{"id":"CVE-2026-54743","state":"PUBLISHED","title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed","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.","publishedAt":"2026-08-19T20:18:43.124Z","updatedAt":"2026-08-21T18:17:01.041Z","severity":{"score":6.4,"label":"Medium","version":"4.0","vector":"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":"GitHub_M"},"assigner":"GitHub_M","products":["LemmyNet lemmy"],"affected":[{"vendor":"LemmyNet","product":"lemmy","versions":[{"version":"< 0.19.19-beta.1","status":"affected"}]}],"references":["https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","https://github.com/LemmyNet/lemmy-ui/pull/4213","https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1"],"weaknesses":["CWE-79"],"ingestedAt":"2026-10-29T20:12:24.861Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/54xxx/CVE-2026-54743.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-54743","state":"PUBLISHED","datePublished":"2026-08-19T20:18:43.124Z","dateUpdated":"2026-08-21T18:17:01.041Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-15T23:12:41.964Z"},"containers":{"cna":{"title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"LemmyNet","product":"lemmy","versions":[{"version":"< 0.19.19-beta.1","status":"affected"}]}],"references":[{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","name":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/LemmyNet/lemmy-ui/pull/4213","name":"https://github.com/LemmyNet/lemmy-ui/pull/4213","tags":["x_refsource_MISC"]},{"url":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","name":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","tags":["x_refsource_MISC"]},{"url":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","name":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"NONE","privilegesRequired":"NONE","userInteraction":"PASSIVE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","subAvailabilityImpact":"NONE","baseScore":6.4,"baseSeverity":"MEDIUM","vectorString":"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","version":"4.0"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-79","lang":"en","description":"CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-08-19T20:18:43.124Z"},"source":{"advisory":"GHSA-2g66-9fr3-ppwj","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","references":[{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","tags":["exploit"]}],"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-08-21T18:16:39.227018Z","id":"CVE-2026-54743","options":[{"Exploitation":"poc"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-08-21T18:17:01.041Z"}}]}}}