CVE-2026-54743
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 [email protected] 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.
How to fix
Sources list no fixed version or patch link for this vulnerability.
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| LemmyNet lemmy | < 0.19.19-beta.1 | — |
Check your version
Choose the product and enter the version you run to see whether it falls in the affected range.
Attack Surface Management
See the assets tied to your domains, the technologies they run and known vulnerabilities in one list.