Timing Attack in Buffalo with Hmac Signatures

A timing attack in Buffalo when Hmac Signatures are used occurs because the comparison of the computed MAC with the MAC transmitted by the client does not execute in constant time. If the comparison short-circuits on the first mismatching byte, an attacker can measure small differences in response time and iteratively infer the correct signature byte by byte. This turns a secret HMAC key into a recoverable value through network-level timing observations.

Buffalo applications that expose endpoints verifying Hmac Signatures are vulnerable when the verification logic relies on a naive equality check (e.g., signature == expected) instead of a constant-time comparison. In a black-box scan, middleBrick tests the unauthenticated attack surface and can detect timing discrepancies that indicate a vulnerable comparison routine. Because the scan runs 12 security checks in parallel, the Authentication and BOLA/IDOR checks can surface endpoints where signature validation is performed and where timing differences are observable.

Real-world attack patterns mirror CVE-adjacent behaviors seen in web frameworks where MAC verification is implemented manually. For example, an attacker can send many requests with slightly altered signatures and measure response times; bytes that cause faster responses indicate correct prefix bytes of the HMAC. This is a practical concern for APIs using query parameters or headers to carry signatures without additional protections like constant-time comparison or message length blinding.

Using middleBrick’s OpenAPI/Swagger spec analysis, the scanner resolves $ref definitions and cross-references spec definitions with runtime findings. This helps identify endpoints that accept signed requests and whether the implementation hints at custom signature handling. The LLM/AI Security module does not apply here, but the scanner’s Authentication checks highlight endpoints where Hmac Signatures are expected but not enforced robustly.

Concrete example in Buffalo with Hmac Signatures:

// vulnerable verification in a Buffalo handler
func VerifyHandler(c buffalo.Context) error {
    payload := c.Request().FormValue("payload")
    receivedSig := c.Request().FormValue("signature")
    secret := []byte(os.Getenv("HMAC_SECRET"))
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    if receivedSig != expected { // non-constant-time comparison
        return c.Error(http.StatusUnauthorized, errors.New("invalid signature"))
    }
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

In this snippet, receivedSig != expected is a standard string comparison that short-circuits. A timing attack can exploit this to learn the correct HMAC over repeated requests. middleBrick would flag this as a high-severity finding in the Authentication check and recommend remediation.

Remediation centers on replacing non-constant-time comparison with a constant-time equality check and ensuring the HMAC generation logic is robust. In Go, you can use hmac.Equal which performs constant-time comparison to prevent timing leakage. Additionally, ensure that the signature is encoded consistently (e.g., always hex or always base64) and that the payload normalization is deterministic to avoid bypasses.

Example of secure verification in Buffalo:

// secure verification using constant-time comparison
func VerifyHandler(c buffalo.Context) error {
    payload := c.Request().FormValue("payload")
    receivedSig := c.Request().FormValue("signature")
    secret := []byte(os.Getenv("HMAC_SECRET"))
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(receivedSig), []byte(expected)) { // constant-time comparison
        return c.Error(http.StatusUnauthorized, errors.New("invalid signature"))
    }
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

If you use base64-encoded signatures, decode both sides before comparison or compare the raw bytes after decoding to avoid encoding inconsistencies. Also, consider adding a random nonce or timestamp to the payload (per-request or per-session) to prevent replay attacks, which complements the timing protection.

middleBrick’s CLI tool can be used to validate the fix: middlebrick scan <url> will re-test the endpoint and confirm that the timing discrepancy is no longer detectable. In the Dashboard, you can track the score improvement over time, and with the Pro plan you can enable continuous monitoring so that future changes to the handler are automatically tested. The GitHub Action can enforce a minimum score threshold, failing the build if a regression introduces a non-constant-time comparison.

When integrating with CI/CD, ensure that the scanned endpoint still returns the expected signature format and that the HMAC secret is not exposed in logs or error messages. The MCP Server allows you to run scans directly from your IDE while you modify the handler, providing rapid feedback during development. These practices reduce the risk of timing-based leakage and align with OWASP API Top 10 authentication and integrity controls.