What’s Vulnerable › Vulnerabilities

CVE-2026-54742: Lemmy: `CollectionAdd::Featured` does not check the post is in the community

• Severity: medium (CVSS 5.1 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-19 · updated 2026-09-09
• Weakness: CWE-863

Description

Lemmy is a link aggregator and forum for the fediverse. From 0.19.18 until 0.19.19 and 1.0.0-alpha.20, a community moderator can feature or unfeature posts in other communities through federated CollectionAdd and CollectionRemove activities using CollectionType::Featured. After verify_mod_action authorizes the actor against self.community(), the receive handlers in crates/apub/activities/src/community/collection_add.rs and crates/apub/activities/src/community/collection_remove.rs dereference self.object as an ApubPost and update featured_community without verifying that post.community_id equals community.id. A moderator can therefore target an unrelated post owned by another community, push it into featured feeds and listings, or undo another community's legitimate curation decision. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.20.

Affected products

• LemmyNet lemmy: before 0.19.19; from 1.0.0-alpha.0, < 1.0.0-alpha.20

References

• https://github.com/LemmyNet/lemmy/commit/108d78c592cc88bac2b0bf52ca6631be54f6fb23
• https://github.com/LemmyNet/lemmy/commit/abe256243822526dcd4bca857f7c0adeb49944f5
• https://github.com/LemmyNet/lemmy/pull/6500
• https://github.com/LemmyNet/lemmy/pull/6501
• https://github.com/LemmyNet/lemmy/releases/tag/0.19.19
• https://github.com/LemmyNet/lemmy/releases/tag/1.0.0-alpha.20
• https://github.com/LemmyNet/lemmy/security/advisories/GHSA-gwfj-h8r7-792v
• https://join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19
• https://github.com/LemmyNet/lemmy/security/advisories/GHSA-gwfj-h8r7-792v

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