CVE-2026-54739: CWE-204: Observable Response Discrepancy in LemmyNet lemmy
Description
CVE-2026-54739 is a vulnerability in LemmyNet's lemmy software where the login endpoint reveals whether a username or email exists by returning different error responses. This discrepancy allows unauthenticated attackers to enumerate valid accounts, facilitating targeted credential attacks or social engineering. The issue affects versions prior to 0.19.19 and versions from 1.0.0-alpha.0 up to but excluding 1.0.0-beta.1. It is fixed in versions 0.19.19 and 1.0.0-beta.1.
CVSS v4.0
Score 6.9medium
Affected software
LemmyNet
lemmy
pkg:github/lemmynet/lemmyRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
LemmyNet's lemmy login endpoint in crates/api/api/src/local_user/login.rs returns distinct error responses based on the existence of the username_or_email parameter. Specifically, a NotFound response (HTTP 404) is returned for unknown accounts, while an existing account with an incorrect password returns LemmyErrorType::IncorrectLogin (HTTP 400). This observable response discrepancy (CWE-204) enables unauthenticated attackers to confirm registered usernames or email addresses. The vulnerability is fixed in versions 0.19.19 and 1.0.0-beta.1.
Potential Impact
An unauthenticated attacker can enumerate valid usernames or email addresses by observing the different error responses from the login endpoint. This information can be leveraged for targeted credential stuffing, brute force attacks, or social engineering campaigns. There is no indication of direct system compromise or privilege escalation from this vulnerability alone.
Mitigation Recommendations
This vulnerability is fixed in Lemmy versions 0.19.19 and 1.0.0-beta.1. Users should upgrade to these or later versions to remediate the issue. No additional mitigation steps are indicated by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-06-15T23:07:33.233Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6a86e0f3acd9273b49838516
Added to database: 08/20/2026, 11:11:47 UTC
Last enriched: 09/11/2026, 05:17:55 UTC
Last updated: 10/04/2026, 18:53:17 UTC
Views: 87
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