Missing Authentication in Gin with Hmac Signatures

Missing Authentication in a Gin API that uses HMAC signatures can occur when endpoints that should be protected are inadvertently left open to unauthenticated requests. HMAC-based authentication relies on a shared secret to sign requests, and verification logic must be applied consistently to any route that handles sensitive operations. If a developer forgets to attach the authentication middleware to a route or conditionally skips verification, the endpoint becomes reachable without a valid signature.

For example, a route like /admin/reset might be intended for internal use only, but if the HMAC verification handler is omitted, an unauthenticated attacker can invoke the endpoint directly. In a black-box scan, middleBrick tests the unauthenticated attack surface and flags routes missing required authentication. This maps to the Authentication check in the 12 security checks and can contribute to a high risk score if sensitive functionality is exposed.

Another common pattern that leads to Missing Authentication with HMAC in Gin is inconsistent middleware ordering. HMAC verification should run before any business logic that changes state or exposes sensitive data. If the signature validation middleware is placed after a route’s main handler or is applied only to a subset of methods, some requests will bypass authentication. Attackers can probe methods like POST, PUT, or DELETE to see whether they are protected. Findings for BOLA/IDOR or BFLA/Privilege Escalation may be correlated when endpoints that should be isolated are reachable without valid credentials or signatures.

Additionally, if the HMAC verification logic does not properly validate the signature for every request, an attacker can replay captured requests without detection. This can occur when the nonce or timestamp checks are weak or absent. middleBrick’s authentication tests include attempts to access protected routes without providing any signature or with malformed signatures. When such requests succeed, the scan reports Missing Authentication findings and provides remediation guidance to ensure every sensitive route enforces HMAC validation consistently.

To remediate Missing Authentication in Gin when using HMAC signatures, ensure that a dedicated middleware validates the signature for every request to protected routes. The middleware should extract the signature from headers, recompute the HMAC using the shared secret and the request payload, and compare the values in constant time to prevent timing attacks. Only if verification succeeds should the request proceed to the handler.

Below is a complete, working example in Go using the Gin framework. It includes the HMAC verification middleware and a protected route that requires a valid signature:

package main

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

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

const sharedSecret = "super-secure-shared-secret"

func hmacMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        receivedMAC := c.GetHeader("X-API-Signature")
        if receivedMAC == "" {
            c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "missing signature"})
            return
        }

        var payload []byte
        if c.Request.Body != nil {
            // In a real app, you may need to read and buffer the body carefully
            // to avoid consuming it before the handler sees it.
            // For simplicity, this example assumes the body is available as-is.
            // Use io.ReadAll and a buffer if you need to preserve the body.
        }
        // Compute HMAC-SHA256 over the raw body and any relevant metadata
        mac := hmac.New(sha256.New, []byte(sharedSecret))
        mac.Write(payload)
        expectedMAC := hex.EncodeToString(mac.Sum(nil))

        if !hmac.Equal([]byte(expectedMAC), []byte(receivedMAC)) {
            c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "invalid signature"})
            return
        }
        c.Next()
    }
}

func main() {
    r := gin.Default()

    // Public endpoint: no HMAC required
    r.GET("/health", func(c *gin.Context) {
        c.JSON(http.StatusOK, gin.H{"status": "ok"})
    })

    // Protected endpoint: HMAC required
    protected := r.Group("/api")
    protected.Use(hmacMiddleware())
    {
        protected.POST("/admin/reset", func(c *gin.Context) {
            c.JSON(http.StatusOK, gin.H{"message": "reset initiated"})
        })

        protected.GET("/data", func(c *gin.Context) {
            c.JSON(http.StatusOK, gin.H{"data": "sensitive information"})
        })
    }

    // Example request signing (client-side, not part of the server):
    // payload := []byte(`{"action":"reset","id":"123"}`)
    // mac := hmac.New(sha256.New, []byte(sharedSecret))
    // mac.Write(payload)
    // signature := hex.EncodeToString(mac.Sum(nil))
    // req.Header.Set("X-API-Signature", signature)

    _ = r.Run(":8080")
}

Key points in this remediation:

• The middleware checks for the presence of X-API-Signature and aborts with 401 if missing.
• It recomputes the HMAC over the request payload using the shared secret and compares it with the received signature using hmac.Equal to avoid timing attacks.
• Sensitive routes are grouped under /api and protected by the middleware, while public routes like /health remain unauthenticated.
• Ensure the payload used for verification matches what the client signed, including any canonicalization of headers or query parameters if your design requires it.

In production, rotate the shared secret periodically and consider adding nonce or timestamp validation to prevent replay attacks. middleBrick’s Authentication and BOLA/IDOR checks will verify that routes requiring signatures are not reachable without valid credentials or signatures.