CVE-2026-54739 Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential
An unauthenticated attacker can enumerate registered usernames or emails by observing HTTP response code differences on the login endpoint.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-54739?
CVE-2026-54739 is a medium-severity vulnerability in LemmyNet lemmy, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-204
Which versions are affected by CVE-2026-54739?
How do I fix CVE-2026-54739?
No fix has been published for CVE-2026-54739 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-54739 being exploited?
There is no known exploitation of CVE-2026-54739 in the wild and no public exploit code.
Sources
Also known as GHSA-xgg7-8hvq-8m65.