open-redirect

Open Redirect Security Check (CWE-601)

What this checks

Protects against open redirect vulnerabilities where an attacker crafts a link that redirects users from a trusted domain to a malicious site. Used in phishing campaigns to make malicious links appear legitimate, and in OAuth flows to steal authorization codes.

Vulnerable patterns

• redirect(request.args["next"]) — redirects to any URL the caller supplies
• http.Redirect(w, r, r.URL.Query().Get("return"), 302) — no validation
• response.sendRedirect(req.getParameter("url")) — Java unvalidated redirect
• window.location = params.get("redirect") — client-side open redirect

Fix immediately

Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties:

1. Every redirect target is either a relative path or a member of an exact host allowlist. Parse the URL, inspect the host, and reject anything outside the allowlist. Prefix matching (startswith) is not sufficient — allowed.com.evil.com passes a prefix check.
2. Scheme-relative URLs are blocked explicitly. A value starting with // parses as a protocol-relative URL and redirects to whatever host follows. Checking for http:// / https:// alone misses this.
3. OAuth and login "return-to" parameters go through the same validator as any other redirect target. These are the highest-value targets because they run in an authenticated context; a redirect here hands the attacker the post-login session or an authorization code.
4. On validation failure, redirect to a safe default (/, home, dashboard) rather than echoing an error containing the malicious URL. Echoing it back gives attackers a reflected-XSS surface.

Anchor — shape, not implementation:

def safe_redirect(target):
    if target.startswith("//"): return "/"       # block scheme-relative
    u = parse(target)
    if u.host and u.host not in ALLOWED_HOSTS: return "/"
    return target

Verification

• Every redirect target derived from user input is validated as either a relative path or a member of an explicit host allowlist
• Scheme-relative URLs (//evil.com) are blocked — not just http:// and https://
• OAuth/login "return_to" parameters are validated before redirect

References

• CWE-601 (URL Redirection to Untrusted Site)