Use After Free with Hmac Signatures

HMAC (Hash-based Message Authentication Code) is a pure function that takes a key, a message, and a hash algorithm to produce a tag. In most high‑level languages the operation is safe because the cryptographic library manages memory internally. However, when developers drop down to the native API of libraries such as OpenSSL, BoringSSL, or libsodium, they must manually allocate and free contexts and key buffers. If a context or buffer is freed and then reused for a subsequent HMAC operation, a use‑after‑free condition arises.

Typical vulnerable code paths include:

• Calling EVP_MD_CTX_free on an OpenSSL HMAC context and then attempting to reuse the same pointer for another HMAC_Init_ex call.
• Freeing the key buffer (e.g., OPENSSL_free(key)) after computing an HMAC but before the library finishes internal processing that may still reference the key.
• Using a stack‑allocated array for the key, returning from the function that allocated it, and then passing the dangling pointer to a HMAC function invoked later via a callback or async worker.

When the freed memory is reallocated for another purpose, the HMAC routine may read or write unintended data, leading to crashes, incorrect tags, or information leakage. Attackers can trigger this condition by sending specially crafted requests that cause the server to allocate, free, and re‑allocate memory in a predictable pattern (e.g., via repeated short‑lived connections that allocate HMAC contexts). Real‑world examples include CVE‑2016-2105 in OpenSSL’s ASN1 parsing, where a use‑after‑free in a verification routine allowed remote code execution; similar patterns have been found in HMAC‑related code paths when developers mismanage EVP_MD_CTX objects.

Detecting a use‑after‑free in HMAC signatures from the outside is challenging because the defect lives in the process’s memory space. However, black‑box scanners like middleBrick can surface symptoms that strongly suggest memory corruption:

• Intermittent 500 Internal Server Error responses when the same HMAC‑protected endpoint is queried repeatedly under load.
• Variations in response timing that correlate with the allocation/free pattern (e.g., occasional spikes when a freed buffer is re‑used).
• Presence of crash dumps or core files in server logs after a burst of requests containing malformed or unusually large payloads.

middleBrick’s unauthenticated scan runs 12 parallel checks, including Data Exposure and Rate Limiting. While it does not instrument the binary, it can be configured to trigger a high volume of HMAC‑signed requests (via the Authorization header or query signature) and monitor for abnormal HTTP status codes or latency outliers. When such anomalies appear, the scanner flags them under the Inventory Management and Unsafe Consumption categories, providing a finding with severity high and remediation guidance to review native cryptographic usage.

For developers who want to verify the issue locally, tools such as AddressSanitizer (ASan) or Valgrind can be added to the build. Running the service under ASan while exercising the HMAC endpoint will produce a clear stack trace pointing to the use‑after‑free location (e.g., ==12345==ERROR: AddressSanitizer: heap-use-after-free on address 0x60200000ff80). This concrete evidence can then be correlated with the findings reported by middleBrick to prioritize fixes.

The safest way to avoid use‑after‑free when working with HMAC is to let the cryptographic library manage the lifetime of its objects. In C/C++ with OpenSSL, the recommended pattern is:

#include <openssl/evp.h>
#include <openssl/hmac.h>
#include <string.h>

/* Compute HMAC‑SHA256 and return a newly allocated buffer. */
unsigned char *hmac_sha256(const unsigned char *key, size_t key_len,
                           const unsigned char *msg, size_t msg_len,
                           size_t *out_len)
{
    EVP_MD_CTX *ctx = EVP_MD_CTX_new();
    if (!ctx) return NULL;

    if (1 != HMAC_Init_ex(ctx, key, key_len, EVP_sha256(), NULL)) {
        EVP_MD_CTX_free(ctx);
        return NULL;
    }
    if (1 != HMAC_Update(ctx, msg, msg_len)) {
        EVP_MD_CTX_free(ctx);
        return NULL;
    }
    unsigned char *out = OPENSSL_malloc(EVP_MAX_MD_SIZE);
    if (!out) { EVP_MD_CTX_free(ctx); return NULL; }
    if (1 != HMAC_Final(ctx, out, out_len)) {
        OPENSSL_free(out);
        EVP_MD_CTX_free(ctx);
        return NULL;
    }
    EVP_MD_CTX_free(ctx);
    return out; /* caller must OPENSSL_free when done */
}

void example(void)
{
    const unsigned char *key = (unsigned char *)"secretkey";
    const unsigned char *msg = (unsigned char *)"hello";
    size_t out_len;
    unsigned char *tag = hmac_sha256(key, strlen((char *)key), msg, strlen((char *)msg), &out_len);
    if (tag) {
        /* use tag … */
        OPENSSL_free(tag);
    }
}

Key points that eliminate use‑after‑free:

• The EVP_MD_CTX is created with EVP_MD_CTX_new and always released with EVP_MD_CTX_free exactly once, even on error paths.
• The key buffer is never freed inside the function; ownership remains with the caller, preventing premature deallocation.
• The output buffer is allocated with OPENSSL_malloc

In higher‑level languages, use the language’s native HMAC APIs which handle memory internally:

// Python 3 – HMAC with the standard library
import hmac, hashlib, os

def verify_signature(key: bytes, msg: bytes, sig: bytes) -> bool:
    expected = hmac.new(key, msg, hashlib.sha256).digest()
    return hmac.compare_digest(expected, sig)

# Usage
key = os.urandom(32)
msg = b'test message'
sig = hmac.new(key, msg, hashlib.sha256).digest()
assert verify_signature(key, msg, sig)

By avoiding manual context management and relying on the library’s allocated objects, the risk of use‑after‑free in HMAC signature code disappears. After applying these fixes, re‑run middleBrick scans; the previously flagged anomalies should vanish, and the scanner will report a clean bill of health for the HMAC‑related checks.