Glossary

Open Redirect

An open redirect is a vulnerability where an application redirects a user to an arbitrary external URL without validation. An attacker crafts a link like https://trusted.com/redirect?url=https://phishing.com — the victim sees a trusted domain and lands on a malicious site.

Typical use in attacks

Phishing — the link looks trustworthy
OAuth redirect hijacking — substitute the redirect_uri in an OAuth flow
Bypassing referer checks

Defence

Whitelist allowed URLs or domains for redirection
Do not accept external URLs in redirect parameters
Compare the URL host with the current domain before redirecting

// Check: only relative URLs
function safeRedirect(string $url): string {
  $parsed = parse_url($url);
  // reject if scheme or host is present
  if (isset($parsed['scheme']) || isset($parsed['host'])) {
    return '/'; // fallback to home
  }
  return $url;
}