LemmyNet Lemmy up to 0.19.18/1.0.0-beta.0 Login Endpoint login.rs find_by_email_or_name username_or_email information exposure

Summaryinfo

A vulnerability, which was classified as problematic, has been found in LemmyNet Lemmy up to 0.19.18/1.0.0-beta.0. Affected by this vulnerability is the function LocalUserView::find_by_email_or_name of the file crates/api/api/src/local_user/login.rs of the component Login Endpoint. This manipulation of the argument username_or_email causes information exposure. This vulnerability is registered as CVE-2026-54739. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in LemmyNet Lemmy up to 0.19.18/1.0.0-beta.0. It has been declared as problematic. Affected by this vulnerability is the function LocalUserView::find_by_email_or_name of the file crates/api/api/src/local_user/login.rs of the component Login Endpoint. The manipulation of the argument username_or_email with an unknown input leads to a information exposure vulnerability. The CWE definition for the vulnerability is CWE-203. The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not. As an impact it is known to affect confidentiality. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-54739 since 06/16/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1592 according to MITRE ATT&CK.

Upgrading to version 0.19.19 or 1.0.0-beta.1 eliminates this vulnerability.

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Productinfo

Vendor

• LemmyNet

Name

• Lemmy

Version

• 0.19.0
• 0.19.1
• 0.19.2
• 0.19.3
• 0.19.4
• 0.19.5
• 0.19.6
• 0.19.7
• 0.19.8
• 0.19.9
• 0.19.10
• 0.19.11
• 0.19.12
• 0.19.13
• 0.19.14
• 0.19.15
• 0.19.16
• 0.19.17
• 0.19.18
• 1.0.0-beta

Website

• Product: https://github.com/LemmyNet/lemmy/

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