Fix JWT Vulnerabilities (Weak Signing, None Algo) in Buffalo

Buffalo apps using JWT middleware often fall victim to critical authentication bypasses. The two most lethal vectors are the 'None' algorithm exploit—where an attacker signs a token with no key—and weak secret keys that are trivial to brute-force. As a researcher, I see these misconfigurations everywhere. If you aren't explicitly validating the 'alg' header, your app is a sitting duck for token forgery.

The Vulnerable Pattern

import "github.com/gobuffalo/mw-jwt"
// VULNERABLE: Hardcoded weak secret and no algorithm enforcement.
// Attackers can change ‘alg’ to ‘none’ or use ‘HS256’ to brute-force the key.
var jwtMiddleware = jwt.New(jwt.Options{
Secret: []byte(“secret”),
})
// In Buffalo’s app.go
app.Use(jwtMiddleware)

The Secure Implementation

The fix addresses two core failures. First, it migrates the signing secret to an environment variable to prevent hardcoded leaks and ensures the key has sufficient entropy. Second, it implements a custom 'ValidationKeyGetter'. This is crucial because it forces the middleware to check the 'alg' header in the JWT. By verifying that the token was signed with 'jwt.SigningMethodHMAC', we effectively kill 'None' algorithm bypasses and 'RS256-to-HS256' confusion attacks. If the header doesn't match our expected method, the token is rejected immediately.

import (
    "os"
    "fmt"
    "github.com/golang-jwt/jwt/v4"
    "github.com/gobuffalo/mw-jwt"
)
// SECURE: Enforce HMAC-SHA256 and use high-entropy secrets from env.
var jwtMiddleware = jwt.New(jwt.Options{
Secret: []byte(os.Getenv(“JWT_SECRET”)),
ValidationKeyGetter: func(token *jwt.Token) (interface{}, error) {
// Explicitly verify the signing method is HMAC
if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
return nil, fmt.Errorf(“Unexpected signing method: %v”, token.Header[“alg”])
}
return []byte(os.Getenv(“JWT_SECRET”)), nil
},
})