Session Fixation Anti-Pattern

Severity: High

Summary

Attackers fix a user's session ID before login. The attacker obtains a valid session ID, tricks the victim into using it, and when authentication fails to regenerate the session ID, hijacks the victim's authenticated session.

The Anti-Pattern

The anti-pattern is reusing the same session ID before and after authentication.

BAD Code Example

# VULNERABLE: Session ID not regenerated after login.
from flask import Flask, session, redirect, url_for, request

app = Flask(__name__)