CVE-2026-100646

SiYuan before v3.8.4 Authentication Bypass via Missing Origin Header

HIGH

8.1

CVSS Score

Published: September 26, 2026 at 01:23 PM UTC

Updated: September 30, 2026 at 03:21 PM UTC

2 products affected across 2 vendorsSiyuanSiyuan

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Severity

HIGH8.1

Exploit prediction

EPSS 0.2%

SSVC

PoC

Description

SiYuan is a self-hosted personal knowledge management system. In versions up to and including 3.8.3, the kernel's authentication guards (CheckAuth in kernel/model/session.go and IsSessionOriginAllowed in kernel/util/net.go) fail open when the HTTP Origin header is absent, on the incorrect assumption that any browser-initiated cross-site request carries an Origin. Because browsers omit Origin on cross-site top-level GET navigations and no-cors GET subresource loads — and the session cookie is SameSite=Lax — a single cross-site GET issued from any attacker-controlled web page is granted RoleAdministrator, both on default installations with no access-authorization code and on password-protected instances with a live session. Combined with content-type sniffing on the /api/network/proxy endpoint, which allows attacker-controlled HTML to be served under SiYuan's own origin, this permits an unauthenticated remote attacker to execute arbitrary script in the SiYuan origin (http://127.0.0.1:6806), invoke administrator APIs, and exfiltrate the persistent kernel API token. This issue is fixed in version 3.8.4.