{"id":"CVE-2026-54743","title":"Lemmy is a link aggregator and forum for the fediverse","summary":"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 thr…","severity":"none","cwe":["CWE-79"],"published":"2026-08-19","updated":"2026-09-09","sourceUpdated":"2026-09-09T21:15:59.613","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-54743","references":[{"url":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","label":"security-advisories@github.com"},{"url":"https://github.com/LemmyNet/lemmy-ui/pull/4213","label":"security-advisories@github.com"},{"url":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","label":"security-advisories@github.com"},{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","label":"security-advisories@github.com"},{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","label":"134c704f-9b21-4f2e-91b3-4a467353bcc0"}],"tags":["nvd"],"epss":0.00509,"epssPercentile":0.42487,"ingestedAt":"2026-09-09T21:22:45.544Z","slug":"CVE-2026-54743","body":"## Overview\n\nLemmy 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":3,"depthScoreParts":{"impact":2.8,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}