Heap Overflow with Api Keys

When an API key is received from a client (e.g., in the X-API-Key header), some services copy the raw value into a fixed‑size heap buffer before further processing such as hashing, lookup, or logging. If the copy operation does not validate the length of the incoming key, an attacker can supply an excessively long key that writes past the allocated buffer. This overwrites adjacent heap metadata or other objects, which can lead to crashes, information leakage, or arbitrary code execution.

Typical vulnerable code path in C/C++ looks like this:

/* Vulnerable: fixed buffer, no length check */
char api_key_buf[64];
const char *header = get_http_header("X-API-Key");
strcpy(api_key_buf, header);   // <-- overflow if header > 63 bytes
/* later use api_key_buf for hashing or comparison */

The strcpy call copies bytes until it encounters a null terminator, ignoring the buffer size. Supplying a key of 200 bytes will overflow api_key_buf by 136 bytes, corrupting the heap’s free‑list pointers or nearby allocations. Subsequent heap operations (e.g., free or another malloc) may then use the corrupted pointers, giving the attacker control over execution flow.

In higher‑level languages the risk is lower, but native extensions or C‑based libraries that handle API keys can still contain the same pattern. For example, a Go program that calls C.strcpy via cgo with a fixed C buffer inherits the vulnerability.

Detecting a heap overflow in API key handling requires observing how the service reacts to oversized key values. Because middleBrick performs unauthenticated, black‑box testing, it can automatically send progressively larger API key values and monitor for signs of memory corruption such as:

• HTTP 500 Internal Server Error responses that appear only after a certain key length.
• Unexpected termination of the backend process (observed as a dropped connection).
• Increased response latency or timeouts caused by the process crashing and being restarted.
• Presence of error strings like "segmentation fault" or "stack smashing detected" in error logs (if logs are exposed).

A practical test using the middleBrick CLI is:

middlebrick scan https://api.example.com \
  --header "X-API-Key:$(python -c 'print("A"*500)')"

If the service returns an error only when the key exceeds a threshold (e.g., 128 bytes), that indicates a length‑dependent fault worth investigating further. Complementary techniques include:

• Running the binary with AddressSanitizer (ASan) or MemorySanitizer (MSan) in a staging environment to catch out‑of‑bounds writes.
• Fuzzing the API key input with libFuzzer or AFL++, feeding random large strings and watching for crashes.
• Static analysis tools that flag unsafe functions like strcpy, gets, or unchecked memcpy on buffers sourced from network input.

Because middleBrick does not require agents or credentials, the test can be run against any publicly reachable endpoint, making it easy to incorporate into a regular security scan schedule.

The fix is to ensure that any copy of an API key into a bounded buffer respects the buffer’s size. In C/C++ replace unchecked functions with their length‑limited counterparts and always validate the input length before copying.

Revised version of the vulnerable snippet:

/* Safe: validate length then copy with a limit */
#define API_KEY_MAX_LEN 63   /* leave room for terminator */
char api_key_buf[64];
const char *header = get_http_header("X-API-Key");
size_t len = strlen(header);
if (len > API_KEY_MAX_LEN) {
    /* reject or truncate according to policy */
    return HTTP_400_BAD_REQUEST;
}
/* strncpy guarantees null‑termination when we limit to sizeof-1 */
strncpy(api_key_buf, header, sizeof(api_key_buf)-1);
api_key_buf[sizeof(api_key_buf)-1] = '\0';   /* explicit terminator */
/* now use api_key_buf safely */

Even better, avoid fixed‑size buffers altogether by allocating memory based on the actual key length:

char *api_key_buf = malloc(len + 1);
if (!api_key_buf) return HTTP_500_INTERNAL_SERVER_ERROR;
memcpy(api_key_buf, header, len);
api_key_buf[len] = '\0';   /* null‑terminate */
/* use api_key_buf */
free(api_key_buf);

In languages that manage memory automatically, the issue rarely appears, but if you must interface with native code, keep the same validation at the boundary. For example, a Go program using cgo should check the length before passing the string to C:

func validateAPIKey(header string) (*C.char, error) {
    if len(header) > 64 {
        return nil, fmt.Errorf("API key too long")
    }
    cstr := C.CString(header)   // C.mallocates len+1 bytes
    defer C.free(unsafe.Pointer(cstr))
    return cstr, nil
}

Finally, combine length validation with constant‑time comparison when checking the key against a stored secret to avoid timing attacks:

int constant_time_compare(const char *a, const char *b) {
    volatile int result = 0;
    for (size_t i = 0; i < API_KEY_MAX_LEN; ++i) {
        result |= a[i] ^ b[i];
    }
    return result;   // 0 means equal
}

By applying these patterns—length‑checked allocation or copying, explicit null‑termination, and constant‑time validation—you eliminate the heap overflow vector while preserving the intended API key functionality.