SiYuan Affected by Zero-Click NTLM Hash Theft and Blind SSRF via Mermaid Diagram Rendering

The primary impact of CVE-2026-40107 is the potential for unauthorized access to internal resources and information disclosure. An attacker can embed a malicious Mermaid diagram within a SiYuan note. When a victim opens this note, the application will attempt to fetch the URL specified in the injected SVG <img> tag. On Windows systems, a protocol-relative URL (e.g., //attacker.com/image.png) is resolved as a UNC path (\\attacker.com\image.png). This triggers an SMB connection attempt, automatically sending the victim's NTLMv2 hash to the attacker's server. This can be used for credential theft and subsequent lateral movement within the network. The blast radius extends to any user of the SiYuan application who opens a malicious note, particularly those on Windows systems.

Users of SiYuan who rely on Mermaid diagrams for note-taking and visualization are at risk. Environments with shared hosting or legacy Windows systems with weak SMB configurations are particularly vulnerable, as the automatic NTLMv2 hash transmission poses a significant threat.

• windows / supply-chain: Monitor PowerShell execution for suspicious URLs or SMB connections initiated by the SiYuan process. Use Windows Defender to search for alerts related to network connections to unusual domains or UNC paths.

Get-Process -Name SiYuan | Select-Object -ExpandProperty CommandLine | Select-String -Pattern '\\attacker.com'

• linux / server: Examine SiYuan's process logs for any unexpected outbound network connections. Use journalctl to filter for errors or warnings related to Mermaid.js or URL fetching.

journalctl -u siyuan -g 'error' -g 'warning'

• generic web: Monitor access logs for requests originating from the SiYuan client to unusual or attacker-controlled domains. Check response headers for unexpected content or redirects. Use curl to test for endpoint exposure.

curl -I https://attacker.com/image.png

1. 2026-04-10Disclosure

   disclosure

1. Reserviert2026-04-09
2. Veröffentlicht2026-04-10
3. EPSS aktualisiert2026-04-17

The primary mitigation for CVE-2026-40107 is to upgrade SiYuan Kernel to version 0.0.0-20260407035653-2f416e5253f1 or later. This version includes a fix that properly sanitizes the SVG content, preventing the injection of malicious URLs. As a temporary workaround, consider disabling Mermaid.js support within SiYuan if upgrading is not immediately possible. While a WAF or proxy cannot directly prevent this vulnerability, they can be configured to block outbound SMB connections to suspicious UNC paths. Monitor network traffic for unusual SMB connections originating from SiYuan instances.