CVE-2026-25540
MEDIUMCVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
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2Description
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.
Analysis
Mastodon versions prior to 4.3.19, 4.4.13, and 4.5.6 are vulnerable to web cache poisoning in ActivityPub endpoints when AUTHORIZED_FETCH is enabled, allowing cached responses to be served across different user contexts regardless of request signing. An attacker could exploit this to view content intended for non-blocked accounts or cause blocked users to receive empty responses meant for them, potentially bypassing access controls. …
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Remediation
Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Monitor vendor channels for patch availability.
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