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CVE-2026-66012 – Unauthenticated RCE via Missing Authorization in MCP Endpoint – SiYuan before v3…

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Summary

CVE-2026-66012 is a critical missing authorization vulnerability (CWE-862) in SiYuan, an open-source self-hosted personal knowledge management system, affecting all versions prior to v3.7.2. The POST /mcp kernel endpoint lacks the required CheckAdminRole and CheckReadonly middleware guards, exposing 31 Model Context Protocol (MCP) tools — including full workspace file operations — to unauthenticated remote attackers when the Publish server is running in anonymous mode. The vulnerability carries a CVSS v4.0 and CVSS v3.1 base score of 10.0 (Critical).

Technical details

  • Root cause: The POST /mcp route is protected only by the general model.CheckAuth check; it is missing both CheckAdminRole and CheckReadonly middleware gates. The MCP tool dispatcher also ignores caller role restrictions, and the file tool permits workspace-wide read/write/delete operations to non-administrator principals.
  • Trigger conditions: The Publish server must be enabled in anonymous mode (Conf.Publish.Enable=true and Conf.Publish.Auth.Enable=false). In this configuration, the Publish reverse proxy automatically attaches an anonymous RoleReader JWT to proxied requests, granting unauthenticated network access to the /mcp endpoint.
  • Attack vector: A remote, unauthenticated attacker sends a crafted POST /mcp request through the Publish server to invoke any of the 31 exposed MCP tools, including the file tool with list, read, write, delete, rename, and copy actions across the entire workspace.
  • Impact:
    • Credential theft: Reading conf/conf.json exposes accessAuthCode, api.token, and cookieKey in plaintext.
    • Arbitrary file write: An attacker can write, rename, copy, or delete any file within the workspace directory.
    • Remote code execution: Planting a malicious plugin in data/plugins/ causes it to execute with nodeIntegration:true and no contextIsolation on the next desktop client launch, resulting in full OS-level code execution and administrator takeover.
    • Data destruction: Arbitrary file deletion across the workspace.

Affected software

  • SiYuan all versions from initial release through v3.7.1 (inclusive)
  • Fixed in v3.7.2

Severity

  • CVSS v4.0: 10.0 (Critical) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
  • CVSS v3.1: 10.0 (Critical) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate — upgrade: Update SiYuan to v3.7.2 or later, which introduces proper CheckAdminRole and CheckReadonly enforcement on the /mcp endpoint.
  • If immediate patching is not possible — disable anonymous Publish mode: Set Conf.Publish.Auth.Enable=true to require authentication on the Publish server, or disable the Publish server entirely (Conf.Publish.Enable=false). Either configuration removes the unauthenticated path to the /mcp endpoint.
  • Network mitigation: Restrict external access to SiYuan’s kernel and Publish server ports (default 6806/6808) at the network perimeter until patching is complete.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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