Information Disclosure in Echo Go with Hmac Signatures

Information disclosure occurs when an API inadvertently exposes data that should remain confidential to an authenticated client. In Echo Go, combining insecure HMAC signature handling with verbose error responses can leak secrets or internal logic. An HMAC is a hash-based message authentication code that should be computed over a canonical representation of the request (method, path, selected headers, and body) using a shared secret. If the server compares signatures in a non-constant-time manner or includes the computed or expected signature in logs, error pages, or JSON responses, an attacker can learn information about the secret or about valid signatures.

A common pattern in Echo Go is to compute an HMAC over a canonical string and then compare it with an incoming header, for example using crypto/hmac. If the comparison uses standard bytes.Equal or string equality, timing attacks are not the primary information disclosure concern; the real risk arises when the application reveals whether a signature was valid through side channels such as error messages, response lengths, or debug output. For instance, returning a 500 Internal Server Error that includes the computed HMAC or the raw secret in the response body or stack trace directly discloses sensitive material. Even non-error paths can leak information: differing response times or lengths when a valid versus invalid signature is supplied can allow an attacker to iteratively guess the secret or a valid signature through adaptive chosen-message attacks.

Echo Go applications that expose an unauthenticated endpoint which echoes the signature or secret in logs or metadata also contribute to information disclosure. In distributed systems, signatures sometimes include nonces, timestamps, or user identifiers; if these values are returned in error payloads or documentation pages, an attacker can correlate them with other observations to infer authentication logic or session details. Additionally, improper handling of malformed requests may cause panics that dump internal variables, including the shared secret or intermediate hash values, to the client. The combination of HMAC-based integrity checks and information leakage through responses or logs undermines the security purpose of the MAC, because an attacker gains knowledge that should remain bound to the server-side secret.

Consider a route that validates an HMAC in X-API-Signature and returns detailed validation messages. A vulnerable Echo Go handler might compute the signature and then explicitly indicate whether the signature matched, or include the computed value in a JSON error. This behavior enables an attacker to perform offline or online inference about the correct signature or secret. Real-world attack patterns such as Security Misconfiguration and Injection can be chained with information disclosure to escalate impact, especially when combined with insufficient logging hygiene that exposes secrets in structured logs or crash dumps.

To observe this in practice, an Echo Go service that does not sanitize responses before returning them can inadvertently include HMAC-related fields in error payloads. For example, a debug mode that appends the computed signature to a human-readable error page is a clear information disclosure. Similarly, inconsistent behavior between valid and invalid signatures—such as returning a 400 with a detailed message for an invalid signature but a generic 401 for a valid one—reveals the outcome of the verification step and assists attackers in refining their guesses.

Defense against information disclosure starts with treating the HMAC secret as confidential and ensuring that no component of the verification flow exposes it or its relationship to the request. In Echo Go, this means carefully controlling what is included in HTTP responses, logs, and panic handlers. The framework should be configured to return uniform error responses that do not distinguish between signature validation outcomes at the application layer. Logging must exclude raw secrets, computed MACs, and nonces used in the signature scope, or at minimum redact them before output. Consistent status codes and response shapes prevent timing and content-based leakage, while robust panic recovery avoids dumping internal state to the client.

Remediation focuses on constant-time comparison, strict separation of validation logic from HTTP semantics, and disciplined logging. In Echo Go, compute the HMAC over a canonical representation and compare it using a constant-time function to avoid timing leaks. Do not include the computed or expected signature in any response body, header, or log line. Return the same HTTP status code and response shape for both valid and invalid signatures to prevent content-based disclosure.

Below is a secure example of HMAC verification in Echo Go. It canonicalizes the request method, path, selected headers, and body, computes an HMAC using crypto/hmac, and performs a constant-time comparison. The handler returns a generic error message without exposing signature details, and logs are sanitized to exclude the secret and computed MAC.

// canonicalize builds a deterministic string for HMAC verification.
// Include method, path, selected headers, and body to ensure replay safety.
func canonicalize(r *echo.Request, headers []string) string {
    var b strings.Builder
    b.WriteString(r.Method)
    b.WriteByte('|')
    b.WriteString(r.Path())
    for _, h := range headers {
        b.WriteByte('|')
        b.WriteString(h)
        b.WriteByte(':')
        b.WriteString(r.Request().Header.Get(h))
    }
    // Important: include body only if your API design requires it,
    // and ensure it is read in a deterministic way.
    b.WriteByte('|')
    b.ReadFrom(r.Body()) // io.ReadCloser may need to be wrapped to allow reuse
    return b.String()
}

// verifyHMAC returns true if the signature is valid using constant-time comparison.
func verifyHMAC(secret []byte, r *echo.Request, header string, selectedHeaders []string) bool {
    msg := canonicalize(r, selectedHeaders)
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(msg))
    expected := mac.Sum(nil)
    incoming, err := base64.StdEncoding.DecodeString(r.Header().Get(header))
    if err != nil {
        // Do not distinguish between malformed header and bad signature.
        return false
    }
    return hmac.Equal(expected, incoming)
}

// Handler example in Echo Go.
func apiHandler(secret []byte, selectedHeaders []string) echo.HandlerFunc {
    return func(c echo.Context) error {
        const headerName = "X-API-Signature"
        if !verifyHMAC(secret, c.Request(), headerName, selectedHeaders) {
            // Uniform response to prevent information disclosure.
            return c.JSON(http.StatusUnauthorized, map[string]string{
                "error": "unauthorized",
            })
        }
        // Proceed with business logic.
        return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
    }
}

Additional remediation practices include rotating the HMAC secret using a secure key management process, enforcing strict header whitelisting to avoid variable signature scopes, and ensuring that panic handlers in Echo Go do not expose variables such as the secret or computed MAC. Configure Echo’s middleware to standardize error shapes and avoid debug routes in production. Combine these measures with rate limiting and monitoring to detect abnormal request patterns that may indicate probing for information disclosure.