Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the /web/* route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft. The issue occurs because URL-encoded slashes (%2F) bypass Rails path normalization and are interpreted as host-relative redirects. Versions 4.5.8, 4.4.15, and 4.3.21 patch the issue.