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CVE-2026-54743— Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed

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-beta.1之前版本存在跨站脚本漏洞，该漏洞源于Markdown渲染时未对插件生成的输出进行清理和转义，特制alt文本可被注入为实时HTML，可能导致跨站脚本攻击，暴露用户会话并执行经过身份验证的操作。

CVSS 6.4 · Medium EPSS 0.40% · P34

Affected Version Matrix 1

VendorProduct Version RangeStatus
LemmyNet lemmy < 0.19.19-beta.1 affected
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I. Basic Information for CVE-2026-54743

Vulnerability Information
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Vulnerability Title
Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed
Source: CVE Program / CVE List V5
Vulnerability Description
Lemmy is a link aggregator and forum for the fediverse. Prior to lemmy-ui 0.19.19-beta.1, LemmyNet/lemmy-ui renders Markdown in src/shared/markdown.ts for post bodies, comment bodies, private messages, and community and site sidebars through mdToHtml, which returns a raw __html object that Inferno injects without a sanitizer pass. setupMarkdown configures html as false but registers markdown-it-html5-embed@1.0.0 with useImageSyntax enabled, so an image targeting video media becomes a video element whose fallback incorporates the image alt text through unescaped string replacement. The html setting does not apply to plugin-generated output, allowing crafted alt text to reach the DOM as live HTML in contexts that do not use mdToHtmlNoImages. An approved member or a remote federated instance can store such content, and a viewer who renders it may execute JavaScript in the lemmy-ui origin, exposing the viewer's session and authenticated actions. The advisory notes that Content Security Policy prevents the described exploit in production, but also states that the tested default self-hosted deployment serves no Content-Security-Policy. This issue is fixed in lemmy-ui version 0.19.19-beta.1.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
在Web页面生成时对输入的转义处理不恰当(跨站脚本)
Source: CVE Program / CVE List V5
Vulnerability Title
LemmyNet lemmy 跨站脚本漏洞
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-beta.1之前版本存在跨站脚本漏洞，该漏洞源于Markdown渲染时未对插件生成的输出进行清理和转义，特制alt文本可被注入为实时HTML，可能导致跨站脚本攻击，暴露用户会话并执行经过身份验证的操作。
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-beta.1 -

II. Public POCs for CVE-2026-54743

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III. Intelligence Information for CVE-2026-54743

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Patches & Fixes for CVE-2026-54743 (1)

Vendor Advisories for CVE-2026-54743 (1)

Other References for CVE-2026-54743 (1)

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

CVE-2026-54739 6.9 MEDIUM Lemmy: Login Endpoint User Enumeration via HTTP Response Code Differential
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-2026-54741 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-79

V. Comments for CVE-2026-54743

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