Open Redirect in Joinmastodon Mastodon
CVE-2026-33868
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.
Vulnerability class: Open Redirect
EPSS: 0.005 (40.9th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N.
Affected products
- Joinmastodon Mastodon
- Mastodon — versions >= 4.5.0, < 4.5.8, >= 4.4.0, < 4.4.15, < 4.3.21
Weakness classification (CWE)
Public proof-of-concept exploits
References
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-33868?
- CVE-2026-33868 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 4.3/10. Published 2026-03-27.
- How severe is CVE-2026-33868?
- Medium severity. CVSS v3 base score is 4.3 out of 10.
- Is CVE-2026-33868 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.