medium4.3
CVE-2026-33868
Mastodon is a free, open-source social network server based on ActivityPub.
Impact
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.
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Joinmastodon | Mastodon | before 4.3.21 from 4.4.0 before 4.4.15 from 4.5.0 before 4.5.8 |