CVE-2026-33868
CVE-2026-33868 is a medium-severity vulnerability in Joinmastodon Mastodon with a CVSS 3.x base score of 4.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-601.
Key facts
- Severity: Medium (CVSS 3.x base score 4.3)
- EPSS exploit prediction: 1% (40th percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-16783
- Weakness: CWE-601
- Affected product: Joinmastodon Mastodon
- Published:
- Last modified:
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.
Frequently asked questions
- What is 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.
- How severe is CVE-2026-33868?
- CVE-2026-33868 has a CVSS 3.x base score of 4.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is none, integrity low, and availability none.
- Is CVE-2026-33868 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (40th percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2026-33868?
- CVE-2026-33868 affects Joinmastodon Mastodon. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2026-33868?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
- Does CVE-2026-33868 have an EU (EUVD) identifier?
- Yes. CVE-2026-33868 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-16783.
- When was CVE-2026-33868 published?
- CVE-2026-33868 was published on 2026-03-27 and last updated on 2026-06-17.
References
Affected products (1)
- cpe:2.3:a:joinmastodon:mastodon:*:*:*:*:*:*:*:*
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- CVE-2024-23832 — Critical (CVSS 9.4): Mastodon is a free, open-source social network server based on ActivityPub Mastodon allows configuration of LDAP for…
- CVE-2023-36459 — Critical (CVSS 9.3): Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 1.3 and prior to…
All CVEs affecting Joinmastodon Mastodon →
Other CWE-601 (Open Redirect) vulnerabilities
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