{"apiVersion":"1.0","identifier":"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 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":"2026-08-19T21:16:58","lastModifiedAt":"2026-08-21T19:17:03","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-54743","cvssScore":6.4,"cvssVector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","epssProbability":0.00401,"riskScore":0.66,"affectedProduct":"lemmy-ui","affectedVersions":"<0.19.19-beta.1","vulnerabilityType":"Web app","operatingSystems":[],"links":{"self":"https://www.redsauce.net/api/cves/CVE-2026-54743","webPages":{"es":"https://www.redsauce.net/es/cves/CVE-2026-54743","en":"https://www.redsauce.net/en/cves/CVE-2026-54743","fr":"https://www.redsauce.net/fr/cves/CVE-2026-54743","pt":"https://www.redsauce.net/pt/cves/CVE-2026-54743","de":"https://www.redsauce.net/de/cves/CVE-2026-54743","sk":"https://www.redsauce.net/sk/cves/CVE-2026-54743","el":"https://www.redsauce.net/el/cves/CVE-2026-54743"},"source":"https://nvd.nist.gov/vuln/detail/CVE-2026-54743"}}