CVE-2026-100646: SiYuan before v3.8.4 Authentication Bypass via Missing Origin Header
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
SiYuanto a version that resolves this vulnerability.Fixed in 3.8.4
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
SiYuan versions up to and including 3.8.3 are affected. Both default installations with no access-authorization code and password-protected instances with a live user session are exposed.
What does an attacker need to exploit it?
An unauthenticated remote attacker needs to induce a victim to load an attacker-controlled web page that issues a cross-site GET request. The attack relies on requests without an Origin header, including cross-site top-level GET navigations and no-cors GET subresource loads.
Does password protection prevent exploitation?
No. Password-protected instances remain vulnerable when there is a live session, because the SameSite=Lax session cookie is sent in the relevant cross-site GET scenario.
What is the impact of successful exploitation?
An attacker can execute arbitrary script under the SiYuan origin, invoke administrator APIs, and exfiltrate the persistent kernel API token. The described attack targets the SiYuan origin at http://127.0.0.1:6806.
What remediation is available?
Upgrade SiYuan to version 3.8.4, which fixes the issue.