Auth Bypass in Buffalo with Hmac Signatures

Buffalo is a Go web framework that encourages rapid development. When HMAC signatures are used for request authentication, the security of the mechanism depends on consistent implementation across client and server. An auth bypass can occur when the signature verification logic is incomplete, uses weak comparison, or fails to include all relevant request components, allowing an attacker to forge valid signatures.

Consider a scenario where a Buffalo application signs only the request body with a shared secret but does not include the HTTP method, the request path, or critical headers. An attacker who knows or can guess the body format can replay the signed body to a different endpoint that also expects the same signature scheme but does not validate the method or path. Because the server only checks the presence and correctness of the signature and not the full context of the request, the attacker’s crafted request may be accepted as authorized, leading to an auth bypass.

A second common pitfall is the use of a weak comparison when validating the signature. If the application compares the computed HMAC with the transmitted signature using a simple string equality check that is vulnerable to timing attacks, an attacker can use timing differences to gradually recover information about the valid signature or force a mismatch that is incorrectly accepted due to improper error handling. Even if the algorithm and key are strong, a non-constant-time comparison can weaken the overall scheme enough to facilitate bypass in certain threat models.

Another variant involves missing canonicalization of the signed payload. If the server and client do not agree on how to serialize the data before signing—for example, differing field ordering in JSON, inconsistent handling of whitespace, or differing timestamp formats—an attacker can alter non-security-critical parts of the request that do not affect the signature verification yet change the semantics of the request. The signature may still verify, but the intent of the request is changed, effectively bypassing intended authorization checks.

Buffalo applications that expose unauthenticated endpoints while also using HMAC for select authenticated routes can mistakenly treat the presence of a valid signature as sufficient for authorization, without checking the broader context such as origin, idempotency keys, or replay windows. An attacker who can observe or guess a valid signature may be able to direct the request to an unauthenticated or less-protected route that still processes the operation because the server trusts the signature alone. This highlights the importance of aligning the scope of what is signed with the actual authorization model enforced by the application.

These patterns illustrate how auth bypass can emerge not from a broken HMAC algorithm, but from inconsistencies in what is signed, how verification is performed, and how the application interprets a valid signature. Security therefore requires that the server validate the method, path, critical headers, and body as a unified request context, enforce constant-time comparison, and ensure that signature verification is one component of a broader authorization strategy rather than a standalone gate.

To remediate auth bypass risks in Buffalo applications using HMAC signatures, adopt a canonical, context-aware signing and verification approach. Always include the HTTP method, request path, selected headers, and a timestamp or nonce in the data that is signed. This ensures that a signature tied to one resource or action cannot be reused against another.

Below is a concrete example of server-side signature verification in a Buffalo action. The code computes the expected HMAC over a canonical string composed of method, path, selected headers, timestamp, and body, then performs a constant-time comparison with the signature provided by the client.

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

func verifyHMAC(next http.Handler) http.Handler {
    secret := []byte(os.Getenv("HMAC_SECRET"))
    return http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
        // Expected header format: "HMAC timestamp=...,headers=...,signature=..."
        auth := req.Header.Get("Authorization")
        if auth == "" {
            http.Error(rw, "authorization header required", http.StatusUnauthorized)
            return
        }
        parts := strings.Split(auth, " ")
        if len(parts) != 2 || parts[0] != "HMAC" {
            http.Error(rw, "invalid authorization scheme", http.StatusUnauthorized)
            return
        }
        paramsStr := strings.TrimPrefix(auth, "HMAC ")
        params := make(map[string]string)
        for _, pair := range strings.Split(paramsStr, ",")) {
            kv := strings.SplitN(pair, "=", 2)
            if len(kv) == 2 {
                params[kv[0]] = kv[1]
            }
        }
        timestamp, ok := params["timestamp"]
        if !ok {
            http.Error(rw, "missing timestamp", http.StatusUnauthorized)
            return
        }
        reqHeaders := params["headers"]
        receivedSig := params["signature"]

        // Replay window protection (example: 5 minutes)
        t, err := time.Parse(time.RFC3339, timestamp)
        if err != nil || time.Since(t) > 5*time.Minute {
            http.Error(rw, "stale or invalid timestamp", http.StatusUnauthorized)
            return
        }

        // Build canonical string
        var headerParts []string
        for _, h := range strings.Split(reqHeaders, ";") {
            h = strings.TrimSpace(h)
            if v := req.Header.Get(h); v != "" {
                headerParts = append(headerParts, h+":"+v)
            }
        }
        canonical := req.Method + "\n" + req.URL.Path + "\n" + strings.Join(headerParts, "\n") + "\n" + timestamp + "\n" + req.BodyToString() // implement BodyToString appropriately

        mac := hmac.New(sha256.New, secret)
        mac.Write([]byte(canonical))
        expected := hex.EncodeToString(mac.Sum(nil))

        if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
            http.Error(rw, "invalid signature", http.StatusUnauthorized)
            return
        }

        next.ServeHTTP(rw, req)
    })
}

On the client side, construct the same canonical string and include it in the Authorization header. Here is an example of how a Buffalo client or proxy can prepare the signed request:

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

func signRequest(req *http.Request, secret string) error {
    timestamp := time.Now().UTC().Format(time.RFC3339)
    headers := []string{"Content-Type"} // include only headers that server expects
    var body string
    // read body safely; for illustration only
    // body, _ = io.ReadAll(req.Body)
    // reset body if needed
    canonical := req.Method + "\n" + req.URL.Path + "\n" + strings.Join(headers, "\n") + "\n" + timestamp + "\n" + body
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    sig := hex.EncodeToString(mac.Sum(nil))
    req.Header.Set("Authorization", "HMAC timestamp="+timestamp+",headers="+strings.Join(headers, ";")+",signature="+sig)
    return nil
}

Key remediation points: include method, path, and critical headers in the signed payload; enforce a short timestamp window to prevent replay; use hmac.Equal for constant-time comparison; validate the Authorization scheme strictly; and ensure the server does not treat a valid HMAC as sufficient proof of authorization without additional checks. These changes reduce the risk of auth bypass caused by incomplete or inconsistent signature validation in Buffalo applications.