HIGH timing attackfiberhmac signatures

Timing Attack in Fiber with Hmac Signatures

Scan for timing attack in fiber

Timing Attack in Fiber with Hmac Signatures — how this specific combination creates or exposes the vulnerability

A timing attack in the Fiber Go web framework can occur when HMAC signature verification is implemented using a naive comparison of signature bytes. In many Go HMAC utilities, the comparison function does not use constant-time logic; it returns early as soon as a mismatching byte is found. Because the function exits early, an attacker who can measure request latency differences can infer how many leading bytes of the supplied signature match the expected value.

In the context of Fiber, this typically arises when a client computes an HMAC over a request (for example, over a JSON body or selected headers) and sends the signature in a custom header such as X-API-Signature. The server recomputes the HMAC using a shared secret and compares it to the client-provided signature. If the comparison is not constant-time, network jitter and server load can be minimized by the attacker, allowing byte-by-byte inference. Successful inference can reveal the shared secret or enable a forgery by producing a valid signature for a modified request.

Real-world analogies exist in the wild: timing differences in HMAC verification have been leveraged to recover secrets used by web frameworks and API gateways. Although Fiber itself does not introduce the flaw, common Go helper packages and hand-rolled verification code are susceptible. The risk is especially relevant when endpoints do not enforce strong input validation, allow variable processing paths based on signature validity, or expose unauthenticated routes that an attacker can probe for timing differences.

An attacker might probe endpoints that accept signed payloads without authentication, measuring response times for crafted requests. Because the scan methodology of middleBrick includes checks for Input Validation and Unsafe Consumption, it can surface endpoints where signature verification logic interacts with malformed or unexpected inputs, highlighting routes where timing differences may be measurable. This aligns with LLM/AI Security checks when prompt-injection-like probes are repurposed to explore behavioral differences that could hint at branching logic in verification code.

To detect such issues, security scans compare the application’s reported behavior (e.g., status codes, response times under controlled conditions) against known patterns of insecure comparison. Because middleBrick performs unauthenticated black-box scanning, it can flag endpoints that appear to process signed data without clear mitigation guidance, prompting developers to audit their HMAC verification implementation in Fiber.

Hmac Signatures-Specific Remediation in Fiber — concrete code fixes

Remediation centers on replacing byte-wise comparison with a constant-time comparison. In Go, the standard approach is to use hmac.Equal, which is designed to compare byte slices in constant time regardless of early mismatches. This eliminates timing leakage by ensuring the comparison always takes the same amount of time for a given input length.

Below is a secure pattern for verifying HMAC signatures in a Fiber handler. The example reads a JSON payload, computes HMAC-SHA256 with a shared secret, and compares the result to the signature provided in a header using hmac.Equal. Note that the handler returns a generic error for invalid signatures to avoid leaking information via timing or error messages.

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

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

func verifySignature(payload []byte, receivedSig string, secret []byte) bool {
	h := hmac.New(sha256.New, secret)
	h.Write(payload)
	expected := h.Sum(nil)
	decoded, err := hex.DecodeString(receivedSig)
	if err != nil || len(decoded) != len(expected) {
		// Use constant-time fallback when lengths differ to avoid length-based leaks
		return false
	}
	return hmac.Equal(expected, decoded)
}

func signedHandler(c *fiber.Ctx) error {
	body := c.Body()
	sig := c.Get("X-API-Signature")
	if sig == "" {
		return c.Status(http.StatusBadRequest).JSON(fiber.Map{"error": "missing signature"})
	}
	if !verifySignature(body, sig, []byte("your-shared-secret")) {
		// Always return the same generic error and avoid branching on signature validity
		return c.Status(http.StatusUnauthorized).JSON(fiber.Map{"error": "invalid request"})
	}
	return c.JSON(fiber.Map{"status": "ok"})
}

Additional best practices include normalizing input before signing (e.g., canonical JSON serialization), using strong random secrets, and ensuring the secret is never logged or exposed. When integrating with middleBrick, developers can use the CLI (middlebrick scan <url>) or the GitHub Action to verify that endpoints accepting signed requests do not exhibit timing-sensitive behavior. For teams needing deeper visibility, the Pro plan’s continuous monitoring can track changes to endpoint behavior over time, while the MCP Server allows scanning API contracts directly from AI coding assistants to catch insecure patterns early.

Finally, always pair HMAC verification with robust input validation and transport-layer protections. Even with constant-time comparison, other weaknesses such as weak randomness, poor secret management, or missing integrity checks on related headers can undermine the security of the signing scheme.

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Frequently Asked Questions

Why does using hmac.Equal prevent timing attacks in Fiber HMAC verification?
hmac.Equal compares all bytes of the two slices regardless of early mismatches, so execution time does not depend on the position of the first differing byte. This removes timing information an attacker could use to infer the correct signature byte-by-byte.
Can middleBrick detect timing vulnerabilities in HMAC verification during a scan?
middleBrick focuses on detecting outcomes and insecure patterns rather than measuring micro-timing behavior. It can identify endpoints that accept signed input and flag missing remediation guidance, but it does not measure timing differences directly; developers should complement scans with code review and constant-time verification practices.

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