Cache Poisoning in Buffalo with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache system to store malicious or incorrect responses, which are then served to other users. In Buffalo, this risk is amplified when Hmac Signatures are used incorrectly, as they may give a false sense of integrity while failing to protect cache keys or vary headers that determine cache storage. If a request is cacheable based on a subset of headers or query parameters and the Hmac Signature is computed only over the request body or a limited set of inputs, an attacker can manipulate non-signed parts of the request to poison the cache under the same signature context.

Consider a Buffalo API endpoint that caches responses based on URL path and query parameters but signs only the JSON payload using Hmac Signatures. An authenticated attacker could alter non-signed query parameters such as locale or tracking IDs to inject malicious content into the cached response. Because the signature remains valid (the body is unchanged), the poisoned entry is stored and later served to other users, leading to BOLA/IDOR-like exposure where different users receive unintended data. This is particularly dangerous when the cache is shared across tenants or when user-specific data is inadvertently cached due to missing vary headers.

Buffalo applications that use Hmac Signatures must ensure that the signature scope includes all inputs that affect the response, including headers used for cache differentiation. If the signature does not cover vary headers (e.g., Accept-Language, Authorization), an attacker can cause the same signed request to produce different cached outputs. Additionally, if the application caches at the CDN or reverse proxy layer without considering cookie or session influences, the Hmac Signature may not prevent cache poisoning because the cache key diverges from the signed context. The vulnerability manifests as inconsistent responses for different users under the same signed request, violating integrity assumptions and potentially exposing private data.

To remediate cache poisoning risks when using Hmac Signatures in Buffalo, ensure that the signature covers all request dimensions that influence the response. This includes headers, query parameters, and body. Below are concrete code examples demonstrating secure Hmac Signature generation and verification in Buffalo using Go. These examples assume you are using the standard crypto/hmac and crypto/sha256 packages.

// GenerateHmacSignature creates a secure HMAC-SHA256 signature over the full request context.
// It includes method, path, query parameters, selected headers, and body to prevent cache poisoning.
func GenerateHmacSignature(secret, method, path, query string, headers map[string]string, body []byte) string {
    h := hmac.New(sha256.New, []byte(secret))
    h.Write([]byte(method + "|" + path + "|" + query + "|"))
    // Include headers that affect caching, such as Accept-Language and Authorization.
    for _, key := range []string{"Accept-Language", "Authorization", "Content-Type"} {
        if v, ok := headers[key]; ok {
            h.Write([]byte(key + ":" + v + "|"))
        }
    }
    h.Write(body)
    return hex.EncodeToString(h.Sum(nil))
}

// VerifyHmacSignature validates the HMAC while ensuring all relevant request parts are checked.
func VerifyHmacSignature(secret, signature, method, path, query string, headers map[string]string, body []byte) bool {
    expected := GenerateHmacSignature(secret, method, path, query, headers, body)
    return hmac.Equal([]byte(expected), []byte(signature))
}

// In a Buffalo action, use the above functions to sign and verify the full request context.
// Example usage inside a beforeAction or around filter:
func ApiFilter(c buffalo.Context) error {
    secret := os.Getenv("HMAC_SECRET")
    sig := c.Request().Header.Get("X-API-Signature")
    method := c.Request().Method
    path := c.Request().URL.Path
    query := c.Request().URL.RawQuery
    body, _ := ioutil.ReadAll(c.Request().Body)
    headers := map[string]string{
        "Accept-Language": c.Request().Header.Get("Accept-Language"),
        "Authorization":   c.Request().Header.Get("Authorization"),
        "Content-Type":    c.Request().Header.Get("Content-Type"),
    }
    if !VerifyHmacSignature(secret, sig, method, path, query, headers, body) {
        return c.Error(http.StatusUnauthorized, errors.New("invalid signature"))
    }
    return c.Next()
}

Additionally, configure cache keys to incorporate the same signed components. If your caching layer uses headers, ensure that the vary header includes all inputs covered by the Hmac Signature. This alignment prevents attackers from altering non-signed dimensions to manipulate cached responses. For shared caches, avoid caching authenticated responses unless the signature or a canonicalized representation of the request is part of the cache key. With these practices, Hmac Signatures in Buffalo effectively protect against cache poisoning by binding the signature to the full request context that determines the response.