Double Free in Buffalo with Hmac Signatures

A Double Free occurs when a program attempts to free the same dynamically allocated memory twice. In the Buffalo web framework, this can intersect with Hmac Signatures when request parsing or routing logic mishandles memory associated with signature parameters. If a developer binds Hmac signature values to request context objects and those objects are freed or reset inconsistently, the same underlying buffer can be freed multiple times during a single request lifecycle or across concurrent requests.

Buffalo does not manage memory manually in application code, but the vulnerability can surface through misuse of C-based extensions or unsafe dependencies that interact with request buffers. For example, if an Hmac signature is extracted from headers, copied into a C string, and later freed by a middleware layer while another layer also attempts to free the same pointer, the runtime may encounter a Double Free. This can corrupt the heap, leading to crashes or potential code execution when an attacker crafts requests that trigger repeated signature validation paths.

An attacker can exploit this by sending requests with malformed or repeated Signature headers, forcing the application to traverse the signature verification logic multiple times within a single request. If the server uses native code via C bindings to optimize Hmac verification, improper reference handling around signature buffers can create use-after-free conditions immediately following the Double Free. The combination of Buffalo’s convention-based routing, which may instantiate multiple context objects per request, and the signature validation workflow increases the surface for memory mismanagement bugs.

Consider a scenario where a custom middleware decodes and verifies Hmac signatures using a C library via cgo. If the middleware allocates a buffer for the signature payload and passes it to a verification function that also allocates and later frees a duplicate reference, the program may crash when both allocations attempt to free the same memory. This is especially dangerous when the signature is included in JSON payloads or URL query parameters that are parsed and re-allocated across different framework layers, each potentially managing its own lifecycle for the data.

Real-world patterns from similar frameworks show that improper handling of cryptographic parameters in request parsing can lead to security-relevant instability. While Buffalo’s default stack is written in pure Go and avoids manual memory management, integrating native code for performance-sensitive Hmac operations reintroduces classic C memory safety issues. The framework’s reliance on middleware chains means that a poorly implemented Hmac verification hook can introduce Double Free paths that are triggered by malformed but technically valid-seeming requests.

To detect this class of issue during automated scans, tools like middleBrick analyze request flows that include Hmac Signatures and look for inconsistencies in how signature-related data is passed between layers. The scanner does not fix the bug but surfaces findings with remediation guidance, helping developers audit their use of native extensions and pointer management around cryptographic inputs.

Remediation focuses on ensuring that memory associated with Hmac signature inputs is managed consistently and that no layer attempts to free the same resource more than once. In Buffalo, this typically involves reviewing custom middleware that interfaces with C-based Hmac libraries and adjusting how signature data is passed and released.

When using Go’s standard crypto/hmac package, memory safety is handled automatically, but if you integrate C libraries via cgo, you must ensure strict ownership semantics. Below is an example of safe Hmac verification in pure Go within a Buffalo application, avoiding manual memory management entirely:

package middleware

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
)

func VerifyHmac(next http.Handler) http.Handler {
    return http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
        secret := []byte("your-secure-secret")
        signature := req.Header.Get("X-Hub-Signature-256")
        if signature == "" {
            http.Error(rw, "missing signature", http.StatusBadRequest)
            return
        }
        body := http.MaxBytesReader(rw, req.Body, 10<sup>6</sup>) // 1MB limit
        defer body.Close()
        data, err := io.ReadAll(body)
        if err != nil {
            http.Error(rw, "request too large", http.StatusRequestEntityTooLarge)
            return
        }
        mac := hmac.New(sha256.New, secret)
        mac.Write(data)
        expected := mac.Sum(nil)
        provided, err := hex.DecodeString(signature[len("sha256="):])
        if err != nil {
            http.Error(rw, "invalid signature format", http.StatusBadRequest)
            return
        }
        if !hmac.Equal(expected, provided) {
            http.Error(rw, "invalid signature", http.StatusUnauthorized)
            return
        }
        next.ServeHTTP(rw, req)
    })
}

If you must use C-based Hmac via cgo, ensure that allocations are freed in a single, well-defined scope. The following example demonstrates correct pairing of malloc and free within a helper function, preventing Double Free by isolating lifetime management to one function:

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

int verify_hmac_c(const char* key, int key_len, const char* data, int data_len, const char* received_sig) {
    unsigned char* computed_sig = NULL;
    unsigned int sig_len = 0;
    
    computed_sig = (unsigned char*)malloc(SHA256_DIGEST_LENGTH);
    if (!computed_sig) {
        return -1;
    }
    
    HMAC(EVP_sha256(), key, key_len, (const unsigned char*)data, data_len, computed_sig, &sig_len);
    
    int result = CRYPTO_memcmp(computed_sig, received_sig, SHA256_DIGEST_LENGTH) == 0;
    
    free(computed_sig); // freed exactly once
    computed_sig = NULL;
    return result;
}

In Buffalo middleware, wrap such C calls so that the allocation and deallocation boundaries are explicit and confined. Avoid passing pointers across multiple middleware layers or storing them in request context beyond the verification step.

Additionally, validate and normalize Hmac signature inputs before use. Ensure that header values are trimmed and checked for format consistency to prevent malformed inputs from causing erratic memory handling in underlying libraries. Combine these practices with code reviews focused on native integrations to eliminate Double Free risks specific to Hmac processing.