CVE-2026-54743: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in LemmyNet lemmy
Description
CVE-2026-54743 is a cross-site scripting (XSS) vulnerability in LemmyNet's lemmy-ui prior to version 0.19.19-beta.1. The issue arises from improper sanitization of Markdown-rendered content, specifically when image alt text is used in video media fallback elements. This allows an approved member or remote federated instance to inject malicious JavaScript that executes in the viewer's browser context. The vulnerability is mitigated in production by Content Security Policy (CSP), but default self-hosted deployments lack CSP, making them vulnerable. The flaw is fixed in lemmy-ui version 0.19.19-beta.1.
CVSS v4.0
Score 6.4medium
Affected software
LemmyNet
lemmy
pkg:github/lemmynet/lemmy-uiRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
LemmyNet's lemmy-ui renders Markdown content using mdToHtml, which returns raw HTML injected without sanitization. Although the html setting disables HTML rendering, the markdown-it-html5-embed plugin with useImageSyntax enabled allows crafted image alt text to be injected as live HTML in video fallback elements. This enables cross-site scripting when such content is viewed, potentially exposing user sessions and authenticated actions. The vulnerability affects versions prior to 0.19.19-beta.1 and is resolved in that version. Content Security Policy can prevent exploitation, but default self-hosted instances do not enforce CSP.
Potential Impact
An attacker with approved membership or from a remote federated instance can store malicious content that executes JavaScript in the context of the lemmy-ui origin when viewed. This can lead to session exposure and unauthorized authenticated actions by the victim. The vulnerability is medium severity with a CVSS 4.0 score of 6.4. Production deployments with CSP are protected, but default self-hosted deployments without CSP are vulnerable.
Mitigation Recommendations
Upgrade lemmy-ui to version 0.19.19-beta.1 or later to apply the official fix. Additionally, deploying a strict Content Security Policy can prevent exploitation by restricting script execution. For existing deployments, enabling CSP is strongly recommended. No other specific mitigations are noted.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-06-15T23:12:41.964Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6a86e0f3acd9273b4983851c
Added to database: 08/20/2026, 11:11:47 UTC
Last enriched: 09/11/2026, 05:17:37 UTC
Last updated: 10/05/2026, 06:48:17 UTC
Views: 48
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.