External risk intelligence

SiYuan Desktop Bazaar Plugin Reflected XSS to RCE via Siyuan Protocol

CVE advisorySeverity: CRITICAL (CVSS 9.4)

CVE-2026-66395

This is a desktop application vulnerability requiring user interaction with a specific deep link handler. It is primarily a client-side issue residing on an individual user's machine, not an internet-facing service, API, or network appliance.

Cross-site Scripting

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory concerns a vulnerability in the SiYuan desktop application that could allow an attacker to execute arbitrary code. The issue stems from a flaw in how the application handles certain links, potentially enabling malicious code injection when a user interacts with a specially crafted link. The primary concern is to confirm if this specific application is used and if any exposure exists.

Flaw in link handling can lead to code execution.
Confirms if this specific software is in use.
Assess relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending a specially crafted `siyuan://` deep link to a user, likely through a social engineering tactic. If the user clicks this link, the application's bazaar plugin readme handler would process it insecurely, allowing the attacker to inject malicious HTML. This could lead to the execution of arbitrary code on the user's machine with Node.js privileges.

User interaction with a malicious link.
Crafted `siyuan://` deep link.
Arbitrary code execution.

Live Threat

Current exploitation, exposure, and threat context

A reflected cross-site scripting vulnerability in the bazaar plugin readme handler could allow attackers to execute arbitrary code by crafting a malicious siyuan:// deep link. This could occur when a user clicks such a link, leading to the injection of HTML payloads via the plugin name parameter, which may then execute with full Node.js access in an insecurely configured Electron renderer.

System data could be compromised.
Malicious deep links could be delivered.
Arbitrary code execution may occur.

Operational Fix

Recommended remediation, mitigation, and detection steps

Real-world action for this vulnerability will likely involve desktop application support or platform teams responsible for managing user-installed software. The first step is to confirm the presence of the affected application on user workstations, assess potential exposure through malicious links, and identify the specific users or teams accountable for remediation.

Identify application owners and user impact.
Verify bazaar plugin readme handler exposure.
Plan user-level remediation and vendor coordination.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

Free EASM Trial See How Signal Works

Frequently asked questions

What is SiYuan desktop?

SiYuan is a privacy-first, local-first personal knowledge management application that functions as a desktop note-taking tool. It uses Electron to run web-based technologies locally, allowing users to organize information, manage local files, and extend functionality through a plugin system called the Bazaar.

What does CVE-2026-66395 mean?

This CVE describes a Reflected Cross-Site Scripting (XSS) vulnerability, classified as CWE-79. In this specific case, the software fails to properly sanitize input in its Bazaar plugin handler. Because the application runs on Electron with elevated Node.js privileges, this weakness allows an attacker to elevate a simple script injection into the execution of arbitrary code on the user's system.

How is this vulnerability triggered?

An attacker must convince a user to interact with a specially crafted 'siyuan://' deep link, often through social engineering. The bug is triggered when the application's readme handler processes this link and injects the malicious payload. Simply having the application installed is not enough; the attack requires the active participation of the user clicking the link for the code to execute.

Is my organization at risk according to Halo Surface Signal?

Halo Surface Signal indicates this is very unlikely to pose a traditional network-level risk. Because the vulnerability exists within a desktop application and requires user interaction, it is a client-side issue rather than an internet-facing service. The primary threat is targeted toward individual workstations where users might click malicious links, rather than a broad exposure of network infrastructure.

How do I respond to this threat?

Start by identifying all workstations where SiYuan is installed. Once you have an inventory of affected users, check the application version to see if it is older than 3.7.2. Coordinate with your team to ensure the software is updated, and educate users on the risks of clicking untrusted deep links, especially those using non-standard protocol handlers like 'siyuan://'.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.