Timing Attack in Gorilla Mux with Hmac Signatures

A timing attack in the context of Gorilla Mux with HMAC signatures arises when signature verification does not use a constant-time comparison. Gorilla Mux is a standard HTTP router; it does not provide cryptographic primitives. If you build HMAC verification as a middleware or handler and compare the computed MAC with the MAC supplied by the client using a regular Go equality check (e.g., computedMAC == receivedMAC), an attacker can learn information about the correct signature byte-by-byte based on response timing differences.

In practice, an attacker can send requests with a guessed HMAC and measure response times. If the comparison short-circuits on the first mismatching byte, earlier bytes that match will take slightly longer than mismatched early bytes. Repeated measurements allow the attacker to infer the correct HMAC byte by byte, potentially leading to signature forgery. This becomes especially relevant when the HMAC is used to authenticate API requests or webhook signatures without additional protections like request-idempotency safeguards that mask timing variance.

Note that the vulnerability is not in Gorilla Mux itself but in how HMAC verification is implemented in user code combined with network characteristics that may allow timing measurements (e.g., load balancing or TLS termination introducing variability). The attack surface is the unauthenticated attack surface tested by middleBrick, where an endpoint accepts HMAC-signed requests and the verification logic contains a data-dependent branching pattern on the signature bytes.

Example of a vulnerable comparison in a Gorilla Mux route handler (do not use):

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"
    "strings"
)

func insecureVerify(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        secret := []byte("my-secret-key")
        receivedMAC := r.Header.Get("X-API-Signature")
        payload := []byte(strings.TrimPrefix(r.URL.Path, "/api/"))
        mac := hmac.New(sha256.New, secret)
        mac.Write(payload)
        computedMAC := fmt.Sprintf("%x", mac.Sum(nil))

        // Vulnerable: data-dependent branching on signature bytes
        if computedMAC == receivedMAC {
            next.ServeHTTP(w, r)
        } else {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
        }
    })
}

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

    secure := insecureVerify(r)
    http.ListenAndServe(":8080", secure)
}

To mitigate timing attacks when using HMAC signatures with Gorilla Mux, replace equality checks with a constant-time comparison. The standard library provides hmac.Equal for this purpose; it compares two byte slices in constant time, preventing attackers from learning partial matches via timing.

Additionally, ensure that the data used in HMAC computation is canonical (e.g., fixed-order query parameters, sorted headers if needed), and avoid including variable timing operations (such as database or filesystem lookups) before the comparison. Below is a secure example using hmac.Equal with Gorilla Mux.

Secure HMAC verification middleware for Gorilla Mux:

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"
    "strings"
)

func secureVerify(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        secret := []byte("my-secret-key")
        receivedMAC := r.Header.Get("X-API-Signature")
        if receivedMAC == "" {
            http.Error(w, "missing signature", http.StatusUnauthorized)
            return
        }

        payload := []byte(strings.TrimPrefix(r.URL.Path, "/api/"))
        mac := hmac.New(sha256.New, secret)
        mac.Write(payload)
        computedMAC := mac.Sum(nil)

        // Decode receivedMAC from hex
        receivedMACBytes, err := hex.DecodeString(receivedMAC)
        if err != nil || len(receivedMACBytes) != sha256.Size {
            http.Error(w, "invalid signature encoding", http.StatusUnauthorized)
            return
        }

        // Constant-time comparison to prevent timing attacks
        if !hmac.Equal(computedMAC, receivedMACBytes) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }

        next.ServeHTTP(w, r)
    })
}

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

    secure := secureVerify(r)
    http.ListenAndServe(":8080", secure)
}

Additional recommendations:

• Use a cryptographically random secret key and rotate it according to your security policy.
• Include a nonce or timestamp in the signed payload and reject requests with stale timestamps to prevent replay attacks.
• Ensure the entire request path used for the HMAC is deterministic (e.g., normalize query parameter ordering) to avoid variability that could aid an attacker.