Mastodon up to 4.4.13/4.5.6 FASP Feature EXPERIMENTAL_FEATURES authorization

Summaryinfo

A vulnerability, which was classified as problematic, has been found in Mastodon up to 4.4.13/4.5.6. This affects an unknown part of the component FASP Feature. This manipulation of the argument EXPERIMENTAL_FEATURES causes authorization. This vulnerability is registered as CVE-2026-27468. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic was found in Mastodon up to 4.4.13/4.5.6. Affected by this vulnerability is an unknown functionality of the component FASP Feature. The manipulation of the argument EXPERIMENTAL_FEATURES with an unknown input leads to a authorization vulnerability. The CWE definition for the vulnerability is CWE-862. The product does not perform an authorization check when an actor attempts to access a resource or perform an action. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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 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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-27468 since 02/19/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation requires an enhanced level of successful authentication. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 4.4.14 or 4.5.7 eliminates this vulnerability. Applying the patch 6ba6285a73c3a8b281123814d45f534e3bcebb96 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Name

• Mastodon

Version

• 4.4.0
• 4.4.1
• 4.4.2
• 4.4.3
• 4.4.4
• 4.4.5
• 4.4.6
• 4.4.7
• 4.4.8
• 4.4.9
• 4.4.10
• 4.4.11
• 4.4.12
• 4.4.13
• 4.5.0
• 4.5.1
• 4.5.2
• 4.5.3
• 4.5.4
• 4.5.5
• 4.5.6

License

• open-source

Website

• Product: https://github.com/mastodon/mastodon/

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