CVE-2026-27468
Published: 24 February 2026
Summary
CVE-2026-27468 is a medium-severity Missing Authorization (CWE-862) vulnerability in Joinmastodon Mastodon. Its CVSS base score is 4.8 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 15.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 AC-3 (Access Enforcement) and AC-6 (Least Privilege).
Deeper analysis
CVE-2026-27468 is a missing authorization vulnerability (CWE-862) in Mastodon, a free, open-source social network server based on ActivityPub. It affects versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, but only on servers that have enabled the experimental FASP (Federated Activity Streaming Protocol) feature by including "fasp" in the EXPERIMENTAL_FEATURES environment variable. The issue arises because FASP registration requires manual administrator approval, but actions performed by a FASP to subscribe to account or content lifecycle events, or to backfill content, fail to properly verify whether the FASP has been approved.
The vulnerability can be exploited remotely over the network by unauthenticated attackers with no privileges required (AV:N/AC:L/PR:N). An attacker can initiate subscriptions and content backfill requests without administrator approval. A single exploitation results in a minor information leak of URIs that are publicly available anyway. However, repeated exploitation serves as a serious denial-of-service vector, overwhelming the Sidekiq worker responsible for processing the "fasp" queue and potentially disrupting server operations. The CVSS v3.1 base score is 8.2 (C:L/I:N/A:H).
The Mastodon security advisory (GHSA-qgmm-vr4c-ggjg) and associated fix commit recommend updating to Mastodon 4.4.14 or 4.5.7, which include patches to enforce proper approval checks. Administrators testing the experimental FASP feature should apply these updates immediately. Servers not enabling the "fasp" flag in EXPERIMENTAL_FEATURES are unaffected.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8546
Vulnerability details
Mastodon is a free, open-source social network server based on ActivityPub. FASP registration requires manual approval by an administrator. In versions 4.4.0 through 4.4.13 and 4.5.0 through 4.5.6, actions performed by a FASP to subscribe to account/content lifecycle events or…
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to backfill content did not check properly whether the FASP was actually approved. This only affects Mastodon servers that have opted in to testing the experimental FASP feature by setting the environment variable `EXPERIMENTAL_FEATURES` to a value including `fasp`. An attacker can make subscriptions and request content backfill without approval by an administrator. Done once, this leads to minor information leak of URIs that are publicly available anyway. But done several times this is a serious vector for DOS, putting pressure on the sidekiq worker responsible for the `fasp` queue. The fix is included in the 4.4.14 and 4.5.7 releases. Admins that are actively testing the experimental "fasp" feature should update their systems. Servers not using the experimental feature flag `fasp` are not affected.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Missing authorization in public-facing Mastodon FASP feature enables remote unauthenticated exploitation (T1190) and repeated abuse of resource-intensive subscribe/backfill actions to exhaust Sidekiq workers (T1499.003).
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
AC-3 requires enforcement of approved authorizations, directly addressing the missing verification of FASP administrator approval before allowing subscriptions and backfill requests.
SC-5 protects against denial-of-service events like the resource exhaustion of the Sidekiq fasp queue from repeated unauthorized requests.
AC-6 enforces least privilege, ensuring FASP-related processes only perform approved actions and mitigating unauthorized subscriptions or backfills.