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TI-2026-39553 Aliases CVE-2026-54741

Lemmy: Blocked users can edit private messages sent before the block

Priority MEDIUM 57CWE-862Published · Updated

Key facts

Attack vector
Network
Privileges required
Low
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
Security bypass
Confidentiality · integrity · availability
None · Low · None
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA. Impact type from CWE-862.

Summary

Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-alpha.18, Lemmy blocks new private messages from a sender after the recipient blocks that sender, but the edit path skips the same block check. create_private_message checks the recipient's block list with PersonActions::read_block before inserting a message, while edit_private_message in crates/api/api_crud/src/private_message/update.rs only checks that the caller is orig_private_message.creator_id. The update then writes new content and returns the modified PrivateMessageView without consulting the recipient's block list, allowing a blocked sender to keep changing an old message that the recipient can still see and providing a post-block harassment path. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.18.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00484 41st percentileScored 2026-11-08
Fix Fixed lemmy (LemmyNet): 0.19.19, 1.0.0-alpha.18

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
LemmyNetlemmybefore 0.19.19; 1.0.0-alpha.0 to before 1.0.0-alpha.180.19.19, 1.0.0-alpha.18Patch availableCNA
  • lemmy VendorLemmyNet Affected before 0.19.19; 1.0.0-alpha.0 to before 1.0.0-alpha.18 Fixed in 0.19.19, 1.0.0-alpha.18 Patch available CNA

Remediation

  • Fix · CNA lemmy (LemmyNet) Fixed in 0.19.19, 1.0.0-alpha.18

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-54741

References

Other 7

  • TI-2026-23295 Lemmy: Unauthenticated SSRF via file_type query parameter injection in image endpoint CVSS 4.0HIGH 7.7
  • TI-2026-26073 Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid() CVSS 3.1MEDIUM 6.5
  • TI-2026-39555 Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed CVSS 4.0MEDIUM 6.4
  • TI-2026-39552 Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators CVSS 3.1MEDIUM 6.5
  • TI-2026-30504 Lemmy: SSRF and internal image disclosure in post link metadata via unvalidated og:image CVSS 3.1MEDIUM 6.5
  • TI-2026-39554 Lemmy: `CollectionAdd::Featured` does not check the post is in the community CVSS 4.0MEDIUM 5.1
See all 10 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences