CVE-2026-33868
Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'
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.
How to fix CVE-2026-33868
To remediate CVE-2026-33868, upgrade the affected package to a fixed version below.
- —upgrade to 4.3.21 or later
Is CVE-2026-33868 being exploited?
Low — EPSS is 1.4%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 4.3.21, >= 4.4.0, < 4.4.15, >= 4.5.0, < 4.5.8
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | MEDIUM6.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N |