Is CVE-2026-100646 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-100646 being exploited in the wild.

How severe is CVE-2026-100646?

High severity, with a CVSS score of 8.1, published 26 September 2026.

Which products and versions are affected?

SiYuan up to and including 3.8.3.

Is there a fix for CVE-2026-100646?

A patch was published on 2026-09-26: Version 3.8.4 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-100646 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-100646?

Check whether you run SiYuan at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-100646: SiYuan authentication bypass via missing Origin header

CVE-2026-100646 · Severity: high · CVSS 8.1 · Published 2026-09-26

SiYuan is a self-hosted note-taking and knowledge management system. Versions up to 3.8.3 have a critical authentication flaw where missing HTTP Origin headers are treated as trusted, combined with improper content-type handling. An attacker can craft a single cross-site request from any web page to gain administrator access to a SiYuan instance, steal API tokens, and execute arbitrary code within the system.

Technologies: SiYuan. Vendors: SiYuan.

Quick answers

Is CVE-2026-100646 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-100646 being exploited in the wild.
How severe is CVE-2026-100646?: High severity, with a CVSS score of 8.1, published 26 September 2026.
Which products and versions are affected?: SiYuan up to and including 3.8.3.
Is there a fix for CVE-2026-100646?: A patch was published on 2026-09-26: Version 3.8.4 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-100646 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-100646?: Check whether you run SiYuan at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability stems from two authentication guards (CheckAuth and IsSessionOriginAllowed) that incorrectly treat an absent Origin header as trustworthy, based on the false assumption that all browser cross-site requests include Origin. Combined with content-type sniffing on the /api/network/proxy endpoint that allows HTML to be served under SiYuan's own origin without X-Content-Type-Options headers, an unauthenticated attacker can issue a cross-site GET request lacking Origin to gain RoleAdministrator, then serve malicious HTML via the proxy endpoint to achieve same-origin JavaScript execution and exfiltrate the kernel API token. The attack works against both unprotected and password-protected instances with active sessions due to the SameSite=Lax cookie policy.

Affected products

SiYuan SiYuan up to and including 3.8.3

Timeline

2026-09-26: disclosed
2026-09-26: patched: Version 3.8.4 released with fix

References

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