CVE-2026-54739: CWE-204: Observable Response Discrepancy in LemmyNet lemmy
LemmyNet lemmy versions prior to 0.19.19 and 1.0.0-beta.1 have a vulnerability in the login endpoint that reveals whether a username or email exists by returning different error responses. This allows unauthenticated attackers to enumerate valid accounts, facilitating targeted credential attacks or social engineering. The issue is fixed in versions 0.19.19 and 1.0.0-beta.1.
AI Analysis
Technical Summary
The vulnerability (CVE-2026-54739) in LemmyNet lemmy affects the login endpoint where the system returns distinct error messages depending on whether the username_or_email exists. Specifically, an unknown account triggers a NotFound response (HTTP 404), while an existing account with an incorrect password returns a LemmyErrorType::IncorrectLogin (HTTP 400). This observable response discrepancy enables unauthenticated attackers to confirm registered usernames or email addresses. The flaw is addressed in versions 0.19.19 and 1.0.0-beta.1.
Potential Impact
An attacker can enumerate valid usernames or email addresses on the Lemmy platform without authentication by analyzing the differing error responses. This information can be leveraged for targeted credential stuffing, password guessing, or social engineering attacks. There is no indication of direct system compromise from this vulnerability alone.
Mitigation Recommendations
Upgrade LemmyNet lemmy to version 0.19.19 or later, or 1.0.0-beta.1 or later, where this issue has been fixed. Patch status is confirmed by the vendor advisory stating the fix is included in these versions. No additional mitigations are specified.
CVE-2026-54739: CWE-204: Observable Response Discrepancy in LemmyNet lemmy
Description
LemmyNet lemmy versions prior to 0.19.19 and 1.0.0-beta.1 have a vulnerability in the login endpoint that reveals whether a username or email exists by returning different error responses. This allows unauthenticated attackers to enumerate valid accounts, facilitating targeted credential attacks or social engineering. The issue is fixed in versions 0.19.19 and 1.0.0-beta.1.
CVSS v4.0
Score 6.9medium
Affected software
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
The vulnerability (CVE-2026-54739) in LemmyNet lemmy affects the login endpoint where the system returns distinct error messages depending on whether the username_or_email exists. Specifically, an unknown account triggers a NotFound response (HTTP 404), while an existing account with an incorrect password returns a LemmyErrorType::IncorrectLogin (HTTP 400). This observable response discrepancy enables unauthenticated attackers to confirm registered usernames or email addresses. The flaw is addressed in versions 0.19.19 and 1.0.0-beta.1.
Potential Impact
An attacker can enumerate valid usernames or email addresses on the Lemmy platform without authentication by analyzing the differing error responses. This information can be leveraged for targeted credential stuffing, password guessing, or social engineering attacks. There is no indication of direct system compromise from this vulnerability alone.
Mitigation Recommendations
Upgrade LemmyNet lemmy to version 0.19.19 or later, or 1.0.0-beta.1 or later, where this issue has been fixed. Patch status is confirmed by the vendor advisory stating the fix is included in these versions. No additional mitigations are specified.
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
- Remediation Level
- null
Threat ID: 6a86e0f3acd9273b49838516
Added to database: 08/20/2026, 11:11:47 UTC
Last enriched: 08/20/2026, 11:23:03 UTC
Last updated: 08/20/2026, 22:52:13 UTC
Views: 3
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