External risk intelligence

SiYuan CSS Injection Allows Remote Code Execution

CVE advisorySeverity: CRITICAL (CVSS 9.9)

CVE-2026-54067

This is a personal knowledge management system primarily used as a local desktop application. The vulnerability requires a malicious payload to be injected into a workspace file, which then executes on the user's local machine when synced and opened. It is not an internet-facing service, API, or gateway reachable from the public internet.

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

A critical vulnerability in SiYuan, a personal knowledge management system, allows attackers to execute arbitrary code on user devices by embedding malicious scripts in shared files. This issue, even in systems configured to block JavaScript, could lead to significant compromise if exploited through synced workspaces. The primary concern is to confirm if this specific technology is in use and assess any potential exposure.

Malicious code can run via shared files.
Affects users who disabled JavaScript execution.
Confirm relevance and assess exposure.

Attack Path

How an attacker could exploit the issue

An attacker with write access to a synced SiYuan workspace can plant a malicious CSS snippet. When any user's device syncs and renders this snippet, it breaks out of its intended tag and runs arbitrary JavaScript. On desktop builds, this JavaScript can then access system commands to achieve remote code execution.

Write access to synced workspace.
Rendering a malicious CSS snippet.
Arbitrary JavaScript execution and RCE.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, arbitrary JavaScript could execute within the SiYuan application. This could occur when a user opens a synced workspace containing a malicious CSS snippet, even if JavaScript execution is disabled by default.

Malicious JavaScript in CSS snippets.
Syncing malicious workspace files.
Host remote code execution possible.

Operational Fix

Recommended remediation, mitigation, and detection steps

SiYuan's vulnerability requires understanding who manages synced workspaces and the Electron desktop builds. Application owners or platform teams are likely responsible for identifying affected instances, especially those that sync workspaces and run on Electron. The first action should be to locate all SiYuan deployments, determine if they sync workspaces, and assess their criticality before planning remediation.

Workspace owners and platform teams.
Verify workspace sync and Electron usage.
Plan remediation based on exposure.

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?

SiYuan is an open-source personal knowledge management system designed for organizing notes and data. It functions primarily as a local desktop application built on Electron, allowing users to create and manage their own private knowledge bases. Users often utilize its synchronization features to share or backup their workspaces across multiple devices, which is the specific functionality involved in this security concern.

What does CVE-2026-54067 mean for SiYuan?

This vulnerability involves Improper Neutralization of Input during Web Page Generation, classified as CWE-79. In plain terms, the application fails to properly secure custom CSS snippets. An attacker can craft a snippet that breaks out of its container and executes unauthorized JavaScript. Because the application processes these snippets, this flaw bypasses intended security boundaries, including settings meant to disable JavaScript, to run arbitrary code.

How does the malicious code trigger in SiYuan?

The trigger requires a malicious CSS snippet to be present within a synced workspace repository. Once planted, the payload executes automatically when the application renders the snippet on any device that synchronizes and opens that workspace. Simply viewing or opening the affected workspace triggers the execution. This behavior occurs even if the user has manually disabled JavaScript, as the CSS rendering path ignores those restrictions.

Is my SiYuan instance at risk?

According to Halo Surface Signal, this is very unlikely to be an internet-facing risk. The software is primarily a local desktop tool. The threat is most relevant to users who utilize shared or synced workspaces where an attacker could gain write access to inject the malicious snippet. If you use SiYuan locally without syncing workspaces with untrusted parties, your risk is significantly lower compared to environments that share workspace files frequently.

What should I do if I use SiYuan?

The primary step is to update your SiYuan installation to version 3.7.0 or later, which contains the fix for this issue. Before updating, identify all devices running the application and verify if they are using workspace synchronization. If you participate in shared workspaces, ensure all collaborators are also running updated versions of the software to prevent the propagation of malicious snippets across your synced repositories.

References

github.com/siyuan-note/siyuan/security/advisories/GHSA-mvjr-vv3c…

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.