Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed
CVSS score
6.4
EPSS probability
%0.5
Risk score
0.0
Published
2 mo ago
Resmî kaynaklarda yeterli kanıt bulunamadı. Bu sonuç “yama yok” anlamına gelmez.
Impact on My Inventory
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Vulnerability description
DayBreach CVE AI Research
CVE-2026-54743 has vendor, distribution, patch and security-source records on file. We can compare them and prepare an explanation linked to the evidence.
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.
IMPACT: Affected Products and Software
Affected Products
The following products are affected by CVE-2026-54743 vulnerability. Where our threat engine knows the exact affected versions, they are listed below.
| ID | VENDOR | PRODUCT | ACTION |
|---|---|---|---|
| 1 | LemmyNet | lemmy | View |
RED HAT CSAF/VEX: Product Impact Check
SCORING: CVSS from Multiple Sources
CVSS Scores
The Common Vulnerability Scoring System is a standardised framework for assessing the severity of vulnerabilities in software and systems. We collect and display CVSS scores from several sources for each CVE.
| SCORE | VERSION | SEVERITY | VECTOR | SOURCE |
|---|---|---|---|---|
| 6.4 | CVSS V4_0 | MEDIUM | 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 | CVE CNA |
| 6.4 | CVSS V40 | MEDIUM | 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 | NVD |
Patch and Remediation Guide
Solution & Remediation Advisory
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Public exploit and active exploitation status
Verified exploit intelligence
No verified public PoC, exploit repository or Metasploit module has been found for this CVE yet.
Weakness Classification and Attack Patterns (CWE & CAPEC)
CWE - Common Weakness Enumeration
While CVE identifies specific instances of vulnerabilities, CWE categorises the common flaws or weaknesses that can lead to vulnerabilities. CVE-2026-54743 is associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2026-54743 weaknesses.
Attack Vector and Accessibility
Official Sources and Advisories
Similar Vulnerabilities
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