What’s Vulnerable › Vulnerabilities
CVE-2026-33868: Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'
- Severity: medium (CVSS 6.1 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.5% probability of exploitation in the next 30 days
- Public exploit or detection template: yes
- Published: 2026-03-27 · updated 2026-06-17
- Weakness: CWE-601
Description
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
- mastodon: before 4.3.21; from 4.4.0 before 4.4.15; from 4.5.0 before 4.5.8 — fixed in 4.3.21, 4.4.15, 4.5.8
How to fix
Upgrade to a fixed version: mastodon 4.3.21, 4.4.15, 4.5.8. Check the vendor advisory for mitigations.
References
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