External risk intelligence

SiYuan Stored Cross-Site Scripting via Asset Upload

CVE advisorySeverity: CRITICAL (CVSS 9.4)

CVE-2026-74800

SiYuan is primarily a local-first note-taking application. While it can be deployed in a server/multi-user configuration, it is generally used as a personal tool or within restricted internal environments, making public internet exposure of the asset management interface uncommon.

Cross-site Scripting

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in SiYuan software could allow unauthorized script execution within a workspace if an attacker can upload and share a crafted HTML file that is then opened by a user. This flaw exists when the software does not properly set certain security headers for uploaded assets, potentially enabling attackers to execute malicious code with elevated privileges. The primary concern at this time is to confirm if this specific software is in use and if the affected functionality is exposed.

Flaw allows script execution via uploaded files.
Affects workspaces and potentially kernel APIs.
Confirm relevance and exposure within your environment.

Attack Path

How an attacker could exploit the issue

An attacker could begin by uploading a specially crafted HTML file as an asset within a SiYuan workspace. If this file is later accessed by another user, such as the workspace owner, the vulnerability could be triggered, leading to the execution of arbitrary scripts with elevated privileges.

Authenticated user can upload assets.
Arbitrary file assets are served without proper headers.
Stored XSS leading to full API access.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an authenticated attacker to execute arbitrary scripts within the workspace owner's browser when they open a specially crafted HTML asset. This could potentially lead to unauthorized access to system functions and data accessible through the kernel API, under conditions where the workspace owner views the malicious asset.

User-uploaded HTML assets.
Authenticated users upload malicious HTML.
Unauthorized access to system functions.

Operational Fix

Recommended remediation, mitigation, and detection steps

SiYuan's asset handling vulnerability requires careful triage by application owners and potentially infrastructure or security teams. The immediate first step is to locate all instances of SiYuan, confirm their exposure and business criticality, and identify the accountable owner for each. Remediation planning should then be prioritized based on this risk assessment.

Identify affected SiYuan instances.
Verify asset reachability and criticality.
Plan remediation based on owner input.

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.

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Frequently asked questions

What is SiYuan?

SiYuan is a local-first note-taking application designed to help users organize knowledge. While often used as a personal tool for individual note management, it supports server-based, multi-user deployments where workspaces can be shared or hosted in network environments.

What does CVE-2026-74800 mean for security?

This vulnerability is a Stored Cross-Site Scripting (XSS) flaw, categorized as CWE-79. It occurs because the software fails to provide security instructions (headers) telling browsers how to safely handle uploaded files. Consequently, the browser may treat a malicious uploaded HTML file as a trusted part of the application, allowing it to run unauthorized scripts.

How is this vulnerability triggered?

An attacker must first have authenticated access to a SiYuan workspace to upload a malicious HTML file as an asset. The vulnerability is only triggered when another user, such as a workspace owner, subsequently opens or views that specific file link. Simply having an account or browsing existing notes does not trigger the bug without that specific interaction.

Is my instance of SiYuan at risk?

Halo Surface Signal notes that SiYuan is typically used as a personal tool or within restricted internal networks, making public internet exposure of its asset management interface uncommon. You should evaluate if your deployment allows unauthorized users to access the platform, as the risk is highest where untrusted parties have authenticated access to workspaces.

What is the first step to address this?

Begin by creating an inventory of all SiYuan instances running in your environment to understand your footprint. Once located, verify which instances are accessible to others and confirm who manages those specific workspaces. This allows you to prioritize updates for any systems that handle shared content or multi-user access.

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.