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

Source: https://junglewise.ai/threats/cve-2026-100646-siyuan-is-a-self-hosted-personal-knowledge-management-system-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-26. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/siyuan). Vendors: [SiYuan](https://junglewise.ai/threats/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

- [https://github.com/siyuan-note/siyuan/security/advisories/GHSA-2w6q-wgc8-q743](https://github.com/siyuan-note/siyuan/security/advisories/GHSA-2w6q-wgc8-q743)
- [https://www.vulncheck.com/advisories/siyuan-before-3.8.4-authentication-bypass-via-missing-origin-header](https://www.vulncheck.com/advisories/siyuan-before-3.8.4-authentication-bypass-via-missing-origin-header)

## Related threats

- [SiYuan path traversal in web fetch](https://junglewise.ai/threats/cve-2026-126776-siyuan-3-7-0-before-3-8-10-on-windows-contains-a-path-traversal)
- [SiYuan publish filter information disclosure in local storage](https://junglewise.ai/threats/go-2026-6836-siyuan-the-publish-filter-for-local-storage-returns-the-7f022e62)
- [SiYuan path traversal via GLOBALROOT alias bypass](https://junglewise.ai/threats/cve-2026-118680-siyuan-3-8-7-through-3-8-8-fails-to-normalize-windows-globalroot)
- [SiYuan SQL injection in history search](https://junglewise.ai/threats/cve-2026-116170-siyuan-before-v3-8-7-contains-an-sql-injection-vulnerability)
- [SiYuan cross-site scripting in /check-auth redirect](https://junglewise.ai/threats/cve-2026-116167-siyuan-from-2-9-0-before-3-8-7-contains-a-cross-site-scripting)