Race Condition in Gin with Hmac Signatures

A race condition in a Gin application that uses HMAC signatures typically arises when signature verification and request processing are not performed atomically. If an endpoint reads or modifies shared state based on a request after only a partial verification, an attacker can manipulate timing to cause the state to change between the signature check and the business logic execution.

Consider a payment or transfer endpoint that first validates an HMAC and then updates balances. An attacker can send two concurrent requests with the same payload but different target accounts. If the first request updates the balance after the second request has been verified, the second request may operate on stale data, effectively bypassing intended authorization or integrity constraints. This is a classic time-of-check-to-time-of-use (TOCTOU) pattern, where the window between verification and action is exploitable.

Insecure patterns often include per-request signature validation without idempotency keys or locking, and relying on in-memory state that can shift between calls. If the HMAC is computed over a subset of headers or body fields that an attacker can partially control, they may craft requests that collide in ways that cause verification to pass under one ordering but fail under another. Shared caches or global mutable objects used during verification (e.g., reused buffers or parsers) can exacerbate the issue by introducing non-determinism.

Even when using strong HMAC algorithms like HMAC-SHA256, the vulnerability is not in the cryptographic primitive but in the surrounding transaction handling. An attacker does not need to break the HMAC; they exploit the asynchronous interleaving of verification and state changes. This can lead to unauthorized transfers, privilege escalation if role assignments are mutable, or inventory oversell scenarios.

middleBrick detects timing-related inconsistencies across parallel scan iterations and surfaces findings tied to BOLA/IDOR and BFLA/Privilege Escalation when such patterns are observed in unauthenticated attack surface testing. The scanner does not infer internal architecture but highlights behavioral indicators that suggest non-atomic handling of signed requests.

Remediation centers on making signature verification and state updates atomic and deterministic. Use explicit locking, idempotency tokens, or database-level constraints to ensure that once a request is authenticated, the state it depends on cannot change until the operation completes.

Below is a concrete, working Gin example that shows secure handling of HMAC signatures. The signature is computed over a canonical string that includes a timestamp and a nonce, verified within a short window, and then the business logic proceeds within a synchronized context.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"strconv"
	"sync"
	"time"

	"github.com/gin-gonic/gin"
)

var (
	secretKey = []byte("super-secret-key-32-bytes-long-12345678")
	mutex     = &sync.Mutex{}
	usedNonces = make(map[string]bool)
)

func verifyHMAC(payload, receivedMAC string) bool {
	mac := hmac.New(sha256.New, secretKey)
	mac.Write([]byte(payload))
	expected := hex.EncodeToString(mac.Sum(nil))
	return hmac.Equal([]byte(expected), []byte(receivedMAC))
}

func transferHandler(c *gin.Context) {
	// Example canonical payload: timestamp:nonce:from:to:amount
	timestamp := c.GetHeader("X-Timestamp")
	rawNonce := c.GetHeader("X-Nonce")
	if timestamp == "" || rawNonce == "" {
		c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "missing timestamp or nonce"})
		return
	}

	// Prevent replay: ensure nonce is unique and recent
	mutex.Lock()
	if usedNonces[rawNonce] {
		mutex.Unlock()
		c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "replay detected"})
		return
	}
	// Accept only recent nonces (e.g., within 2 minutes)
	nonceTime, err := strconv.ParseInt(rawNonce[:13], 10, 64) // assuming millisecond prefix
	if err != nil || time.Since(time.Unix(0, nonceTime*1e6)) > 2*time.Minute {
		mutex.Unlock()
		c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "invalid nonce"})
		return
	}
	usedNonces[rawNonce] = true
	mutex.Unlock()

	body := c.Request.Body
	// In real code, read and restore body if needed; here we assume a string for simplicity
	// payload := io.ReadAll(body)
	payload := timestamp + ":" + rawNonce + ":alice:bob:100"
	signature := c.GetHeader("X-Signature")
	if !verifyHMAC(payload, signature) {
		c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
		return
	}

	// Critical section: perform state change atomically
	mutex.Lock()
	defer mutex.Unlock()
	// Here update balances or perform other actions
	// Ensure no other goroutine can interleave on shared resources
	c.JSON(http.StatusOK, gin.H{"status": "processed"})
}

func main() {
	r := gin.Default()
	r.POST("/transfer", transferHandler)
	r.Run()
}

Key points in this remediation:

• Canonical payload construction ensures the HMAC covers all relevant data, preventing selective field omission attacks.
• Nonce and timestamp validation with a short window mitigates replay and ensures freshness.
• A mutex protects shared state (e.g., nonce cache and balance updates) so verification and mutation occur as a single atomic unit, closing the race window.
• HMAC verification uses hmac.Equal to prevent timing attacks on the signature comparison itself.

For production, consider moving shared state to a transactional database with unique constraints (e.g., unique nonce columns) to achieve atomicity without in-process locks, and always validate the full request body rather than a subset.

middleBrick’s CLI can be used to scan this service from the terminal with middlebrick scan <url>, while the GitHub Action can add API security checks to your CI/CD pipeline to fail builds if risk scores drop below your chosen threshold. The MCP Server allows you to scan APIs directly from your AI coding assistant within your development environment.