Cache Poisoning in Fiber with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache to store malicious responses that are served to other users. In the Fiber web framework for Go, using Hmac Signatures for request authentication can inadvertently expose cache poisoning risks if the cache key does not incorporate the signature or related authenticated context. When a cache layer (e.g., CDN or in-memory cache) uses only the request path and query parameters as the cache key, authenticated requests that include an Hmac signature in headers or cookies may be cached separately per unique signature. This can lead to one user’s signed response being reused for another user if the cache key does not differentiate by user or signature, or worse, signed and unsigned requests may map to the same key, bypassing intended access controls.

Specifically in Fiber, Hmac Signatures are often used to ensure request integrity and authenticity, for example via a custom Authorization header such as Authorization: Hmac :. If responses for different users or different signature values share the same cache key, an attacker who can observe or influence one user’s signed request might be able to retrieve or manipulate cached content meant for another user. This violates separation of authenticated contexts and can lead to information disclosure or privilege escalation. Additionally, if query parameters that are not part of the cache key are used to carry Hmac-related metadata (such as timestamps or nonces), cache normalization rules may strip or ignore them, causing different signed requests to map to the same entry and amplifying the poisoning surface.

Real-world attack patterns mirror OWASP API Top 10 API1:2023 — Broken Object Level Authorization, where lack of proper cache segregation enables horizontal privilege exposure. For example, an authenticated request to GET /api/v1/users/me with a valid Hmac signature could be cached and subsequently served to another authenticated user whose signature differs, exposing private user data. Similarly, SSRF or input validation issues combined with caching can lead to cache poisoning via malicious query parameters that influence backend responses but are not considered in the cache key. Therefore, when using Hmac Signatures in Fiber, it is critical to ensure that cache keys incorporate user identity or request-specific authenticated context to prevent cross-user cache contamination.

To mitigate cache poisoning when using Hmac Signatures in Fiber, ensure that cache keys include elements that differentiate authenticated contexts, such as user ID or a canonical representation of the signed request. Avoid caching responses where the Hmac signature or user identity affects authorization unless the cache key explicitly includes that identity. Below are concrete code examples demonstrating secure handling of Hmac Signatures in Fiber with proper cache-aware design.

Example 1: Hmac Signature Verification in Fiber

Verify the Hmac signature on each request and extract the user ID to be used in cache key construction. Do not rely on query parameters or headers that may be normalized away by caching layers.

package main

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

	"github.com/gofiber/fiber/v2"
)

// Secret key should be stored securely, e.g., from environment variable
var hmacSecret = []byte("your-secure-secret")

func verifyHmac(c *fiber.Ctx) error {
	auth := c.Get("Authorization")
	if auth == "" {
		return c.Status(http.StatusUnauthorized).SendString("Missing Authorization header")
	}
	// Expected format: "Hmac userID:signature"
	if !strings.HasPrefix(auth, "Hmac ") {
		return c.Status(http.StatusUnauthorized).SendString("Invalid Authorization scheme")
	}
	parts := strings.SplitN(auth[5:], ":", 2)
	if len(parts) != 2 {
		return c.Status(http.StatusUnauthorized).SendString("Invalid Authorization format")
	}
	userID, signature := parts[0], parts[1]
	computed := hmac.New(sha256.New, hmacSecret)
	computed.Write([]byte(userID))
	expected := hex.EncodeToString(computed.Sum(nil))
	if !hmac.Equal([]byte(expected), []byte(signature)) {
		return c.Status(http.StatusUnauthorized).SendString("Invalid signature")
	}
	// Attach userID to context for downstream use in cache key
	c.Locals("userID", userID)
	return c.Next()
}

func handler(c *fiber.Ctx) error {
	userID := c.Locals("userID").(string)
	// Construct a cache key that includes userID to prevent cross-user poisoning
	cacheKey := "user_data:" + userID
	// ... retrieve or compute response for cacheKey
	return c.SendString("Secure data for " + userID)
}

func main() {
	app := fiber.New()
	app.Use(verifyHmac)
	app.Get("/api/v1/users/me", handler)
	app.Listen(":3000")
}

Example 2: Cache Key Construction with User Context

When integrating with a caching layer, ensure the cache key incorporates the user identity extracted from the Hmac signature. This prevents one user’s cached response from being served to another user.

// Assume a caching interface with Get/Set
var cache = map[string][]byte{}

func getWithCache(c *fiber.Ctx) error {
	userID := c.Locals("userID").(string)
	cacheKey := "profile:" + userID
	if val, ok := cache[cacheKey]; ok {
		return c.Send(val)
	}
	// Simulate fetching user-specific data
	data := []byte("profile data for " + userID)
	cache[cacheKey] = data
	return c.Send(data)
}

By anchoring cache keys to user identity derived from the Hmac signature, you ensure that authenticated caches remain isolated per user, effectively mitigating cache poisoning risks. Combine this with input validation and canonical header handling to further reduce edge cases where cache normalization might otherwise blur distinctions between authenticated requests.