Out Of Bounds Read with Hmac Signatures

Out Of Bounds Read (OOB Read) in HMAC signature implementations occurs when code attempts to read beyond the allocated buffer boundaries, typically when parsing or validating cryptographic signatures. In HMAC contexts, this often happens during signature extraction, length validation, or buffer handling in cryptographic operations.

The most common manifestation involves improper length validation when extracting the HMAC signature from a message. Consider a scenario where an API expects a message with an appended HMAC signature. If the implementation doesn't properly validate the message length before attempting to read the signature, an attacker can craft messages that cause the parser to read beyond the allocated memory.

// Vulnerable HMAC signature extraction
function verifyHmac(message, key) {
    const signatureLength = 32; // SHA-256 HMAC length
    const messageLength = message.length;
    const signature = message.slice(messageLength - signatureLength); // Potential OOB read
    const data = message.slice(0, messageLength - signatureLength);
    
    const expectedSignature = crypto.createHmac('sha256', key)
        .update(data)
        .digest();
    
    return crypto.timingSafeEqual(signature, expectedSignature);
}

In this vulnerable implementation, if message.length < 32, the slice() operation will still return a buffer, but the subsequent timingSafeEqual() comparison will read beyond the actual message boundaries, potentially exposing memory contents or causing undefined behavior.

Another Hmac Signatures-specific OOB Read scenario occurs in length-prefixed HMAC implementations where the message includes a length field followed by the actual data and signature. An attacker can manipulate the length field to cause the parser to read beyond the allocated buffer:

// Vulnerable length-prefixed HMAC verification
function verifyLengthPrefixedHmac(buffer, key) {
    const length = buffer.readUInt32BE(0); // Read length prefix
    const data = buffer.slice(4, 4 + length); // Potential OOB read if length is manipulated
    const signature = buffer.slice(4 + length, 4 + length + 32);
    
    const expectedSignature = crypto.createHmac('sha256', key)
        .update(data)
        .digest();
    
    return crypto.timingSafeEqual(signature, expectedSignature);
}

If an attacker sends a buffer where the length field is larger than the actual remaining data, the slice() operation will read beyond the buffer's actual allocated size, potentially exposing sensitive memory contents or causing the verification to succeed incorrectly.

Base64-encoded HMAC signatures present another OOB Read vector. When implementations decode base64 signatures without proper length validation, malformed or oversized base64 strings can cause buffer overflows during the decoding process:

// Vulnerable base64 HMAC verification
function verifyBase64Hmac(base64Message, key) {
    const decoded = Buffer.from(base64Message, 'base64'); // No length validation
    const separatorIndex = decoded.indexOf(0x3A); // Colon separator
    const message = decoded.slice(0, separatorIndex);
    const signature = decoded.slice(separatorIndex + 1);
    
    const expectedSignature = crypto.createHmac('sha256', key)
        .update(message)
 .digest();
    
}

If the base64 string decodes to a buffer smaller than expected, or if the separator is missing, the signature extraction will read beyond the actual decoded buffer boundaries.

Detecting OOB Read vulnerabilities in HMAC signature implementations requires both static analysis and dynamic testing. The most effective approach combines code review for unsafe buffer operations with runtime fuzzing to trigger boundary violations.

Static analysis should focus on identifying unsafe buffer operations in HMAC verification code. Look for patterns where buffer lengths are not properly validated before slicing or reading operations. Key indicators include:

// Code patterns to flag for OOB Read risk
function analyzeForOobRead(code) {
    const riskyPatterns = [
    
    return riskyPatterns.some(pattern => pattern.test(code));
}

Dynamic testing should include fuzzing with boundary cases: messages shorter than the expected signature length, length fields that exceed actual data size, and malformed base64 encodings. A comprehensive fuzzing suite might include:

// Fuzzing test cases for HMAC OOB Read detection
const fuzzTestCases = [
    { name: 'short_message', data: 'short' },
    { name: 'zero_length', data: '' },
    { name: 'excessive_length', data: 'A'.repeat(1000) },
    { name: 'invalid_base64', data: '!!!invalid!!!' },

For production API scanning, tools like middleBrick can automatically detect OOB Read vulnerabilities in HMAC implementations by sending boundary-testing payloads and analyzing responses for memory disclosure or incorrect validation behavior. The scanner tests unauthenticated endpoints by sending crafted messages that attempt to trigger buffer overreads, then analyzes the timing and error responses to identify potential vulnerabilities.

middleBrick's approach includes testing HMAC signature extraction with messages of varying lengths, including those shorter than the expected signature size, and analyzing whether the implementation properly rejects these cases or exhibits timing differences that suggest unsafe memory access.