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CVE-2026-85581 – Unauthenticated Denial of Service – SiYuan before v3.8.2

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Summary

CVE-2026-85581 is an unauthenticated denial-of-service vulnerability affecting SiYuan Note, a self-hosted note-taking application. The flaw resides in the /api/system/uiproc endpoint, which accepts and retains attacker-supplied process identifiers without any authentication or size restriction. The issue carries a CVSS v4.0 base score of 8.7 (High) and has been fixed in SiYuan 3.8.2.

Technical details

  • Root cause: The /api/system/uiproc endpoint stores attacker-controlled, unique process identifiers server-side without enforcing any limit on the number of entries retained.
  • Trigger conditions: An attacker sends repeated requests to the endpoint, each with a unique identifier, causing the server to accumulate state indefinitely.
  • Attack vector: Network-based; the endpoint requires no authentication, so any remote, unauthenticated actor can reach it.
  • Impact: Repeated exploitation exhausts server process memory, degrading or crashing the SiYuan service and denying availability to legitimate users.

Affected software

  • SiYuan (b3log/siyuan-note) — all versions prior to 3.8.2
  • Fixed in: SiYuan 3.8.2 and later

Severity

  • CVSS v3.1: 7.5 (High) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  • CVSS v4.0: 8.7 (High) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade SiYuan to version 3.8.2 or later, which addresses the unbounded resource allocation in the /api/system/uiproc endpoint.
  • If immediate patching is not possible: Restrict network exposure of the SiYuan kernel/API port (do not expose it directly to the internet), place it behind an authenticated reverse proxy or VPN, and monitor for abnormal memory growth or repeated requests to /api/system/uiproc as an indicator of exploitation attempts.

References

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