---
title: "CVE-2026-100639: SiYuan - Stored XSS"
description: "SiYuan v3.8.3 contains a stored XSS caused by improper HTML-escaping of the data-subtype attribute in gutter-button markup from pasted Markdown with Kramdown IAL, letting attackers execute JavaScript with Node.js privileges in the Electron app, exploit requires victim to paste crafted Markdown and focus the gutter control."
url: https://panop.io/threat-heartbeat/CVE-2026-100639/
---

# CVE-2026-100639 SiYuan - Stored XSS

SiYuan v3.8.3 contains a stored XSS caused by improper HTML-escaping of the data-subtype attribute in gutter-button markup from pasted Markdown with Kramdown IAL, letting attackers execute JavaScript with Node.js privileges in the Electron app, exploit requires victim to paste crafted Markdown and focus the gutter control.

CVSS base score 8.8 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.47% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-100639)

## Details

Type

Stored XSS

Impact

Remote Code Execution

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Published

26 September 2026

Updated

15 October 2026

Weakness

[CWE-79](https://cwe.mitre.org/data/definitions/79.html)

## Vendor fix

Update to version 3.8.4 or later.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-100639](https://nvd.nist.gov/vuln/detail/CVE-2026-100639)
- [https://github.com/siyuan-note/siyuan/security/advisories/GHSA-2cmm-9x9j-rvff](https://github.com/siyuan-note/siyuan/security/advisories/GHSA-2cmm-9x9j-rvff)
- [https://www.vulncheck.com/advisories/siyuan-before-3.8.4-cross-site-scripting-via-kramdown-ial](https://www.vulncheck.com/advisories/siyuan-before-3.8.4-cross-site-scripting-via-kramdown-ial)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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