CVE-2026-100803

HIGHPre-NVD 8.18.1—
EchelonGraph scoreHIGH confidence

Score 8.1 from GitHub Security Advisory (severity: HIGH) published 2026-09-29. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 8.1; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: cisa-adp, epss, ghsa
Trending — 5 sources updated this week
8.1EG
EchelonGraph verdictPlan mitigationSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0.2%CVSS: 8.1Exploit: None knownExposed services: Not assessed

No fix is confirmed yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for the fix.

Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

CVSS v3
8.1
EG Score
8.1HIGHhigh confidence
EG Risk
37
EG Risk 37/100CISA 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%
Exploitation0% × 40%
Automatability0% × 15%
CISA SSVC: Track at low or medium mission impact; Track* at high (mission-essential systems).
Action: No fix is confirmed yet. Restrict network exposure of the affected system or apply the vendor's mitigation within your standard update timelines, and watch the vendor's advisory for the fix.
EPSS PROB
0.2%
EPSS %ILE
7th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track* at high (mission-essential systems).

No fix is confirmed yet. Restrict network exposure of the affected system or apply the vendor's mitigation within your standard update timelines, and watch the vendor's advisory for the fix.

Exploitation none (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 29, 2026

Last Modified

October 1, 2026

Advisory Details (8)

Auto-updated Oct 1, 2026
No patch confirmed yet.
generic

Security Vulnerabilities fixed in Firefox ESR 140.17 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-99/
generic

Security Vulnerabilities fixed in Firefox ESR 115.42 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-98/
generic

Security Vulnerabilities fixed in Firefox 157 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-97/
generic

Security Vulnerabilities fixed in Thunderbird 153.4 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-103/
generic

Security Vulnerabilities fixed in Thunderbird 140.17 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-102/
generic

Security Vulnerabilities fixed in Thunderbird 157 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-101/
generic

Security Vulnerabilities fixed in Firefox ESR 153.4 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-100/
generic

Access Denied

https://bugzilla.mozilla.org/show_bug.cgi?id=2057988

Vendor Advisories for CVE-2026-100803(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 25× in last 7d / 25× 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. 2026-10-04 10:37 UTCGHSA enrichment
  2. 2026-10-03 23:34 UTCEG score recompute
  3. 2026-10-03 23:34 UTCGHSA enrichment
  4. 2026-10-03 14:25 UTCEPSS rescore
  5. 2026-10-03 12:36 UTCGHSA enrichment
  6. 2026-10-03 01:37 UTCEG score recompute
  7. 2026-10-03 01:37 UTCGHSA enrichment
  8. 2026-10-02 14:38 UTCGHSA enrichment
  9. 2026-10-02 03:38 UTCEG score recompute
  10. 2026-10-02 03:38 UTCGHSA enrichment
  11. 2026-10-01 19:49 UTCEPSS rescore
  12. 2026-10-01 16:39 UTCGHSA enrichment
  13. 2026-10-01 15:26 UTCEG score recompute▲ 8.10
  14. 2026-10-01 15:26 UTCGHSA enrichment
  15. 2026-10-01 15:25 UTCMITRE cvelistV5CVSS v3 → 8.1 · severity → HIGH
  16. 2026-09-30 18:40 UTCGHSA enrichment
  17. 2026-09-30 18:24 UTCNVD update
  18. 2026-09-30 17:22 UTCEG score recompute
  19. 2026-09-30 17:22 UTCGHSA enrichment
  20. 2026-09-30 17:20 UTCMITRE cvelistV5
  21. 2026-09-30 15:02 UTCEPSS rescore
  22. 2026-09-29 22:03 UTCGHSA enrichment
  23. 2026-09-29 13:35 UTCNVD update
  24. 2026-09-29 12:49 UTCEG score recompute
  25. 2026-09-29 12:48 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-100803?
CVE-2026-100803 is a high vulnerability published on September 29, 2026. Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
When was CVE-2026-100803 disclosed?
CVE-2026-100803 was first published on September 29, 2026, with the most recent update on October 1, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-100803 actively exploited?
CVE-2026-100803 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.2% probability of exploitation in the next 30 days (7th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-100803?
CVE-2026-100803 has a CVSS v3.1 base score of 8.1 (CISA-ADP / Vulnrichment enrichment; NVD's own analysis pending).
How do I remediate CVE-2026-100803?
No fix for CVE-2026-100803 is confirmed yet. Until one is published, restrict network exposure of the affected system or apply the vendor's mitigation — for example, keep it off the internet or limit it to trusted networks — and watch the vendor's advisory for the fix. The vendor advisories EchelonGraph has for CVE-2026-100803 are linked in the Vendor Advisories panel on this page.

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