Today we’re going deep on a CVE-2017-11882 exploit that’s a showcase in evasion. This sample uses dynamic XOR encryption, position-independent shellcode, and enough obfuscation to make analysts pause.

VirusTotal link to the sample

The Target: Microsoft’s Ancient Equation Editor

CVE-2017-11882 targets a stack-based buffer overflow in Microsoft Equation Editor (EQNEDT32.EXE). We’re dealing with legacy code that predates modern memory protections like ASLR and DEP.

The vulnerability is simple: when processing font names in embedded equation objects, the code copies user data into a 40-byte stack buffer without length validation. Game over.

Buffer Layout:
┌─────────────────┐
│  Return Address │ ← Overwritten to jump to shellcode
├─────────────────┤
│  Saved EBP      │
├─────────────────┤
│  Font Name      │ ← Buffer overflow starts here
└─────────────────┘

RTF Container: The Art of Fragmentation

The malicious document uses RTF as its container, but not in any straightforward way. The attackers fragmented the embedded OLE object:

# RTF structure discovered via rtfdump

rtfdump.py sample.rtf

# Stream 3: Main objdata

The hex data is nibble-misaligned, meaning standard extraction tools fail. You need the -S (hex-shift) parameter to reconstruct the object properly:

# This fails

rtfdump.py -s 3 -H sample.rtf -d > failed.bin

# This works - corrects the nibble alignment

rtfdump.py -s 3 -S -H -c 0x64: sample.rtf -d > success.bin

Fragmentation breaks signature-based detection, while nibble corruption may defeat automated analysis tools.

OLE Object Structure: Hiding in Plain Sight

Once reconstructed, the object reveals a corrupted OLE1 Native Stream:

Offset 0x60: 02 00 00 00 0B 00 00 00  65 71 75 41 70 36 C9 E4
[OLE1 Type] [Class Len]  [Corrupted "Equation.3"]

The class name corruption—”Equation.3” becomes “equAp6…IoN.3”—is intentional. It’s anti-analysis obfuscation to break tools that look for specific class names.

The MTEF (Math Type Equation Format) header follows standard structure:

Offset 0x84: 00 00 03 7E 81 EB 47 62  01 05 3B 1E BF EC 90 00
[MTEF Ver] [Header]      [Font Record Marker]

That 01 05 pattern is the font record marker triggering the vulnerable parsing code.

The Overflow: Precision Engineering

The malicious font data extends exactly to offset 0xDA, overwriting the return address with:

E9 65 01 00 00    ; JMP +0x165 (jumps to shellcode at offset 0x244)

Jump calculation: 0xDA + 5 + 0x165 = 0x244 (shellcode entry point)

This lands at the position-independent code prologue:

seg000:00000244  E8 00 00 00 00    call $+5      ; Push return address
seg000:00000249  5A                pop  edx      ; Get current EIP
seg000:0000024A  81 C2 36 01 00 00 add  edx, 136h ; Calculate data offset

Classic PIC technique—the shellcode calculates its runtime location without hardcoded addresses.

Dynamic XOR Encryption: Adaptive Keying

Instead of static XOR, the shellcode uses dynamic key generation:

; Key evolution: ECX_new = (ECX_old × 0x787F3959) + 0x5EB59C59
seg000:000002BB  imul ecx, 787F3959h   ; Multiply by LCG constant
seg000:000002C1  add  ecx, 5EB59C59h   ; Add LCG increment
seg000:00000290  xor  [edx], ecx      ; XOR 4 bytes with dynamic key
seg000:00000292  add  edx, 4           ; Advance pointer

Each 4-byte block uses a different XOR key. Python equivalent:

def dynamic_xor_decrypt(encrypted_data):
    decrypted = bytearray()
    ecx = 0  # Initial key state

    for i in range(0, len(encrypted_data), 4):
        ecx = ((ecx * 0x787F3959) + 0x5EB59C59) & 0xFFFFFFFF
        chunk = encrypted_data[i:i+4]
        key_bytes = ecx.to_bytes(4, 'little')
        for j in range(len(chunk)):
            decrypted.append(chunk[j] ^ key_bytes[j])

    return bytes(decrypted)

Static analysis can’t decrypt the strings, but the algorithm is deterministic enough for reconstruction.

Shellcode Obfuscation: Junk Code

; Save registers and flags
seg000:00000308  pushf               ; Save flags
seg000:00000309  push    edi         ; Save EDI register
seg000:0000030A  push    edi         ; Save EDI again

; Extensive junk math operations on the saved register
seg000:0000030B  lea     edi, [edi+5CD7h] ; Add 0x5CD7 to EDI
seg000:00000311  lea     edi, [edi+6C02h] ; Add 0x6C02 to EDI  
seg000:00000317  sub     edi, 4E03h       ; Subtract 0x4E03
seg000:0000031D  sub     edi, 1D71h       ; Subtract 0x1D71
seg000:00000323  sub     edi, 4EA1h       ; Subtract 0x4EA1
seg000:00000329  lea     edi, [edi+5A49h] ; Add 0x5A49 back

; Restore registers and flags to original state
seg000:0000032F  pop     edi         ; Restore original EDI
seg000:00000330  pop     edi         ; Restore again  
seg000:00000331  popf                ; Restore flags

The actual functionality—address calculation and control transfer—is buried in over 300 bytes of noise.

The Payload: No PowerShell Required

Decrypted strings reveal the complete attack chain.

The payload downloads petitzx.exe from the C2 server, saves it as petitgxk8523.exe in the user’s AppData folder, and executes it with CreateProcessW.

Wrapping Up

This CVE-2017-11882 exploit layers multiple evasion techniques:

• Document fragmentation breaks signature detection
• Nibble corruption defeats automated extraction
• Dynamic encryption prevents static string analysis
• Code obfuscation wastes analyst time

Stay tuned for more malware-related tips and tricks!