Broken Access Control in Gin with Hmac Signatures

Broken Access Control (BOLA/IDOR) in the Gin framework when Hmac Signatures are used incorrectly can allow an authenticated user to access or modify resources that belong to another user. Hmac Signatures are commonly employed to ensure request integrity and authenticity, but if the signature is computed over an insufficient scope of the request, an attacker can tamper with identifiers such as :id or resource parameters without invalidating the signature.

Consider a typical Gin route where a signature is generated from selected parts of the request, for example the HTTP method, path, selected headers, and a timestamp. If the signature does not include the user identity or a strict scope that binds the request to a specific resource owner, an attacker who knows or guesses another user’s resource ID can simply replace the ID in the URL or body while keeping the same valid Hmac. Gin will validate the signature successfully because the signature components remain unchanged, but the authorization check that maps the resource to the requesting user is missing or incomplete.

A concrete example is an endpoint like GET /users/:id/orders/:order_id. If the Hmac is computed only on method, path prefix, and timestamp, and the server fails to verify that the authenticated user (e.g., via session or JWT subject) actually owns the provided :id or :order_id, this constitutes Broken Access Control. The vulnerability maps to OWASP API Top 10 2023 — Broken Object Level Authorization (BOLA), and can lead to unauthorized viewing or manipulation of other users’ data. Inadequate signature scope effectively decouples the cryptographic integrity check from the authorization decision, allowing an attacker to iterate over IDs and observe differences in behavior or data, a classic IDOR pattern.

Real-world analogies include cases where a signature covers a subset of parameters but an attacker modifies an included parameter such as user_id by leveraging weak binding between the signature and the resource ownership model. For instance, if a timestamp is used to prevent replay but ownership is not enforced, an attacker can reuse a valid signed request across users. This is especially risky in administrative or high-privilege endpoints where one signed request can trigger actions on behalf of another user. The root issue is treating Hmac Signatures as a universal access control mechanism rather than a complementary integrity check that must be paired with explicit ownership validation.

To detect such issues during scanning, middleBrick runs checks that correlate endpoint behavior, signature coverage, and authorization logic across the unauthenticated attack surface. It does not fix the code, but findings include precise remediation guidance to align Hmac usage with strict access control in Gin.

Remediation focuses on ensuring that Hmac Signatures bind the request to a specific user or tenant and that authorization checks are applied after signature validation. The signature scope must include user identity or a resource ownership token, and the server must enforce that the subject of the request matches the resource being accessed.

Below are concrete, working examples in Go using the Gin framework. The first example shows a vulnerable pattern where the Hmac covers only method, path, and timestamp, and the second shows a corrected version that includes user ID and enforces ownership.

// Vulnerable: Hmac does not include user identity
func CalculateHmacVulnerable(data string, secret string) string {
    h := hmac.New(sha256.New, []byte(secret))
    h.Write([]byte(data))
    return hex.EncodeToString(h.Sum(nil))
}

// Handler with only signature check, missing ownership validation
func vulnerableOrderHandler(c *gin.Context) {
    orderID := c.Param("order_id")
    userID := c.GetHeader("X-User-ID") // not enforced against order ownership
    // signature verification omitted for brevity
    var order Order
    if db.Where("id = ?", orderID).First(&order).Error != nil {
        c.AbortWithStatusJSON(404, gin.H{"error": "order not found"})
        return
    }
    c.JSON(200, order)
}

// Corrected: include user identity in Hmac scope and enforce ownership
type SignedRequest struct {
    UserID   string `json:"user_id"`
    OrderID  string `json:"order_id"`
    Method   string `json:"method"`
    Path     string `json:"path"`
    Timestamp int64  `json:"timestamp"`
}

func CalculateHmacSecure(payload SignedRequest, secret string) string {
    h := hmac.New(sha256.New, []byte(secret))
    bin, _ := json.Marshal(payload)
    h.Write(bin)
    return hex.EncodeToString(h.Sum(nil))
}

func secureOrderHandler(c *gin.Context) {
    var req SignedRequest
    if err := c.BindJSON(&req); err != nil {
        c.AbortWithStatusJSON(400, gin.H{"error": "invalid request"})
        return
    }
    // Enforce that the requesting user matches the resource owner
    if req.UserID != c.GetString("auth_user_id") {
        c.AbortWithStatusJSON(403, gin.H{"error": "forbidden"})
        return
    }
    verified := verifyHmac(req, c.GetHeader("X-Signature"), os.Getenv("HMAC_SECRET"))
    if !verified {
        c.AbortWithStatusJSON(401, gin.H{"error": "invalid signature"})
        return
    }
    var order Order
    if db.Where("id = ? AND user_id = ?", req.OrderID, req.UserID).First(&order).Error != nil {
        c.AbortWithStatusJSON(404, gin.H{"error": "order not found or access denied"})
        return
    }
    c.JSON(200, order)
}

func verifyHmac(req SignedRequest, receivedSig, secret string) bool {
    expected := CalculateHmacSecure(req, secret)
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

Key principles in the corrected approach:

• Include user identity (or tenant ID) directly in the data signed by Hmac so that a signature cannot be reused across users.
• Always enforce ownership on the server side after signature validation by scoping database queries to the authenticated user (e.g., WHERE user_id = ?).
• Avoid including only mutable or attacker-influenced values (such as only path or timestamp) in the signature scope; couple them with immutable identifiers that reflect authorization boundaries.
• Use constant-time comparison (hmac.Equal) to prevent timing attacks on the signature verification.

These steps ensure that Hmac Signatures complement rather than replace access control, mitigating BOLA/IDOR risks in Gin APIs while preserving the integrity guarantees of cryptographic signatures.