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

Quick assessment

Affected
LemmyNet lemmy
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

LemmyNet lemmy是LemmyNet组织的一个去中心化的链接聚合与讨论社区平台。 LemmyNet lemmy 0.19.19之前版本和1.0.0-beta.1之前版本存在侧信道信息泄露漏洞，该漏洞源于登录端点根据用户名或邮箱是否存在返回不同错误响应，导致未认证攻击者能够确认已注册的用户名或邮箱地址，并用于定向凭证攻击或社会工程学。

CVSS 6.9 · Medium EPSS 0.43% · P36

Affected Version Matrix 2

VendorProduct Version RangeStatus
LemmyNet lemmy < 0.19.19 affected
>= 1.0.0-alpha.0, < 1.0.0-beta.1 affected
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I. Basic Information for CVE-2026-54739

Vulnerability Information
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Vulnerability Title
Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
响应差异性信息暴露
Source: CVE Program / CVE List V5
Vulnerability Title
LemmyNet lemmy 侧信道信息泄露漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
LemmyNet lemmy是LemmyNet组织的一个去中心化的链接聚合与讨论社区平台。 LemmyNet lemmy 0.19.19之前版本和1.0.0-beta.1之前版本存在侧信道信息泄露漏洞，该漏洞源于登录端点根据用户名或邮箱是否存在返回不同错误响应，导致未认证攻击者能够确认已注册的用户名或邮箱地址，并用于定向凭证攻击或社会工程学。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
LemmyNet lemmy < 0.19.19 -

II. Public POCs for CVE-2026-54739

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III. Intelligence Information for CVE-2026-54739

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Patches & Fixes for CVE-2026-54739 (4)

Vendor Advisories for CVE-2026-54739 (1)

Vendor Pages for CVE-2026-54739 (3)

Same Patch Batch · LemmyNet · 2026-08-19 · 6 CVEs total

CVE-2026-54740 6.5 MEDIUM Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators
CVE-2026-54738 6.5 MEDIUM Lemmy: Rate limit bypass via X-Forwarded-For header spoofing in actix-web ConnectionInfo
CVE-2026-54743 6.4 MEDIUM Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed
CVE-2026-54741 5.3 MEDIUM Lemmy: Blocked users can edit private messages sent before the block
CVE-2026-54742 5.1 MEDIUM Lemmy: `CollectionAdd::Featured` does not check the post is in the community

IV. Related Vulnerabilities

Same product: lemmy
Same vendor: LemmyNet
Same weakness: CWE-204

V. Comments for CVE-2026-54739

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