CVE-2026-54743

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.

• CWE-79

• https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff
• https://github.com/LemmyNet/lemmy-ui/pull/4213
• https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1
• https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj
• https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj

Published

Aug 19, 2026

Last Modified

Sep 9, 2026

Status

Deferred

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