Heap Overflow with Hmac Signatures

Heap overflow vulnerabilities in HMAC signature verification usually arise when native cryptographic APIs receive attacker‑controlled data that exceeds internal buffer limits. Many low‑level HMAC implementations (e.g., OpenSSL’s HMAC_Init_ex, HMAC_Update, or Windows CNG BCryptHash) allocate internal buffers based on the length of the key or the message supplied by the caller. If the application does not validate the size of these inputs before passing them to the HMAC function, an attacker can supply an excessively long key or message, causing the library to write past the end of a heap‑allocated buffer.

Typical vulnerable code paths include:

• Accepting a raw JWT or API‑signature header and directly passing the key field to HMAC_Init_ex without checking its length.
• Using a custom HMAC wrapper that concatenates user‑provided data into a fixed‑size stack buffer before hashing.
• Parsing binary protocols where the length field is trusted and used to allocate a buffer for the HMAC key via malloc.

When the overflow occurs, the attacker may overwrite adjacent heap metadata or function pointers, leading to arbitrary code execution, privilege escalation, or service crash. Because HMAC verification is often performed on unauthenticated endpoints (e.g., webhook signature validation), the attack surface is exposed without any credentials.

Detecting this issue requires looking for places where user‑controlled data is fed directly into HMAC primitives without length validation. Static analysis can flag calls to native HMAC APIs where the key or message length originates from request parameters, headers, or body payloads. Dynamic black‑box scanners such as middlebrick can help by exercising the unauthenticated attack surface and observing whether excessively large inputs trigger abnormal behavior (e.g., 500 errors, crashes, or delayed responses) that may indicate a heap overflow.

Example using the middleBrick CLI to scan an endpoint that verifies a webhook signature:

middlebrick scan https://api.example.com/webhook --header "Signature: $(python3 -c "import sys; print('a'*1000000)")"

The command sends a signature value consisting of one million ‘a’ characters. If the underlying HMAC implementation allocates a buffer based on this length without bounds checking, the request may cause a crash or timeout, which middleBrick’s Input Validation check will surface as a finding with severity high and a recommendation to enforce input length limits.

Additionally, middleBrick’s OpenAPI/Swagger analysis can reveal operations that accept a string or binary parameter destined for HMAC verification without a maxLength constraint, highlighting a potential missing validation.

The most reliable fix is to validate and limit the size of any data that will be used as an HMAC key or message before invoking the cryptographic function. Use language‑native HMAC helpers that internally manage buffers safely, or enforce explicit length checks when calling lower‑level APIs.

Example in C using OpenSSL (vulnerable)

/* Vulnerable: key length taken directly from user input */
int verify_signature(const char *user_key, size_t user_key_len,
                     const char *msg, size_t msg_len,
                     const unsigned char *sig)
{
    unsigned char *computed = malloc(EVP_MAX_MD_SIZE);
    unsigned int outlen;
    HMAC_CTX *ctx = HMAC_CTX_new();
    HMAC_Init_ex(ctx, user_key, user_key_len, EVP_sha256(), NULL);  // <-- potential overflow
    HMAC_Update(ctx, (unsigned char *)msg, msg_len);
    HMAC_Final(ctx, computed, &outlen);
    HMAC_CTX_free(ctx);
    return (outlen == sig_len && memcmp(computed, sig, outlen) == 0);
}

Fixed version with length validation

#define MAX_HMAC_KEY_LEN 64  // reasonable limit for SHA‑256 keys

int verify_signature_fixed(const char *user_key, size_t user_key_len,
                           const char *msg, size_t msg_len,
                           const unsigned char *sig)
{
    if (user_key_len > MAX_HMAC_KEY_LEN) {
        return 0;  // reject overly long key
    }
    /* msg length can also be limited if protocol defines a max */
    unsigned char *computed = malloc(EVP_MAX_MD_SIZE);
    unsigned int outlen;
    HMAC_CTX *ctx = HMAC_CTX_new();
    HMAC_Init_ex(ctx, user_key, user_key_len, EVP_sha256(), NULL);
    HMAC_Update(ctx, (unsigned char *)msg, msg_len);
    HMAC_Final(ctx, computed, &outlen);
    HMAC_CTX_free(ctx);
    return (outlen == sig_len && memcmp(computed, sig, outlen) == 0);
}

Example in Java using javax.crypto.Mac (safe by default)

// Java’s Mac API copies the supplied key into an internal buffer,
// so excessive key length does not cause a heap overflow.
// Nevertheless, enforce a policy limit to avoid DoS.
private static final int MAX_KEY_BYTES = 64;

public boolean verify(String keyBase64, String payload, String signatureBase64) {
    byte[] key = Base64.getDecoder().decode(keyBase64);
    if (key.length > MAX_KEY_BYTES) {
        throw new IllegalArgumentException("HMAC key too large");
    }
    Mac mac = Mac.getInstance("HmacSHA256");
    mac.init(new SecretKeySpec(key, "HmacSHA256"));
    byte[] macBytes = mac.doFinal(payload.getBytes(StandardCharsets.UTF_8));
    return MessageDigest.isEqual(macBytes, Base64.getDecoder().decode(signatureBase64));
}

Example in Python using the built‑in hmac module

import hmac, hashlib, base64
MAX_KEY_LEN = 64

def verify_signature(key_b64: str, payload: str, sig_b64: str) -> bool:
    key = base64.b64decode(key_b64)
    if len(key) > MAX_KEY_LEN:
        return False  # reject oversized key
    mac = hmac.new(key, payload.encode('utf-8'), hashlib.sha256)
    return hmac.compare_digest(mac.digest(), base64.b64decode(sig_b64))

By applying these checks, you eliminate the heap overflow vector while preserving the cryptographic integrity of the HMAC verification. Remember to communicate the validation failure via a generic error message (e.g., "Invalid signature") to avoid leaking implementation details.