# CVE-2026-100646: SiYuan before v3.8.4 Authentication Bypass via Missing Origin Header

> An unauthenticated remote attacker can bypass authentication via missing Origin headers to execute arbitrary script and access administrator APIs in SiYuan.

- **Severity:** High 8.6 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 26 Sep 2026
- **Updated:** 30 Sep 2026
- **Also known as:** GHSA-2w6q-wgc8-q743, GHSA-r2fg-xpcg-8v48

## What is CVE-2026-100646?

CVE-2026-100646 is a high-severity authentication bypass vulnerability in siyuan-note siyuan, published on 26 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-346 (Origin Validation Error)

## Which versions are affected by CVE-2026-100646?


## How do I fix CVE-2026-100646?

No fix has been published for CVE-2026-100646 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-100646 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-100646 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-100646 and fix it if it is affected.

What it is: CVE-2026-100646 is a high-severity authentication bypass vulnerability in siyuan-note siyuan, published on 26 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-100646 exists.

Affected product: siyuan-note siyuan. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-100646/ (Markdown: https://insights.sbomify.com/CVE-2026-100646/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-100646
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-100646
- OSV: https://osv.dev/vulnerability/CVE-2026-100646

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-100646/
Summaries are generated and may contain mistakes; the official description above is the record.