CVE-2026-42180: Lemmynet Lemmy

Medium severity, CVSS 6.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Lemmy is a link aggregator and forum for the fediverse. Prior to version 0.19.18, Lemmy allows an authenticated low-privileged user to create a link post through POST /api/v3/post. When a post is created in a public community, the backend asynchronously sends a Webmention to the attacker-controlled link target. The submitted URL is checked for syntax and scheme, but the audited code path does not reject loopback, private, or link-local destinations before the Webmention request is issued. This lets a normal user trigger server-side HTTP requests toward internal services. This issue has been patched in version 0.19.18.

Affected products

• Lemmynet Lemmy: before 0.19.18 (fixed in 0.19.18)

Published 2026-05-08. Last modified 2026-06-17.