Mastodon up to 4.3.18/4.4.12/4.5.5 ActivityPub Endpoint Rails.cache cache containing sensitive information

Summaryinfo

A vulnerability was found in Mastodon up to 4.3.18/4.4.12/4.5.5. It has been classified as critical. Affected by this issue is the function Rails.cache of the component ActivityPub Endpoint. Performing a manipulation results in use of cache containing sensitive information. This vulnerability was named CVE-2026-25540. The attack may be initiated remotely. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as critical, was found in Mastodon up to 4.3.18/4.4.12/4.5.5. This affects the function Rails.cache of the component ActivityPub Endpoint. The manipulation with an unknown input leads to a use of cache containing sensitive information vulnerability. CWE is classifying the issue as CWE-524. The code uses a cache that contains sensitive information, but the cache can be read by an actor outside of the intended control sphere. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.3.19, 4.4.13, 4.5.6, Mastodon is vulnerable to web cache poisoning via `Rails.cache. When AUTHORIZED_FETCH is enabled, the ActivityPub endpoints for pinned posts and featured hashtags have contents that depend on the account that signed the HTTP request. However, these contents are stored in an internal cache and reused with no regards to the signing actor. As a result, an empty response generated for a blocked user account may be served to requests from legitimate non-blocked actors, or conversely, content intended for non-blocked actors may be returned to blocked actors. This issue has been patched in versions 4.3.19, 4.4.13, 4.5.6.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-25540 since 02/02/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1592 for this issue.

Upgrading to version 4.3.19, 4.4.13 or 4.5.6 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-5329). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• Mastodon

Version

• 4.3.0
• 4.3.1
• 4.3.2
• 4.3.3
• 4.3.4
• 4.3.5
• 4.3.6
• 4.3.7
• 4.3.8
• 4.3.9
• 4.3.10
• 4.3.11
• 4.3.12
• 4.3.13
• 4.3.14
• 4.3.15
• 4.3.16
• 4.3.17
• 4.3.18
• 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
• 4.5.0
• 4.5.1
• 4.5.2
• 4.5.3
• 4.5.4
• 4.5.5

Website

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

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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