CVE-2026-54739
Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential
- State
- Published
- Severity
- 6.9 Medium
- Published
- Aug 19, 2026
- Updated
- Aug 21, 2026
- Reserved
- Jun 15, 2026
Description
Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-beta.1, Lemmy's login endpoint in crates/api/api/src/local_user/login.rs returns different errors depending on whether the username_or_email value exists. LocalUserView::find_by_email_or_name propagates a NotFound response for an unknown account, while an existing account with a wrong password returns LemmyErrorType::IncorrectLogin. This observable response discrepancy, including HTTP 404 for a nonexistent account and HTTP 400 for an incorrect password on an existing account, allows an unauthenticated attacker to confirm registered usernames or email addresses and use the results for targeted credential attacks or social engineering. This issue is fixed in versions 0.19.19 and 1.0.0-beta.1.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 6.9 Medium | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N | GitHub_M |
Weaknesses
- CWE-204 Observable Response Discrepancy
Affected Products
| Product | Versions |
|---|---|
lemmyLemmyNet |
|
References
- https://github.com/LemmyNet/lemmy/security/advisories/GHSA-xgg7-8hvq-8m65
- https://github.com/LemmyNet/lemmy/pull/6531
- https://github.com/LemmyNet/lemmy/pull/6535
- https://github.com/LemmyNet/lemmy/commit/4d235a1b668b7dda6b73dbbc9ca289bb40acfa83
- https://github.com/LemmyNet/lemmy/commit/59e968a5ac36d36d08a59028e2dbb92641536964
- https://github.com/LemmyNet/lemmy/releases/tag/0.19.19
- https://github.com/LemmyNet/lemmy/releases/tag/1.0.0-beta.1
- https://join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19
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