CVE-2025-27399: Joinmastodon Mastodon

Medium severity, CVSS 5.3. EPSS: 0.4% chance of exploitation in the next 30 days.

Mastodon is a self-hosted, federated microblogging platform. In versions prior to 4.1.23, 4.2.16, and 4.3.4, when the visibility for domain blocks/reasons is set to "users" (localized English string: "To logged-in users"), users that are not yet approved can view the block reasons. Instance admins that do not want their domain blocks to be public are impacted. Versions 4.1.23, 4.2.16, and 4.3.4 fix the issue.

Affected products

• Joinmastodon Mastodon: before 4.1.23 (fixed in 4.1.23); from 4.2.0, before 4.2.16 (fixed in 4.2.16); from 4.3.0, before 4.3.4 (fixed in 4.3.4)

Published 2025-02-27. Last modified 2026-06-17.