Vulnerability · Published May 8, 2026 · Assigned by GitHub_M

CVE-2026-42180Lemmy: SSRF in /api/v3/post via Webmention dispatch

Medium 6.3

In plain English

A medium-severity server-side request forgery in lemmy from LemmyNet. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities).

Severity (CVSS 3.1)6.3Medium

Exploit odds (30 days)0.2%top 92.0% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

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.

Affected products (as reported)

Vendor	Product	Versions
LemmyNet	lemmy	< 0.19.18

Weakness: CWE-918 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

References

https://github.com/LemmyNet/lemmy/security/advisories/GHSA-3jvj-v6w2-h948 x_refsource_CONFIRM
https://github.com/LemmyNet/lemmy/releases/tag/0.19.18 x_refsource_MISC