Private Lemmy instances expose multi-community metadata without authentication

Exploitation

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

CVSS

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

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

NOTE: Only affects development version. Summary readmulticommunity() does not enforce the private-instance setting. On a private instance, an unauthenticated visitor can read multi-community names, titles, summaries, sidebars, owner identities, and member community lists. Details Other read handlers load localsite and call checkprivateinstance() before returning data to unauthenticated callers. readmulticommunity() does not call that helper: getcommunity(), listposts(), listcomments(), readperson(), search(), and resolveobject() all enforce the private-instance guard. Proof of Concept The script creates a multi-community whose metadata contains a marker, turns on privateinstance, confirms a guarded control endpoint blocks unauthenticated callers, then reads the same multi-community over GET /multicommunity without authentication. Output: The control request shows the privacy setting is active. The multi-community endpoint still returns the private metadata. Impact An unauthenticated…

Published by the source

6 May 2026, 22:12 UTC

Last modified by the source

8 Jun 2026, 23:39 UTC

Source last checked

26 Oct 2026, 15:42 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-jmxc-hhwx-gvv3

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# Patch Next advisory brief: GHSA-jmxc-hhwx-gvv3

Advisory: Private Lemmy instances expose multi-community metadata without authentication
IDs: GHSA-jmxc-hhwx-gvv3
Record: https://patchnext.com/advisories/ghsa-jmxc-hhwx-gvv3 (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 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io lemmy_api: affected <= 0.19.1-rc.1

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

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on lemmy_api.
- 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-jmxc-hhwx-gvv3 (GitHub-reviewed advisory GHSA-jmxc-hhwx-gvv3 lists affected ranges: crates.io lemmy_api >=0 <=0.19.1-rc.1)

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