CVE-2026-50128: Mastodon

Medium severity, CVSS 5.3. EPSS: 0.2% chance of exploitation in the next 30 days.

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.

Affected products

• Mastodon Mastodon: from 4.5.0-beta.1, before 4.5.11 (fixed in 4.5.11); from 4.3.0, before 4.4.18 (fixed in 4.4.18)

Published 2026-06-24. Last modified 2026-06-25.