CVE-2026-100646

HIGHPre-NVD 8.18.1—
EchelonGraph scoreHIGH confidence

Score 8.1 from GitHub Security Advisory (severity: HIGH) published 2026-09-26. A secondary CVSS source baseline 8.1; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 3 sources updated this weekElevated
8.1EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0.2%CVSS: 8.1Exploit: Elevated riskExposed services: Not assessed

A fix is available — apply it.

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.

CVSS v3
8.1
EG Score
8.1HIGHhigh confidence
EG Risk
52
EG Risk 52/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity81% × 45%
Exploitation40% × 40%
Automatability0% × 15%
CISA SSVC: Track at low, Track* at medium and Attend at high mission impact.
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
0.2%
EPSS %ILE
8th
KEV
Not listed

CISA SSVCTrack at low, Track* at medium and Attend at high mission impact.

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation public PoC (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact total (CISA Vulnrichment). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

September 26, 2026

Last Modified

September 30, 2026

Advisory Details (2)

Auto-updated Sep 26, 2026
🔬 Proof of concept available. Patch available. Sources: github.
generic Patch Available

SiYuan before v3.8.4 Authentication Bypass via Missing Origin Header | Advisories | VulnCheck

https://www.vulncheck.com/advisories/siyuan-before-3.8.4-authentication-bypass-via-missing-origin-header
github Patch Available🟡 PoC Available

Missing-Origin fail-open in `CheckAuth` / `IsSessionOriginAllowed`, composed with content-type sniffing on `/api/network/proxy`: any web page obtains administrator API access and same-origin script execution in SiYuan · Advisory · siyuan-note/siyuan · GitHub

https://github.com/siyuan-note/siyuan/security/advisories/GHSA-2w6q-wgc8-q743

Vendor Advisories for CVE-2026-100646(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 30× in last 7d / 38× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-10-05 11:48 UTCEG score recompute
  2. 2026-10-05 11:48 UTCGHSA enrichment
  3. 2026-10-04 23:21 UTCEPSS rescore
  4. 2026-10-04 22:54 UTCGHSA enrichment
  5. 2026-10-04 10:00 UTCGHSA enrichment
  6. 2026-10-03 21:06 UTCEG score recompute
  7. 2026-10-03 21:06 UTCGHSA enrichment
  8. 2026-10-03 14:25 UTCEPSS rescore
  9. 2026-10-03 08:12 UTCGHSA enrichment
  10. 2026-10-02 19:16 UTCEG score recompute
  11. 2026-10-02 19:16 UTCGHSA enrichment
  12. 2026-10-02 06:22 UTCEG score recompute
  13. 2026-10-02 06:22 UTCGHSA enrichment
  14. 2026-10-01 19:49 UTCEPSS rescore
  15. 2026-10-01 17:28 UTCGHSA enrichment
  16. 2026-10-01 04:34 UTCGHSA enrichment
  17. 2026-09-30 15:40 UTCEG score recompute
  18. 2026-09-30 15:39 UTCGHSA enrichment
  19. 2026-09-30 15:02 UTCEPSS rescore
  20. 2026-09-30 14:53 UTCEG score recompute
  21. 2026-09-30 14:53 UTCGHSA enrichment
  22. 2026-09-30 08:59 UTCGHSA enrichment
  23. 2026-09-29 20:05 UTCEG score recompute
  24. 2026-09-29 20:05 UTCGHSA enrichment
  25. 2026-09-29 15:20 UTCEPSS rescore
Show 13 more
  1. 2026-09-29 07:11 UTCEG score recompute
  2. 2026-09-29 07:11 UTCGHSA enrichment
  3. 2026-09-28 18:14 UTCEG score recompute
  4. 2026-09-28 18:13 UTCGHSA enrichment
  5. 2026-09-28 13:50 UTCEPSS rescore
  6. 2026-09-28 05:17 UTCGHSA enrichment
  7. 2026-09-27 16:23 UTCEG score recompute
  8. 2026-09-27 16:23 UTCGHSA enrichment
  9. 2026-09-27 03:29 UTCEG score recompute
  10. 2026-09-27 03:29 UTCGHSA enrichment
  11. 2026-09-26 14:34 UTCEG score recompute
  12. 2026-09-26 13:38 UTCEG score recompute
  13. 2026-09-26 13:30 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-100646?
CVE-2026-100646 is a high vulnerability published on September 26, 2026. 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…
When was CVE-2026-100646 disclosed?
CVE-2026-100646 was first published on September 26, 2026, with the most recent update on September 30, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-100646 actively exploited?
CVE-2026-100646 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.2% probability of exploitation in the next 30 days (8th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-100646?
CVE-2026-100646 has a CVSS base score of 8.1 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-100646?
A fix for CVE-2026-100646 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-100646 are linked in the Vendor Advisories panel on this page.

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