Dns Cache Poisoning in Buffalo with Hmac Signatures

Buffalo is a web framework for the Go programming language. When Hmac Signatures are used to protect routing or form parameters, cache poisoning can occur if the cached response contains a valid HMAC but was generated for a different request context, such as a different path, method, or user. Because Buffalo does not automatically bind the HMAC scope to the request path and method, an attacker can sometimes reuse a cached response that carries a valid signature but was intended for another resource.

Consider an endpoint that caches a redirect or a JSON response with an HMAC derived from the URL path and a server secret. If the cache key omits the full request context (e.g., method and host), a cached response for a benign GET route might be served for a different route that requires stricter authorization. The HMAC remains valid because the signature was computed over a subset of request attributes that the attacker can predict or observe, enabling cache poisoning via a crafted link or shared cache.

In a Buffalo application, this can surface in scenarios where signed URLs are cached at the edge or within a shared caching layer. If the signature does not include a nonce or timestamp, and the cache key does not incorporate the full request fingerprint, an attacker who tricks a victim into visiting a poisoned link may receive a response that the server previously signed for another action. This can lead to unauthorized state changes or information disclosure, aligning with OWASP API Top 10 API1:2023 Broken Object Level Authorization when the cached response is interpreted as authoritative for a different resource.

SSRF and external cache interactions can exacerbate this risk. If Buffalo routes trigger backend calls whose results are cached with HMACs, an attacker may induce the server to cache a malicious response from an internal endpoint and then serve it to other users. Because the HMAC is tied only to the payload and a shared secret, the server may treat the poisoned cache entry as valid. This illustrates why cache keys must include the full request context and why HMAC usage must consider replay and scope boundaries.

To mitigate Dns Cache Poisoning risks when using Hmac Signatures in Buffalo, bind the signature scope to the full request context and avoid caching responses that depend on mutable request attributes. Include the HTTP method, host, and an optional nonce or timestamp in the data that is signed, and ensure cache keys incorporate these elements.

Example: signing the canonical request data in Buffalo middleware.

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

func SignMiddleware(secret string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            // Include method, host, path, and a short timestamp to prevent replay
            timestamp := time.Now().Unix()
            payload := strings.Join([]string{
                r.Method,
                r.Host,
                r.URL.Path,
                string(timestamp),
            }, "|")
            mac := hmac.New(sha256.New, []byte(secret))
            mac.Write([]byte(payload))
            signature := hex.EncodeToString(mac.Sum(nil))

            // Add signature and timestamp to headers for downstream caches or clients
            w.Header().Set("X-Request-Signature", signature)
            w.Header().Set("X-Request-Timestamp", string(rune(timestamp)))

            next.ServeHTTP(w, r)
        })
    }
}

Example: verifying the signed request in a Buffalo route to ensure the context matches.

func VerifySignature(secret string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            receivedSig := r.Header.Get("X-Request-Signature")
            receivedTs := r.Header.Get("X-Request-Timestamp")
            if receivedSig == "" || receivedTs == "" {
                http.Error(w, "missing signature or timestamp", http.StatusBadRequest)
                return
            }

            timestamp := receivedTs // in a real app, parse and validate freshness
            payload := strings.Join([]string{
                r.Method,
                r.Host,
                r.URL.Path,
                timestamp,
            }, "|")
            mac := hmac.New(sha256.New, []byte(secret))
            mac.Write([]byte(payload))
            expected := hex.EncodeToString(mac.Sum(nil))

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

            next.ServeHTTP(w, r)
        })
    }
}

When using caching, ensure the cache key includes the signature scope components. For example, use a composite key such as method:host:path:timestamp so that cached entries cannot be reused across different methods or hosts. Avoid caching responses that contain sensitive or user-specific data unless the HMAC scope and cache key explicitly cover the differentiating attributes.

Finally, prefer short-lived nonces or timestamps and enforce freshness checks to reduce replay window. Combine these measures with secure secret rotation and restrict caching of responses that carry HMACs computed over mutable inputs. This approach reduces the risk of Dns Cache Poisoning in Buffalo applications that rely on Hmac Signatures for integrity.