Is it possible to substitute or intercept a JWT token, and how can this be done?

Substitution or hijacking of JWT tokens is possible if an attacker gains access to the token or the keys used for signing it. Main methods:

1. Intercepting the token over the network — if the connection is not secure (e.g., without HTTPS), the token can be intercepted via a MITM attack.
2. XSS attacks — if there are vulnerabilities on the site, an attacker can execute a script to get the token from localStorage or cookies.
3. Token forgery — if the secret key for signing JWT is known or compromised, a valid token with arbitrary data can be created.

To protect, use HTTPS, HttpOnly and Secure cookies, Content Security Policy (CSP), and regularly change and protect signing secrets. In Go, when verifying JWT, it is important to check the signature and token validity:

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

func validateToken(tokenString string, secret []byte) (*jwt.Token, error) {
  return jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
    if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
      return nil, fmt.Errorf("unexpected signing method")
    }
    return secret, nil
  })
}

Thus, substitution is possible with weak security, but proper use of JWT and security measures minimize risks.