Mojic: Observable Timing Discrepancy in HMAC Verification

Summary

The CipherEngine in Mojic v2.1.3 uses a standard equality operator (!==) to verify the HMAC-SHA256 integrity seal during the decryption phase. This creates an Observable Timing Discrepancy (CWE-208), allowing a potential attacker to bypass the file integrity check via a timing attack.

Details

In lib/CipherEngine.js, the footer check validates the HMAC signature using a standard string comparison: if (footerHex !== calcDigest) { ... }

Standard string comparisons in JavaScript short-circuit; they return false the moment a character mismatch occurs. Because the time taken to evaluate the comparison is proportional to the number of matching leading bytes, an attacker can measure the exact microseconds it takes for the engine to throw the FILE_TAMPERED error. By repeatedly altering the signature byte-by-byte and analyzing these minute timing differences, a malicious actor can theoretically forge a valid HMAC signature without possessing the decryption password.

PoC

The vulnerable implementation is located in lib/CipherEngine.js, within the getDecryptStream() flush method (approximately line 265):

// Vulnerable Code
if (footerHex !== calcDigest) {
    this.emit('error', new Error("FILE_TAMPERED"));
    return;
}

Recommended Remediation:

Replace the standard equality operator with Node.js's built-in constant-time comparison utility, crypto.timingSafeEqual().

// Remediated Code
const footerBuffer = Buffer.from(footerHex, 'hex');
const calcBuffer = Buffer.from(calcDigest, 'hex');

if (footerBuffer.length !== calcBuffer.length || !crypto.timingSafeEqual(footerBuffer, calcBuffer)) {
    this.emit('error', new Error("FILE_TAMPERED"));
    return;
}

Impact

If successfully exploited, an attacker could tamper with the encrypted .mojic payload and forge a valid HMAC signature. This bypasses the integrity seal, tricking the decryption engine into processing maliciously injected emoji streams. Because the engine translates these emojis back into C keywords and raw data chunks, this could ultimately result in arbitrary Code Injection into the restored .c source code when an unsuspecting user decrypts the tampered file.