CVE-2026-33868: Joinmastodon Mastodon

Medium severity, CVSS 6.1. EPSS: 0.6% chance of exploitation in the next 30 days.

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

• Joinmastodon Mastodon: before 4.3.21 (fixed in 4.3.21); from 4.4.0, before 4.4.15 (fixed in 4.4.15); from 4.5.0, before 4.5.8 (fixed in 4.5.8)

Published 2026-03-27. Last modified 2026-06-17.