CVE-2026-66012: SiYuan before v3.7.2 Unauthenticated Administrator Takeover via MCP

What it is

SiYuan before v3.7.2 contains a missing authorization vulnerability in the POST /mcp kernel endpoint, which is gated only by a general auth check (model.CheckAuth) with no admin-role or read-only enforcement. This exposes 31 MCP tools, including a file tool with list/read/write/delete/rename/copy actions across the entire workspace. When the Publish server is enabled in anonymous mode (Conf.Publish.Enable=true and Conf.Publish.Auth.Enable=false), the Publish reverse proxy attaches an anonymous RoleReader JWT to proxied requests, allowing a remote unauthenticated attacker to reach /mcp. The attacker can read conf/conf.json to extract accessAuthCode, api.token, and cookieKey in plaintext, write arbitrary files in the workspace, and plant a plugin into data/plugins/ that executes with nodeIntegration:true and no contextIsolation on the next desktop launch, leading to administrator takeover.

Description quoted from the CVE record (assigned by VulnCheck, published 2026-07-25).

Weakness: Missing Authorization

How it can be exploited

Exploitable over the network with low attack complexity with no special attack requirements needing no privileges and no user interaction. Impact: confidentiality high, integrity high, availability high. 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

CISA assessment (SSVC): CISA assesses that a public proof of concept exists. Automatable: yes. Technical impact: total.

Public exploit evidence (Exploit Observer)

Exploit Observer found 6 code repositories naming this CVE.

Code repositories that name a CVE are often scanners, copies or fakes, and some are malware. Read the code before running anything.

See all 11 references on Exploit Observer

Live scores

EPSS

0.008
low predicted chance of attack

VEDAS

0.987
strong exploit evidence

On CISA KEV

Not listed

Detection rule

Nuclei · Suricata

Who is affected

Version ranges exactly as the CVE record states them. “Before X” means X is the first fixed version.

How to fix it

Fixed versions stated in the CVE record: 3.7.2.

Vendor guidance:

• GitHub Security Advisory (GHSA-cvhv-7xhj-xjp8) (vendor-advisory)
• https://github.com/siyuan-note/siyuan/commit/c72ca4cd09019e5f64afdee8f8c6ec5ef34858db (patch)

How to detect it

Nuclei template (active check)

SiYuan - Missing Authorization in MCP Kernel Endpoint

SiYuan before 3.7.2 exposes the POST /mcp kernel endpoint protected only by a general auth check with no admin-role or read-only enforcement. When the Publish server runs in anonymous mode the endpoint becomes reachable unauthenticated, exposing 31 MCP tools including a file tool able to list, read, write, delete, rename and copy workspace files, leading to administrator takeover. This template confirms the flaw non-destructively by performing an unauthenticated MCP initialize handshake against the endpoint.

Name and description quoted from the template, written by VEDAS.

The template sends 1 request (POST /mcp); it decides using status, word matchers. Severity in the template: critical. Marked as not yet verified against a live vulnerable target.

Run it against a host you own or are authorised to test:

nuclei -u https://your-host -t CVE-2026-66012.yaml

id: CVE-2026-66012

info:
  name: "SiYuan - Missing Authorization in MCP Kernel Endpoint"
  author: vedas-arpsyndicate
  severity: critical
  description: |
    SiYuan before 3.7.2 exposes the POST /mcp kernel endpoint protected only by a general auth
    check with no admin-role or read-only enforcement. When the Publish server runs in anonymous
    mode the endpoint becomes reachable unauthenticated, exposing 31 MCP tools including a file
    tool able to list, read, write, delete, rename and copy workspace files, leading to
    administrator takeover. This template confirms the flaw non-destructively by performing an
    unauthenticated MCP initialize handshake against the endpoint.
  reference:
    - https://www.exploit.observer/?keyword=CVE-2026-66012&match=exact
    - https://subdomain.center/?engine=ammonites&keyword=siyuan&match=exact
  classification:
    cvss-metrics: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
    cvss-score: 10.0
    cve-id: CVE-2026-66012
    cwe-id: CWE-862
  metadata:
    verified: false
    max-request: 1
  tags: cve,cve2026,siyuan,mcp,missing-auth,unauth
http:
  - raw:
      - |
        POST /mcp HTTP/1.1
        Host: {{Hostname}}
        Content-Type: application/json
        Accept: application/json, text/event-stream

        {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"vedas","version":"1"}}}

    matchers-condition: and
    matchers:
      - type: word
        part: body
        words:
          - "jsonrpc"
          - "protocolVersion"
          - "result"
        condition: and
      - type: status
        status:
          - 200

CVE-2026-66012.yaml on GitHub

Suricata rules (network detection)

1 rule that alert on traffic matching this exploit: VEDAS CVE-2026-66012 SiYuan - Missing Authorization MCP Tool Invocation Attempt. Add them to your Suricata rule set and reload.

alert http $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"VEDAS CVE-2026-66012 SiYuan - Missing Authorization MCP Tool Invocation Attempt"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"/mcp"; nocase; http.request_body; content:"tools/call"; nocase; fast_pattern; content:"file"; nocase; classtype:web-application-attack; reference:cve,CVE-2026-66012; reference:url,https://www.exploit.observer/?keyword=CVE-2026-66012&match=exact; reference:url,https://subdomain.center/?engine=ammonites&keyword=siyuan&match=exact; sid:1080801; rev:1;)

CVE-2026-66012.rules on GitHub

References

Vendor advisories and patches

• GitHub Security Advisory (GHSA-cvhv-7xhj-xjp8) (vendor-advisory)
• https://github.com/siyuan-note/siyuan/commit/c72ca4cd09019e5f64afdee8f8c6ec5ef34858db (patch)

Technical analysis and advisories

• VulnCheck Advisory: SiYuan before v3.7.2 Unauthenticated Administrator Takeover via MCP (third-party-advisory)

Other databases

• CVE record (cve.org)
• NVD
• Exploit Observer

Where this comes from

This advisory is generated automatically by VEDAS from: CVE.org record, cve-scores (EPSS and VEDAS), Exploit Observer, vedas-signatures. Text in quotes is reproduced from the source; everything else is a fixed template filled with source fields. Nothing here is written by a language model. Check the vendor advisory before acting, and read how VEDAS and Exploit Observer work, and their limits.