_code()
0.001721 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001729 006981       - case OP_PUSH_8: r1.i=248 // yr_execute_code()
0.001741 006981       - case OP_PUSH_8: r1.i=153 // yr_execute_code()
0.001745 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001753 006981       - case OP_INT_EQ: // yr_execute_code()
0.001762 006981       - case OP_AND: // yr_execute_code()
0.001767 006981       - case OP_JFALSE: // yr_execute_code()
0.001772 006981       - case OP_PUSH_8: r1.i=3 // yr_execute_code()
0.001777 006981       - case OP_INT8: // yr_execute_code()
0.001782 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001790 006981       - case OP_PUSH_8: r1.i=83 // yr_execute_code()
0.001799 006981       - case OP_PUSH_8: r1.i=52 // yr_execute_code()
0.001806 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001813 006981       - case OP_INT_EQ: // yr_execute_code()
0.001819 006981       - case OP_AND: // yr_execute_code()
0.001828 006981       - case OP_JFALSE: // yr_execute_code()
0.001839 006981       - case OP_PUSH_8: r1.i=4 // yr_execute_code()
0.001849 006981       - case OP_INT8: // yr_execute_code()
0.001856 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001865 006981       - case OP_PUSH_8: r1.i=248 // yr_execute_code()
0.001876 006981       - case OP_PUSH_8: r1.i=131 // yr_execute_code()
0.001885 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001892 006981       - case OP_INT_EQ: // yr_execute_code()
0.001899 006981       - case OP_AND: // yr_execute_code()
0.001906 006981       - case OP_JFALSE: // yr_execute_code()
0.001913 006981       - case OP_PUSH_8: r1.i=5 // yr_execute_code()
0.001922 006981       - case OP_INT8: // yr_execute_code()
0.001929 006981       - _yr_get_first_block() {} = 0x7ffc0c7f9650 // default iterator; single memory block, blocking
0.001937 006981       - case OP_PUSH_8: r1.i=154 // yr_execute_code()
0.001944 006981       - case OP_PUSH_8: r1.i=247 // yr_execute_code()
0.001952 006981       - case OP_BITWISE_XOR: // yr_execute_code()
0.001959 006981       - case OP_INT_EQ: // yr_execute_code()
0.001964 006981       - case OP_AND: // yr_execute_code()
0.001969 006981       - case OP_JFALSE: // yr_execute_code()
0.001974 006981       - case OP_JFALSE: // yr_execute_code()
0.001979 006981       - case OP_JFALSE: // yr_execute_code()
0.001983 006981       - case OP_JFALSE: // yr_execute_code()
0.001988 006981       - case OP_JFALSE: // yr_execute_code()
0.001993 006981       - case OP_JFALSE: // yr_execute_code()
0.001998 006981       - case OP_JFALSE: // yr_execute_code()
0.002003 006981       - case OP_JFALSE: // yr_execute_code()
0.002008 006981       - case OP_JFALSE: // yr_execute_code()
0.002013 006981       - case OP_JFALSE: // yr_execute_code()
0.002017 006981       - case OP_JFALSE: // yr_execute_code()
0.002022 006981       - case OP_JFALSE: // yr_execute_code()
0.002028 006981       - case OP_JFALSE: // yr_execute_code()
0.002033 006981       - case OP_JFALSE: // yr_execute_code()
0.002037 006981       - case OP_JFALSE: // yr_execute_code()
0.002042 006981       - case OP_JFALSE: // yr_execute_code()
0.002047 006981       - case OP_JFALSE: // yr_execute_code()
0.002051 006981       - case OP_JFALSE: // yr_execute_code()
0.002056 006981       - case OP_JFALSE: // yr_execute_code()
0.002061 006981       - case OP_JFALSE: // yr_execute_code()
0.002066 006981       - case OP_MATCH_RULE: // yr_execute_code()
0.002071 006981       - case OP_HALT: // yr_execute_code()
0.002093 006981     } = 0 AKA ERROR_SUCCESS 0 // yr_execute_code()
0.002108 006981     - _yr_scanner_clean_matches() {}
0.002122 006981   } = 0 AKA ERROR_SUCCESS 0 // yr_scanner_scan_mem_blocks()
0.002130 006981 } = 0 AKA ERROR_SUCCESS 0 // yr_scanner_scan_mem()
0.002146 006981 - yr_scanner_destroy() {}
0.002156 006981 + yr_finalize() {
0.002162 006981   - hash__finalize() {}
0.002167 006981 } // yr_finalize()

