Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-54739
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-19 21:16:58 UTC
Updated2026-08-21 22:16:40 UTC
DescriptionLemmy 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.

Risk And Classification

Primary CVSS: v4.0 6.9 MEDIUM from [email protected]

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS: 0.004320000 probability, percentile 0.358300000 (date 2026-08-24)

Problem Types: CWE-204 | CWE-204 CWE-204: Observable Response Discrepancy

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/C...
4.0CNADECLARED6.9MEDIUMCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
Confidentiality
Low
Integrity
None
Availability
None
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA LemmyNet Lemmy affected < 0.19.19 Not specified
CNA LemmyNet Lemmy affected >= 1.0.0-alpha.0, < 1.0.0-beta.1 Not specified

References

ReferenceSourceLinkTags
github.com/LemmyNet/lemmy/commit/59e968a5ac36d36d08a59028e2dbb92641536964 [email protected] github.com
join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19 [email protected] join-lemmy.org
github.com/LemmyNet/lemmy/pull/6531 [email protected] github.com
github.com/LemmyNet/lemmy/releases/tag/1.0.0-beta.1 [email protected] github.com
github.com/LemmyNet/lemmy/commit/4d235a1b668b7dda6b73dbbc9ca289bb40acfa83 [email protected] github.com
github.com/LemmyNet/lemmy/pull/6535 [email protected] github.com
github.com/LemmyNet/lemmy/releases/tag/0.19.19 [email protected] github.com
github.com/LemmyNet/lemmy/security/advisories/GHSA-xgg7-8hvq-8m65 [email protected] github.com
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