Cache Poisoning in Echo Go with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache to store malicious content, leading other users to receive that content unintentionally. In Echo Go, combining HTTP caching with Hmac Signatures can introduce a risk if the cache key does not incorporate the Hmac signature or related integrity metadata. When responses are cached based only on the request URI and query parameters, and the application uses an Hmac Signature primarily for request authentication (e.g., to sign a token or payload but not to protect the cache key), an attacker may be able to poison the cache by serving a manipulated response that gets cached under a benign-looking key.

For example, consider an endpoint that returns public data but also includes a signed query parameter (e.g., resource_id=123&sig=...) used by the backend to validate request integrity. If the caching layer uses only resource_id as the cache key and does not include the signature or a normalized representation of the authenticated context, two different signed requests for the same resource_id could map to the same cache entry. An attacker who can get a legitimate response cached, or can inject a crafted response through other means (e.g., SSRF or a compromised upstream), may cause subsequent unauthenticated or low-privilege users to receive the poisoned response. Because Echo Go does not inherently manage cache semantics, the developer must ensure that cache keys incorporate all dimensions that affect response authenticity, including signature presence or a hash of the signed payload.

This situation is distinct from standard Hmac verification, which ensures integrity and authenticity of a message or token. The vulnerability arises when the cache layer is unaware of the Hmac context and treats authenticated and unauthenticated requests as cache-equivalent. To detect such misconfiguration during scans, tools like middleBrick run checks across Authentication, Input Validation, and Security Headers, flagging cases where caching behavior may undermine integrity controls. Developers should treat Hmac signatures as part of the request context that must influence caching, and avoid using unsigned or partially signed inputs as cache keys.

To remediate cache poisoning risks when using Hmac Signatures in Echo Go, ensure the cache key includes a canonical representation of the signed context. Do not rely solely on the resource identifier. Instead, incorporate the signature value (or a hash of the signed payload) into the cache key, or bypass caching for requests that include sensitive or variable authenticated parameters.

Below is a concrete example of Hmac signing in Echo Go, followed by a safe caching approach that includes the signature in the cache key.

// Hmac signing helper
func signPayload(payload string, secret []byte) string {
    h := hmac.New(sha256.New, secret)
    h.Write([]byte(payload))
    return hex.EncodeToString(h.Sum(nil))
}

// Example route with signed query parameter and safe cache key construction
func getData(c echo.Context) error {
    resourceID := c.QueryParam("resource_id")
    sig := c.QueryParam("sig")
    secret := []byte(os.Getenv("HMAC_SECRET"))

    // Recompute expected signature for validation
    expected := signPayload(resourceID, secret)
    if !hmac.Equal([]byte(expected), []byte(sig)) {
        return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
    }

    // Build a cache key that includes both resource ID and signature
    cacheKey := fmt.Sprintf("resource:%s:sig:%s", resourceID, sig)

    // Pseudo-cache lookup; ensure the cache respects the signed context
    if cached, found := cache.Get(cacheKey); found {
        return c.String(http.StatusOK, cached)
    }

    // Produce and cache the response
    data := fetchResource(resourceID)
    cache.Set(cacheKey, data, cache.DefaultExpiration)
    return c.String(http.StatusOK, data)
}

Key points in this remediation:

• The cache key explicitly includes the signature value, so responses signed with different tokens are stored separately.
• Signature validation uses hmac.Equal to avoid timing attacks.
• If caching is not required for authenticated or variable-signed requests, skip caching when a signature is present, or use a private cache key that incorporates user or session context.

For broader protection, combine this with security headers and input validation checks. middleBrick can surface missing or inconsistent controls across Authentication, Input Validation, and Security Headers, helping you identify scenarios where cache behavior does not align with Hmac-based integrity guarantees.