Fix JWT Vulnerabilities (Weak Signing, None Algo) in Go Fiber
JWT implementations in Go Fiber often fail at the most critical step: signature verification. Attackers can exploit 'alg: none' to bypass authentication entirely or brute-force weak HMAC keys. If your middleware blindly trusts the header without enforcing the signing algorithm, you're essentially handing out root access. Let's harden the implementation.
The Vulnerable Pattern
func AuthMiddleware(c *fiber.Ctx) error {
tokenString := c.Get("Authorization")[7:]
// VULNERABLE: No algorithm validation.
// An attacker can set 'alg: none' or switch HMAC to RSA to confuse the parser.
token, _ := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
return []byte("hardcoded-secret"), nil
})
if token.Valid {
return c.Next()
}
return c.SendStatus(401)
}The Secure Implementation
The 'none' algorithm vulnerability occurs when the library allows an unsigned token to be treated as valid. By implementing a strict type check on `token.Method` using a type assertion (e.g., `*jwt.SigningMethodHMAC`), we ensure the parser rejects any token that doesn't match our expected security profile. Furthermore, replacing hardcoded secrets with environment variables prevents credential leakage in source control and allows for high-entropy keys that resist brute-force attacks.
func SecureAuthMiddleware(c *fiber.Ctx) error { authHeader := c.Get("Authorization") if len(authHeader) < 8 { return c.SendStatus(401) } tokenString := authHeader[7:]token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) { // SECURE: Explicitly validate the signing method if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok { return nil, fmt.Errorf("Unexpected signing method: %v", token.Header["alg"]) } secret := os.Getenv("JWT_SECRET") if secret == "" { return nil, errors.New("Internal error") } return []byte(secret), nil }) if err != nil || !token.Valid { return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "Invalid token"}) } return c.Next()
}
Your Go Fiber API
might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)
74% of Go Fiber apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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