CVE-2026-42180 Lemmy: SSRF in /api/v3/post via Webmention dispatch
Authenticated users can trigger server-side requests to internal services via Webmention dispatch in Lemmy, potentially exposing internal data.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42180?
CVE-2026-42180 is a medium-severity server-side request forgery vulnerability in LemmyNet lemmy, published on 8 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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.
Weakness: CWE-918
Which versions are affected by CVE-2026-42180?
2 open source packages ship an affected version, including lemmy_api_common.
| Package | Affected | Fixed in |
|---|---|---|
lemmy_api_common crates.io | All versions | 0.19.18 |
lemmy_api_common crates.io | All versions | 0.19.18 |
How do I fix CVE-2026-42180?
Upgrade lemmy_api_common to 0.19.18 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-42180 being exploited?
There is no known exploitation of CVE-2026-42180 in the wild and no public exploit code.
Sources
Also known as GHSA-3jvj-v6w2-h948.