CVE-2026-16387

CRITICALPre-NVD 9.89.8—
EchelonGraph scoreHIGH confidence

Score 9.8 from GitHub Security Advisory (severity: CRITICAL) published 2026-07-21. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 9.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: cisa-adp, epss, ghsa
Trending — 5 sources updated this week
9.8EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 9.8Exploit: None knownExposed: 0

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

Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.

CVSS v3
9.8
EG Score
9.8(high)
EG Risk
59(Track)
EG Risk 59/100SSVC: Track

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
Severity98% × 45%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
11%
KEV
Not listed

Published

July 21, 2026

Last Modified

July 24, 2026

Advisory Details (5)

Auto-updated Jul 25, 2026
⚠️ Active exploitation confirmed. No patch confirmed yet.
generic🔴 Active Exploitation

Security Vulnerabilities fixed in Thunderbird 140.13 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-72/
generic

Security Vulnerabilities fixed in Thunderbird 153 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-71/
generic🔴 Active Exploitation

Security Vulnerabilities fixed in Firefox ESR 140.13 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-70/
generic

Security Vulnerabilities fixed in Firefox 153 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-68/
generic

Access Denied

https://bugzilla.mozilla.org/show_bug.cgi?id=2043200

Vendor Advisories for CVE-2026-16387(1)

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

Weakness Classification(3)

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

Data Freshness Timeline

(refreshed 33× in last 7d / 33× 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. 2026-07-28 06:22 UTCGHSA enrichment
  2. 2026-07-28 02:01 UTCGHSA enrichment
  3. 2026-07-27 21:41 UTCGHSA enrichment
  4. 2026-07-27 17:06 UTCEG score recompute
  5. 2026-07-27 17:05 UTCGHSA enrichment
  6. 2026-07-27 14:13 UTCEPSS rescore
  7. 2026-07-27 12:45 UTCGHSA enrichment
  8. 2026-07-27 08:25 UTCGHSA enrichment
  9. 2026-07-27 04:04 UTCGHSA enrichment
  10. 2026-07-26 23:41 UTCGHSA enrichment
  11. 2026-07-26 19:21 UTCGHSA enrichment
  12. 2026-07-26 15:01 UTCEG score recompute
  13. 2026-07-26 15:01 UTCGHSA enrichment
  14. 2026-07-26 14:54 UTCEPSS rescore
  15. 2026-07-26 14:54 UTCEPSS rescore
  16. 2026-07-26 10:42 UTCEG score recompute
  17. 2026-07-26 10:42 UTCGHSA enrichment
  18. 2026-07-26 04:22 UTCEG score recompute
  19. 2026-07-26 04:22 UTCGHSA enrichment
  20. 2026-07-25 14:17 UTCEPSS rescore
  21. 2026-07-25 14:17 UTCEPSS rescore
  22. 2026-07-23 14:18 UTCEPSS rescore
  23. 2026-07-23 14:18 UTCEPSS rescore
  24. 2026-07-23 03:05 UTCEG score recompute
  25. 2026-07-22 23:13 UTCEG score recompute
Show 8 more
  1. 2026-07-22 23:13 UTCGHSA enrichment
  2. 2026-07-22 17:37 UTCEG score recompute▲ 9.80
  3. 2026-07-22 17:36 UTCGHSA enrichment
  4. 2026-07-22 17:35 UTCMITRE cvelistV5CVSS v3 → 9.8 · severity → CRITICAL
  5. 2026-07-22 14:08 UTCEPSS rescore
  6. 2026-07-21 14:51 UTCNVD update
  7. 2026-07-21 12:48 UTCEG score recompute
  8. 2026-07-21 12:48 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-16387?
CVE-2026-16387 is a critical vulnerability published on July 21, 2026. Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.
When was CVE-2026-16387 disclosed?
CVE-2026-16387 was first published in the National Vulnerability Database on July 21, 2026, with the most recent update on July 24, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-16387 actively exploited?
CVE-2026-16387 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 89.4% of all scored CVEs.
What is the CVSS score of CVE-2026-16387?
CVE-2026-16387 has a CVSS v3.1 base score of 9.8 (CISA-ADP / Vulnrichment enrichment; NVD's own analysis pending).
How do I remediate CVE-2026-16387?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-16387, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2026-16387

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