Log Injection in Fiber with Hmac Signatures

Log injection occurs when untrusted input is written directly into application logs without proper sanitization, allowing an attacker to forge log entries or hide malicious activity. In the Go Fiber framework, this risk is amplified when HMAC signatures are used for request authentication or integrity checks but are logged insecurely. Consider a scenario where the signature value or related headers (e.g., X-Request-Signature) are included in structured logs without validation or escaping. Because HMAC values often contain high-entropy strings, they may be mistakenly treated as safe and logged verbatim. An attacker can supply crafted input—such as newlines or control characters—in parameters that are reflected in logs, effectively splitting log lines or injecting additional entries. This can corrupt log-based monitoring, bypass detection rules that rely on log format consistency, and assist in log forging attacks. For example, a malicious actor might embed a fake preceding log entry by injecting a newline and timestamp-like text, making it appear as though a prior authenticated request occurred. Since HMAC signatures are often logged for audit purposes, this creates a false chain of trust in log reviews. The interaction between Fiber middleware that validates HMACs and logging components that do not sanitize inputs creates a blind spot: the application correctly verifies integrity but inadvertently records attacker-controlled data as authoritative. This can obscure real security events or trigger log injection–based denial of confusion in SIEM systems. Real-world patterns include logging the entire request context including signature headers without normalization, which violates secure logging practices. The Open Web Application Security Project (OWASP) API Security Top 10 lists log injection under security misconfiguration and insufficient monitoring controls. Unlike generic log injection, HMAC-related cases are particularly dangerous because the integrity mechanism itself becomes part of the forged evidence, potentially bypassing integrity checks in log analysis pipelines.

To remediate log injection when using HMAC signatures in Fiber, treat signature values and any data used to compute them as untrusted. Do not log raw signature strings directly; instead, log a sanitized representation or a reference identifier. Below are concrete, secure patterns for a Fiber application using HMAC authentication.

Secure HMAC verification and logging in Fiber

Use the following approach to validate HMACs and log safely. This example uses the crypto/hmac and crypto/sha256 standard libraries and avoids logging sensitive or attacker-controlled data verbatim.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"net/http"
	"strings"

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

// Securely compute HMAC for comparison (do not log this secret).
var signingKey = []byte("your-256-bit-secret")

func computeHmac(data string) string {
	h := hmac.New(sha256.New, signingKey)
	h.Write([]byte(data))
	return hex.EncodeToString(h.Sum(nil))
}

// sanitizeLogValue removes or replaces newlines and control characters.
func sanitizeLogValue(input string) string {
	// Replace newlines and carriage returns to prevent log injection.
	sanitized := strings.ReplaceAll(input, "\n", "\\n")
	sanitized = strings.ReplaceAll(sanitized, "\r", "\\r")
	// Optionally, hash the value for logging if you must retain uniqueness without risk.
	// return computeHmac(sanitized)
	return sanitized
}

func VerifyAndLog(c *fiber.Ctx) error {
	providedSignature := c.Get("X-Request-Signature", "")
	body := c.Body() // or specific payload string used for signing
	expectedSignature := computeHmac(string(body))

	if !hmac.Equal([]byte(providedSignature), []byte(expectedSignature)) {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
	}

	// Safe logging: sanitize before writing to logs.
	fmt.Printf("[secure] request_id=%s signature=%s\n", c.Get("X-Request-Id", "unknown"), sanitizeLogValue(providedSignature))
	return c.Next()
}

func main() {
	app := fiber.New()
	app.Post("/api/order", VerifyAndLog, func(c *fiber.Ctx) error {
		return c.JSON(fiber.Map{"status": "ok"})
	})
	app.Listen(":3000")
}

Key remediation principles:

• Never log raw signatures or secrets: Signatures should be hashed or replaced with opaque tokens in logs. If you need traceability, log a request-scoped identifier instead.
• Sanitize all inputs that reach logs: Escape or replace newline (\n) and carriage return (\r) characters in any user-controlled data, including headers and query parameters.
• Use constant-time comparison: Always use hmac.Equal to prevent timing attacks during signature verification.
• Structured logging with safe fields: When emitting JSON logs, ensure fields that may contain user input are escaped or omitted.

These steps ensure that HMAC-based integrity checks remain intact while reducing the attack surface for log injection in Fiber applications.