---
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: 0.00547
epssPercentile: 0.43477
ingestedAt: '2026-09-09T21:22:45.544Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.