Cache Poisoning in Gin with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache to store malicious content, which is then served to other users. In the Gin framework, using Hmac Signatures for request authentication can inadvertently contribute to cache poisoning when the signature is included in the cache key or when the cache layer does not differentiate between authenticated and unauthenticated inputs.

Gin is a high-performance HTTP web framework written in Go. When Hmac Signatures are used, the typical pattern is to sign a subset of request headers or the request body to verify integrity and origin. If the cache key incorporates the signed headers or the signature itself without stripping or normalizing them, two different clients with different signatures but identical request paths may map to different cache entries. More critically, if the cache key excludes the signature but the response depends on signed claims (such as user roles or permissions), an attacker can manipulate unsigned or weakly signed parameters to poison the cache with content intended for a different principal. For example, an attacker could send a request with a valid Hmac signature but inject a query parameter like user_id that influences the response body. If the cache key does not include user_id, the poisoned response may be cached and served to other users who do not have permission to see that data.

Additionally, if the Hmac signature is verified after cache lookup, a cache hit may return a response that was generated from an unsigned or tampered request path. This breaks the integrity guarantees that Hmac Signatures are meant to provide, because the cached response may not correspond to the verified request. Real-world attack patterns such as CVE-2021-23364 (template injection) demonstrate how improperly scoped caching can lead to privilege escalation or information disclosure. In the context of Gin, this means developers must ensure that cache keys are canonical and exclude any attacker-controlled parameters that influence authorization or content, while also ensuring that signature verification occurs before any cached response is served.

To mitigate cache poisoning when using Hmac Signatures in Gin, you must ensure that cache keys are independent of signature values and that signed claims that affect authorization are not used to drive cache selection. Below are concrete code examples demonstrating secure handling in Gin.

First, define a middleware that computes a canonical cache key by excluding signature-related headers and normalizing query parameters. Use a consistent ordering for query parameters and exclude any parameter that influences authorization or user-specific content.

// canonicalKey builds a cache-safe key from the request
func canonicalKey(r *http.Request) string {
    u := *r.URL
    // Remove signature headers to avoid key variance
    r.Header.Del("X-Signature)
    // Normalize query parameters
    q := u.Query()
    q.Del("signature")
    q.Del("token")
    // Sort keys to ensure consistent ordering
    keys := make([]string, 0, len(q))
    for k := range q {
        keys = append(keys, k)
    }
    sort.Strings(keys)
    parts := url.Values{}
    for _, k := range keys {
        parts[k] = q[k]
    }
    u.RawQuery = parts.Encode()
    return u.Path + "?" + u.RawQuery
}

Second, verify the Hmac signature before using any cached response. This ensures that a cached object is only served when the request integrity is confirmed and that the cache key does not leak sensitive claims.

// verifyHmac validates the Hmac signature of the request
func verifyHmac(r *http.Request, secret []byte) bool {
    provided := r.Header.Get("X-Signature")
    if provided == "" {
        return false
    }
    body, _ := io.ReadAll(r.Body)
    defer r.Body.Close()
    // Re-create the payload used for signing (e.g., method + path + body)
    payload := []byte(r.Method + "\n" + r.URL.Path + "\n" + string(body))
    mac := hmac.New(sha256.New, secret)
    mac.Write(payload)
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(provided))
}

// Example Gin handler using cache key and signature verification
func getData(c *gin.Context) {
    if !verifyHmac(c.Request, []byte(os.Getenv("HMAC_SECRET"))) {
        c.AbortWithStatusJSON(401, gin.H{"error": "invalid signature"})
        return
    }
    key := canonicalKey(c.Request)
    // pseudo-cache lookup
    if val, ok := cache.Get(key); ok {
        c.Data(200, "application/json", val)
        return
    }
    // generate response, store in cache, then respond
    resp := generateResponse(c.Request)
    cache.Set(key, resp, time.Hour)
    c.JSON(200, resp)
}

These examples ensure that the cache key does not vary with signature or authorization-specific parameters and that the signature is validated before serving cached content. This approach aligns with remediation guidance for mitigating cache poisoning while preserving the integrity checks provided by Hmac Signatures.