Auth Bypass in Fiber with Jwt Tokens

Auth bypass in a Fiber API that uses JWT tokens typically occurs when token validation is incomplete, optional, or inconsistently applied across routes. Fiber is a fast HTTP web framework written in Go, and it does not enforce authentication by default. If developers integrate JWT handling manually and skip verification for certain endpoints, or accept tokens from untrusted sources without proper validation, an attacker can make authenticated requests without a valid token.

Specific patterns that create or expose the vulnerability include:

• Missing or optional middleware: If JWT verification middleware is not applied to all protected routes, or is conditionally skipped (for example, only applied when a specific header is present), unauthenticated access becomes possible.
• Insecure token source: Accepting tokens from query parameters, non-HTTPS headers, or unverified cookies can expose tokens to interception or leakage, enabling an attacker to reuse them.
• Weak validation logic: Failing to validate the issuer (iss), audience (aud), expiration (exp), or signature properly allows tampered or unsigned tokens to be accepted.
• CORS misconfiguration: Permitting untrusted origins or credentials can enable cross-origin requests that carry valid tokens, effectively bypassing intended auth boundaries.
• Overprivileged tokens: Using a single JWT for multiple users or roles without scoping means that if one token leaks, broader access is possible.

These issues map to common weaknesses and are observable during unauthenticated scans. For example, an endpoint that returns sensitive data without requiring a token, or that accepts a modified token with an altered payload, would be flagged. Because JWTs are often used for stateless authorization, incorrect implementation in Fiber can lead to privilege escalation or unauthorized data access, aligning with BOLA/IDOR and Authentication check results.

To remediate JWT-related auth bypass in Fiber, apply strict validation consistently and use secure coding patterns. Below are concrete, working examples for a secure setup.

1. Use a maintained JWT middleware and enforce it globally

Prefer a well-maintained JWT middleware for Fiber instead of custom validation. Apply the middleware to protected routes and avoid optional checks.

import (
    "github.com/gofiber/fiber/v2"
    "github.com/gofiber/contrib/jwt"
)

func main() {
    app := fiber.New()

    // JWT middleware with strict validation
    config := jwt.Config{
        SigningKey:   []byte("your-256-bit-secret-key-min-32-bytes-long-secure-random"),
        SigningMethod: "HS256",
        ContextKey:   "user", // where the parsed claims will be placed
        TokenLookup:  "header:Authorization", // only accept from Authorization header
        AuthScheme:   "Bearer",
        Validate: func(c *fiber.Ctx) error {
            // additional custom claims validation can go here
            return nil
        },
    }

    protected := app.Group("/api")
    protected.Use(jwt.New(config))

    protected.Get("/profile", func(c *fiber.Ctx) error {
        user := c.Locals("user").(*jwt.Token)
        claims := user.Claims.(jwt.MapClaims)
        return c.JSON(fiber.Map{
            "userID": claims["sub"],
            "role":   claims["role"],
        })
    })

    app.Listen(":3000")
}

2. Validate all required claims and reject unsigned tokens

Ensure the middleware validates the signature, exp, nbf, iss, and aud. Never accept tokens with "none" algorithm.

import (
    "github.com/golang-jwt/jwt/v5"
)

func validateToken(raw string, secret []byte) (jwt.MapClaims, error) {
    token, err := jwt.Parse(raw, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
        }
        return secret, nil
    })
    if err != nil {
        return nil, err
    }
    if claims, ok := token.Claims.(jwt.MapClaims); ok && token.Valid {
        // enforce required claims
        if _, exists := claims["exp"]; !exists {
            return nil, fmt.Errorf("missing exp claim")
        }
        if _, exists := claims["iss"]; !exists {
            return nil, fmt.Errorf("missing iss claim")
        }
        // optionally validate aud, sub, roles here
        return claims, nil
    }
    return nil, fmt.Errorf("invalid token")
}

3. Secure token transmission and storage

Only transmit JWTs over HTTPS, prefer Authorization header, and avoid query parameters or local storage in frontend consumers. Configure CORS strictly.

corsConfig := middleware.CORSConfig{
    Middleware: middleware.CORS({
        Middleware: fiber.HandlerFunc(func(c *fiber.Ctx) error {
            if c.Method() == fiber.MethodOptions {
                return c.Next()
            }
            origin := c.Get("Origin")
            if origin == "https://trusted.example.com" {
                c.Set("Access-Control-Allow-Origin", origin)
                c.Set("Access-Control-Allow-Credentials", "true")
                c.Set("Access-Control-Allow-Headers", "Authorization,Content-Type")
                if c.Method() == fiber.MethodGet {
                    return c.Next()
                }
            }
            return c.SendStatus(fiber.StatusForbidden)
        }),
    }),
}
app.Use(corsConfig)

4. Apply principle of least privilege and token scoping

Use short-lived access tokens and scope claims to permissions. Rotate secrets and consider using asymmetric keys (RS256) for better key management.

// Example of role-based check in a handler
func requireRole(role string) fiber.Handler {
    return func(c *fiber.Ctx) error {
        user := c.Locals("user").(*jwt.Token)
        claims := user.Claims.(jwt.MapClaims)
        if claims["role"] != role {
            return c.SendStatus(fiber.StatusForbidden)
        }
        return c.Next()
    }
}

app.Get("/admin", requireRole("admin"), func(c *fiber.Ctx) error {
    return c.JSON(fiber.Map{"adminOnly": true})
})

By combining strict middleware, claim validation, secure transmission, and least-privilege tokens, you reduce the risk of auth bypass when using JWTs with Fiber.