SiYuan note-taking software prior to version 3.8.0 contains a vulnerability in its http_request MCP tool where secret placeholders are interpolated into the destination URL parameter. An attacker controlling an MCP client can exploit this by sending a request with a URL containing these placeholders, causing the software to send plaintext secret data to an attacker-controlled external server without any user confirmation or interaction.

The vulnerability enables attackers with MCP client access to exfiltrate stored secrets in plaintext to arbitrary external hosts. This compromises the confidentiality of sensitive information stored within SiYuan, potentially leading to data leakage. There is no indication of required user interaction or privilege escalation beyond MCP client privileges.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or workaround has been documented at this time. Users should monitor vendor communications for updates and avoid using untrusted MCP clients until a fix is available.