CVE-2026-54739 is a vulnerability in LemmyNet's lemmy software where the login endpoint reveals whether a username or email exists by returning different error responses. This discrepancy allows unauthenticated attackers to enumerate valid accounts, facilitating targeted credential attacks or social engineering. The issue affects versions prior to 0.19.19 and versions from 1.0.0-alpha.0 up to but excluding 1.0.0-beta.1. It is fixed in versions 0.19.19 and 1.0.0-beta.1.
CVE-2026-54740 is a missing authorization vulnerability in LemmyNet's lemmy software that allows a lower-ranked remote moderator to remove a higher-ranked moderator in federated communities. This occurs because the verification logic for moderator removal does not properly check the actor's rank against the target moderator's rank. The issue affects versions prior to 0.19.19 and 1.0.0-alpha.18 and has been fixed in those versions.
CVE-2026-54741 is a missing authorization vulnerability in LemmyNet's lemmy software that allows blocked senders to edit private messages after being blocked by the recipient. The create_private_message function properly checks the recipient's block list before allowing message creation, but the edit_private_message function does not perform this check. This flaw enables a blocked sender to modify existing messages visible to the recipient, potentially facilitating harassment. The issue is fixed in versions 0.19.19 and 1.0.0-alpha.18.
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.
Lemmy is a link aggregator and forum for the fediverse. Starting in version 0.17.0 and prior to version 0.19.1, users can report private messages, even when they're neither sender nor recipient of the message. The API response to creating a private message report contains the private message itself, which means any user can just iterate over message ids to (loudly) obtain all private messages of an instance. A user with instance admin privileges can also abuse this if the private message is removed from the response, as they're able to see the resulting reports.
Creating a private message report by POSTing to `/api/v3/private_message/report` does not validate whether the reporter is the recipient of the message.lemmy-ui does not allow the sender to report the message; the API method should likely be restricted to accessible to recipients only. The API response when creating a report contains the `private_message_report_view` with all the details of the report, including the private message that has been reported:
Any authenticated user can obtain arbitrary (untargeted) private message contents. Privileges required depend on the instance configuration; when registrations are enabled without application system, the privileges required are practically none. When registration applications are required, privileges required could be considered low, but this assessment heavily varies by instance.
Version 0.19.1 contains a patch for this issue. A workaround is available. If an update to a fixed Lemmy version is not immediately possible, the API route can be blocked in the reverse proxy. This will prevent anyone from reporting private messages, but it will also prevent exploitation before the update has been applied.