CVE-2026-50128
Description
Mastodon is a free, open-source social network server based on ActivityPub. From 4.3.0 until 4.5.11 and 4.4.18, Mastodon has a feature to let websites credit authors of their articles. To prevent false attribution claims, Mastodon uses the attributionDomains JSON-LD term, however, an error in how it is defined makes Linked Data Signatures on the toot:attributionDomains property ineffective. An attacker can arbitrarily modify the attributionDomains value of a legitimately signed Update activity and bypass Mastodon’s signature verification. This vulnerability is fixed in 4.5.11 and 4.4.18.
Summary dbcve.org
Mastodon's attributionDomains JSON-LD term, used to prevent false attribution claims in its author credit feature, is incorrectly defined. This error renders Linked Data Signatures on the toot:attributionDomains property ineffective, allowing attackers to arbitrarily modify the attributionDomains value of a legitimately signed Update activity and bypass Mastodon's signature verification.
Mitigation
Upgrade Mastodon to version 4.5.11, 4.4.18, or later to apply the security fix.