It produces quite an interesting execution trace, expecially these blocks:

0.001604 006981       - case OP_PUSH_8: r1.i=95 // yr_execute_code()
0.001612 006981       - case OP_PUSH_8: r1.i=57 // yr_execute_code()
0.001619 006981       - case OP_BITWISE_XOR: // yr_execute_code()
$
0.001664 006981       - case OP_PUSH_8: r1.i=51 // yr_execute_code()
0.001671 006981       - case OP_PUSH_8: r1.i=95 // yr_execute_code()
0.001678 006981       - case OP_BITWISE_XOR: // yr_execute_code()

0.001729 006981       - case OP_PUSH_8: r1.i=248 // yr_execute_code()
0.001741 006981       - case OP_PUSH_8: r1.i=153 // yr_execute_code()
0.001745 006981       - case OP_BITWISE_XOR: // yr_execute_code()

0.001790 006981       - case OP_PUSH_8: r1.i=83 // yr_execute_code()
0.001799 006981       - case OP_PUSH_8: r1.i=52 // yr_execute_code()
0.001806 006981       - case OP_BITWISE_XOR: // yr_execute_code()

0.001865 006981       - case OP_PUSH_8: r1.i=248 // yr_execute_code()
0.001876 006981       - case OP_PUSH_8: r1.i=131 // yr_execute_code()
0.001885 006981       - case OP_BITWISE_XOR: // yr_execute_code()

0.001937 006981       - case OP_PUSH_8: r1.i=154 // yr_execute_code()
0.001944 006981       - case OP_PUSH_8: r1.i=247 // yr_execute_code()
0.001952 006981       - case OP_BITWISE_XOR: // yr_execute_code()

They push 2 values and XOR them together:

>>> chr(95 ^ 57)
'f'
>>> chr(51 ^ 95)
'l'
>>> chr(248 ^ 153)
'a'
>>> chr(83 ^ 52)
'g'
>>> chr(248 ^ 131)
'{'
>>> chr(154 ^ 247)
'm'

Interestingly they match the begining of our file, which we know is a valid flag start.

We can guess it checks the characters one by one and exits on the first non-matching character: it expected a m but got a t (test) from our test file.

We can use to that to bruteforce the flag char by char, by always checking the last xor sequence of the trace:

#!/bin/bash

# get these last 3 lines from the trace:
# 0.001937 006981       - case OP_PUSH_8: r1.i=154 // yr_execute_code()
# 0.001944 006981       - case OP_PUSH_8: r1.i=247 // yr_execute_code()
# 0.001952 006981       - case OP_BITWISE_XOR: // yr_execute_code()
#
# extract the 2 values (154 and 247), xor them and convert to char

get_next() {
    ./yara-4.2.3/yara -C flag.yarc test.txt 2>&1 | grep -B2 OP_BITWISE_XOR: | tail -3 \
        | sed -n 's/.*r1.i=\([0-9]*\).*$/\1/p' | xargs \
        | python -c "x,y=[int(_) for _ in input().split()]; print(chr(x^y), end='')" 2>/dev/null
}

rm -f test.txt

#
# get flag char by char
#
flag=""

for x in $(seq 32); do
    c="`get_next`"
    flag="${flag}${c}"
    echo $flag > test.txt
    cat test.txt
done

running this script gives the flag:

% sh bruteforce.sh

f
fl
fla
flag
flag{
flag{m
flag{m3
flag{m33
flag{m33t
flag{m33t_
flag{m33t_m
flag{m33t_me
flag{m33t_me_
flag{m33t_me_1
flag{m33t_me_1n
flag{m33t_me_1n_
flag{m33t_me_1n_7
flag{m33t_me_1n_7h
flag{m33t_me_1n_7h3
flag{m33t_me_1n_7h3_
flag{m33t_me_1n_7h3_a
flag{m33t_me_1n_7h3_ar
flag{m33t_me_1n_7h3_ar3
flag{m33t_me_1n_7h3_ar3n
flag{m33t_me_1n_7h3_ar3n4
flag{m33t_me_1n_7h3_ar3n4}
flag{m33t_me_1n_7h3_ar3n4}}
flag{m33t_me_1n_7h3_ar3n4}}}
flag{m33t_me_1n_7h3_ar3n4}}}}
flag{m33t_me_1n_7h3_ar3n4}}}}}
flag{m33t_me_1n_7h3_ar3n4}}}}}}