Rate Limiting Bypass in Fiber with Hmac Signatures

Rate limiting is a common control to reduce abuse and ensure fair usage. When Hmac signatures are used for request authentication in Fiber, implementation choices can unintentionally weaken rate limiting. A typical pattern is to compute an Hmac over selected request components (method, path, selected headers, and body) and include the signature in a header. If the rate limiter is applied before signature validation or uses input that an attacker can influence independently, the control can be bypassed.

Consider a scenario where the rate limiter key is derived from a client-supplied identifier, such as a key ID included in the request body or a query parameter, without tying it to the authenticated identity established via Hmac. An attacker can rotate keys or generate new key IDs while keeping the same Hmac secret unknown, evading per-key limits. Another bypass arises from inconsistent normalization: if the rate limiter uses a different request representation than the one covered by the Hmac (e.g., different header casing, omitted headers, or unnormalized query parameter ordering), the server may accept a request as valid under the signature while treating it as a distinct request under the rate limiter, allowing an attacker to send many variant requests that share the same effective identity but bypass aggregate limits.

Timing differences can also contribute to bypass. If signature validation short-circuits on early failures and rate limiting is enforced afterward, an attacker can send大量 invalid-signature requests that consume rate limit capacity without consuming significant backend resources. Even when signature validation precedes rate limiting, insufficient bound on request size or body content can enable bypass via compression or encoding manipulations that produce semantically equivalent but distinct representations, causing the rate limiter to lose count accuracy.

These issues are detectable by middleBrick, which runs 12 security checks in parallel including Authentication, BOLA/IDOR, and Rate Limiting. The scanner validates the unauthenticated attack surface, comparing runtime behavior against the OpenAPI specification (including full $ref resolution). For Hmac-based APIs, middleBrick examines whether rate limiting keys align with authenticated identities, whether request normalization is consistent, and whether controls execute in a robust order. Findings include severity-ranked guidance to help teams close the gap between intended and actual protection.

To securely combine Hmac signatures with rate limiting in Fiber, bind the rate limiter to the authenticated identity derived from the signature and ensure consistent request normalization. Below are concrete Go examples using the Fiber framework and a standard Hmac approach.

1. Compute and verify Hmac over a canonical request representation

Define a canonical representation that includes the HTTP method, the request path without query parameters, selected normalized headers, and the request body. Use this same representation for both signature generation and verification, and for rate limiting key construction.

func computeCanonicalRequest(method, path, body string, headers http.Header) string {
    // Select and normalize headers; use a stable order
    const relevant = "content-type:x-request-body-sha256"
    values := headers[relevant]
    normalized := strings.ToLower(strings.TrimSpace(values[0]))
    return method + "\n" + path + "\n" + normalized + "\n" + bodyHash(body)
}

func bodyHash(body string) string {
    h := sha256.New()
    h.Write([]byte(body))
    return hex.EncodeToString(h.Sum(nil))
}

func computeHmac(key, message string) string {
    mac := hmac.New(sha256.New, []byte(key))
    mac.Write([]byte(message))
    return hex.EncodeToString(mac.Sum(nil))
}

2. Enforce rate limiting after authenticated identity is established

In your Fiber handler, first validate the Hmac signature, extract the client identifier from the canonical request or from a verified header, and then apply rate limiting using an authenticated key. This prevents unauthenticated requests from consuming limits.

func verifyHmac(next fiber.Handler) fiber.Handler {
    return func(c *fiber.Ctx) error {
        secret := os.Getenv("HMAC_SECRET")
        signature := c.Get("X-Request-Signature")
        canonical := computeCanonicalRequest(c.Method(), c.Path(), string(c.Body()), c.Request().Header)
        expected := computeHmac(secret, canonical)
        if !hmac.Equal([]byte(signature), []byte(expected)) {
            return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
        }
        // Attach authenticated identity for downstream use and rate limiting
        clientID := extractClientID(c)
        c.Locals("clientID", clientID)
        return next(c)
    }
}

func rateLimitMiddleware() fiber.Handler {
    store := middleware.NewRedisStore(...)
    return middleware.RateLimiterWithConfig(middleware.RateLimiterConfig{
        Store: store,
        KeyLookup: "locals:clientID",
        Max: 100,
        Expiration: 60,
    })(verifyHmac(c))
}

3. Normalize inputs consistently across signature and rate limiter

Ensure that any query parameter ordering, header casing, or body encoding differences are normalized before both signature computation and rate limiter key derivation. If your API accepts query parameters that do not affect the canonical request, exclude them from the signature and rate limiter key to prevent bypass via benign variations.

func extractClientID(c *fiber.Ctx) string {
    // Use verified request metadata, not user-supplied mutable values
    return c.Locals("clientID").(string)
}

By tying the rate limiter key to the authenticated identity established through Hmac verification and by normalizing inputs consistently, you reduce the risk of rate limiting bypass. middleBrick’s scans can validate these patterns by checking whether rate limiting checks depend on authenticated context and whether request normalization is aligned across security controls.