Http Request Smuggling in Fiber with Jwt Tokens

HTTP request smuggling arises when an API endpoint processes requests differently depending on whether they are parsed as front-end (e.g., a load balancer or reverse proxy) versus back-end requests. In Go Fiber, this can occur when the application relies on non-standard or inconsistent parsing of headers such as Content-Length and Transfer-Encoding while also enforcing JWT-based authentication. If the middleware that validates JWT tokens runs before the request body is normalized, an attacker can craft a request that is accepted by the JWT validator but interpreted differently by the underlying HTTP layer, leading to request smuggling.

Consider a Fiber route that expects a JSON body and uses JWT middleware to authenticate requests:

// Example JWT middleware in Fiber
func JWTMiddleware(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if auth == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header required"})
    }
    // Expects "Bearer <token>"
    tokenString := strings.TrimPrefix(auth, "Bearer ")
    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method")
        }
        return []byte("secret"), nil
    })
    if err != nil || !token.Valid {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid token"})
    }
    return c.Next()
}

If this middleware is placed before body parsing, a request with both Content-Length: 0 and Transfer-Encoding: chunked might pass JWT validation but be interpreted differently by the server or an upstream proxy. For example, a request that appears valid to the application layer may be processed with a different body by an intermediary, allowing an authenticated user to smuggle a request that bypasses intended routing or leaks data across requests. This becomes particularly risky when JWT tokens are used to assert identity but the request boundary is ambiguous, enabling authenticated smuggling where the attacker’s token is accepted but the request path is altered.

Another scenario involves APIs that accept both token-based auth and optional body parameters. If the JWT check passes but the body is parsed inconsistently (for instance, accepting both application/json and form data without strict validation), an authenticated attacker can inject smuggling headers that change how the request is interpreted by downstream services. The presence of JWT tokens may give a false sense of security, as the vulnerability lies in how the request is parsed and normalized rather than in authentication itself.

To identify such issues, scans should compare authenticated and unauthenticated paths while looking for discrepancies in how headers like Content-Length, Transfer-Encoding, and Host are handled when JWT tokens are present. The goal is to ensure that request parsing is consistent regardless of authentication state and that no trust is placed in header values without strict normalization.

Remediation focuses on normalizing request parsing before JWT validation and ensuring that header interpretation is consistent across all paths. In Fiber, you should parse and canonicalize the request body and headers before checking the token, and avoid relying on potentially ambiguous header combinations.

First, enforce strict content-type handling and explicitly reject requests that contain both Content-Length and Transfer-Encoding:

// Reject requests with both headers to prevent smuggling
func SecureMiddleware(c *fiber.Ctx) error {
    if c.Get("Content-Length") != "" && c.Get("Transfer-Encoding") != "" {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid headers"})
    }
    return c.Next()
}

Then apply JWT validation only after the request body has been safely read and normalized:

// Parse body first, then validate JWT
app.Post("/secure", func(c *fiber.Ctx) error {
    // Ensure consistent body reading
    body, err := c.BodyParser(new(MyStruct))
    if err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid body"})
    }
    // Now validate JWT
    auth := c.Get("Authorization")
    if auth == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header required"})
    }
    tokenString := strings.TrimPrefix(auth, "Bearer ")
    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method")
        }
        return []byte("secret"), nil
    })
    if err != nil || !token.Valid {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid token"})
    }
    // Safe to proceed with authenticated, normalized request
    return c.JSON(fiber.Map{"data": body})
})

Additionally, configure strict header normalization and avoid accepting ambiguous transfer encodings. If your deployment uses proxies, ensure that they do not forward Transfer-Encoding to the application when it is not expected. This reduces the risk of authenticated requests being interpreted differently at different layers.

By combining these steps—rejecting ambiguous header combinations, normalizing the request before authentication, and validating JWT tokens only after body parsing—you mitigate the risk of HTTP request smuggling in Fiber applications that rely on JWT tokens.