Validate post_logout_redirect_uri to prevent open redirect#71
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Previously, the logout handler accepted any post_logout_redirect_uri and redirected to it without validation. This was an open redirect vulnerability — an attacker could craft a logout link that redirects the user to a phishing site after logout. Now, post_logout_redirect_uri is only honored when a client_id is also provided and the URI matches one of that client's registered redirect URIs. If validation fails (missing client_id, unknown client, or unregistered URI), the handler falls back to /oauth/login. Closes #59. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
post_logout_redirect_uriwas accepted and redirected to without any validation — an attacker could craft a link like/oauth/logout?post_logout_redirect_uri=https://evil.com/phishingthat logs the user out and sends them to a fake login page.client_idparameter alongsidepost_logout_redirect_uri, looks up the client, and validates the URI is in the client's registeredredirect_uris. Falls back to/oauth/loginif validation fails for any reason.Test plan
Five integration tests covering all validation paths (written red/green TDD — confirmed failures before fix):
/oauth/login/oauth/login/oauth/login/oauth/loginCloses #59.
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