Lemmy has SSRF in /api/v3/post via Webmention dispatch

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L

EPSS

• CVE-2026-42180: 0.18% estimated exploitation activity in the next 30 days; percentile 8.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Details The entry point is the normal post creation API. The user-controlled url field is accepted, normalized with dieselurlcreate(), and only validated with isvalidurl(). That validation allows http and https but does not implement internal address rejection. The post creation flow then schedules Webmention delivery for public communities. This creates a direct source-to-sink path from an externally supplied post URL to a server-side outbound HTTP request. Core vulnerable code path:…

Published by the source

24 Apr 2026, 15:22 UTC

Last modified by the source

27 May 2026, 06:11 UTC

Source last checked

26 Oct 2026, 15:42 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-3jvj-v6w2-h948

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# Patch Next advisory brief: CVE-2026-42180

Advisory: Lemmy has SSRF in /api/v3/post via Webmention dispatch
IDs: CVE-2026-42180, GHSA-3jvj-v6w2-h948
Record: https://patchnext.com/advisories/cve-2026-42180 (revision 1, source checked 26 Oct 2026, 15:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lemmy_api_common 0.19.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io lemmy_api_common: affected < 0.19.18; fixed in 0.19.18

## Suggested inspection (read-only)
- crates.io lemmy_api_common: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on lemmy_api_common.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-3jvj-v6w2-h948 (GitHub-reviewed advisory GHSA-3jvj-v6w2-h948 lists affected ranges: crates.io lemmy_api_common >=0 <0.19.18)

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