CVE-2026-54742

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation. Prior to version 0.19.16, the GET /api/v4/image/{filename} endpoint is vulnerable to unauthenticated SSRF through parameter injection in the file_type query parameter. An attacker can inject arbitrary query parameters into the internal request to pict-rs, including the proxy parameter which causes pict-rs to fetch arbitrary URLs.

Lemmy-ui before version 0.19.19-beta.1 renders Markdown without sanitization, allowing HTML injection and JavaScript execution in the lemmy-ui origin. An approved member or a remote federated instance can store malicious content, and a viewer who renders it may execute script, potentially exposing the viewer's session and authenticated actions.

Lemmy before versions 0.19.19 and 1.0.0-alpha.18 does not check the block list when editing private messages. A blocked sender can still edit old messages that the recipient can see, enabling post-block harassment.

Lemmy before versions 0.19.19 and 1.0.0-alpha.18 allows a lower-ranked moderator to remove a higher-ranked moderator by sending a signed ActivityPub Remove activity. The verification does not check moderator rank, enabling a junior moderator to strip senior moderators from the community moderator list.

Lemmy before versions 0.19.19 and 1.0.0-beta.1 returns different errors during login depending on whether the username or email exists. A nonexistent account returns 404, while an existing account with a wrong password returns 400. This discrepancy allows an unauthenticated attacker to confirm registered usernames or email addresses.

Lemmy before versions 0.19.19 and 1.0.0-beta.1 uses the first X-Forwarded-For value as the client address for rate limiting. The bundled nginx config appends the real client address to a client-supplied value, allowing an attacker to spoof the address and bypass limits on multiple API endpoints.