Buffer Overflow in Gorilla Mux with Hmac Signatures

Gorilla Mux is a widely used HTTP router for Go that supports route variables and middleware patterns. When HMAC signatures are introduced to authenticate request payloads or headers, integration mistakes can expose a buffer overflow risk, especially when signature handling involves fixed-size buffers or unchecked input lengths.

The vulnerability arises when the application reads the HMAC signature from headers or query parameters and copies it into a fixed-size byte array without validating length. For example, if a developer uses a fixed-size byte array to store a signature and the attacker provides an overly long or malformed signature, a classic off-by-one or pure buffer overflow can occur during the copy operation. This can corrupt adjacent memory, leading to crashes or potentially allowing an attacker to influence execution flow.

In the context of Gorilla Mux, route variables and middleware interceptors may pass user-controlled data directly into signature verification logic. If the signature is expected to be a base64-encoded HMAC, but the application decodes it into a fixed buffer, an oversized payload can overflow the destination buffer. Even when using higher-level functions, improper use of byte slices with pre-allocated backing arrays can create similar risks if length checks are omitted before writing.

Additionally, parsing multiple headers or concatenating strings to form the signed payload without length constraints can expand the attack surface. An attacker may send a long header value that, when processed by the HMAC verification code, triggers an overflow in an intermediate buffer. Because Gorilla Mux often chains multiple handlers, a vulnerable handler can compromise the safety of downstream handlers.

These issues map to common weaknesses in the CWE/SANS Top 25, particularly CWE-120 (Classic Buffer Overflow) or CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The combination of a high-performance router and cryptographic verification increases the stakes: a single unchecked copy can undermine the entire authentication mechanism.

middleBrick scans such integrations as part of its 12 security checks, including Input Validation and Unsafe Consumption, to detect patterns that may lead to buffer overflow conditions. By correlating runtime behavior with OpenAPI specifications and route definitions, the scanner highlights risky handling of HMAC-related inputs in Gorilla Mux-based services.

To remediate buffer overflow risks when using HMAC signatures with Gorilla Mux, always validate input lengths, use bounded-safe operations, and avoid fixed-size buffers for variable-length data. Below are concrete, safe patterns for HMAC handling.

Example 1: Safe HMAC verification with length checks

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"github.com/gorilla/mux"
)

func verifyHMAC(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		signature := r.Header.Get("X-API-Signature")
		if signature == "" {
			http.Error(w, "missing signature", http.StatusUnauthorized)
			return
		}
		// Limit signature length to prevent overflow attempts
		if len(signature) > 1024 {
			http.Error(w, "signature too long", http.StatusBadRequest)
			return
		}
		// Use HMAC-SHA256 constant-time comparison
		mac := hmac.New(sha256.New, []byte("my-secret-key"))
		mac.Write([]byte(r.URL.Path))
		expected := hex.EncodeToString(mac.Sum(nil))
		if !hmac.Equal([]byte(expected), []byte(signature)) {
			http.Error(w, "invalid signature", http.StatusUnauthorized)
			return
		}
		next.ServeHTTP(w, r)
	})
}

func main() {
	r := mux.NewRouter()
	r.HandleFunc("/api/resource", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("secure data"))
	}).Methods("GET")

	// Apply middleware globally
	http.ListenAndServe(":8080", verifyHMAC(r))
}

Example 2: Using bounded byte slices when decoding signatures

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"net/http"
	"github.com/gorilla/mux"
)

func safeDecodeAndVerify(w http.ResponseWriter, r *http.Request) {
	sigB64 := mux.Vars(r)["signature"] // route variable, user-controlled
	// Enforce max length before decoding
	if len(sigB64) > 512 {
		http.Error(w, "signature too long", http.StatusBadRequest)
		return
	}
	sigBytes, err := base64.StdEncoding.DecodeString(sigB64)
	if err != nil {
		http.Error(w, "invalid encoding", http.StatusBadRequest)
		return
	}
	// Ensure decoded length is within expected bounds
	if len(sigBytes) != sha256.Size {
		http.Error(w, "invalid signature length", http.StatusBadRequest)
		return
	}
	mac := hmac.New(sha256.New, []byte("my-secret-key"))
	mac.Write([]byte(r.Body))
	if !hmac.Equal(mac.Sum(nil), sigBytes) {
		http.Error(w, "signature mismatch", http.StatusUnauthorized)
		return
	}
	w.Write([]byte("verified"))
}

func main() {
	r := mux.NewRouter()
	r.HandleFunc("/verify/{signature}", safeDecodeAndVerify).Methods("POST")
	http.ListenAndServe(":8080", r)
}

Key practices: enforce maximum lengths, use constant-time comparison (hmac.Equal), avoid fixed-size buffers for decoded data, and validate decoded byte lengths against expected sizes. These steps prevent buffer overflow conditions and ensure robust HMAC verification in Gorilla Mux routes.