CVE-2026-16387
published 2026-07-21CVE-2026-16387: Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.
PriorityP353critical9.8CVSS 3.1
AVNACLPRNUINSUCHIHAH
EPSS
0.20%
10.6th percentile
Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153, Firefox ESR 140.13, Thunderbird 153, and Thunderbird 140.13.
Affected
12 ranges
| Vendor | Product | Version range | Fixed in |
|---|---|---|---|
| mozilla | firefox | < Firefox 153 | Firefox 153 |
| mozilla | firefox | < 140.13.0 | 140.13.0 |
| mozilla | firefox | < 153.0.0 | 153.0.0 |
| mozilla | firefox | — | — |
| mozilla | firefox_esr | < Firefox ESR 140.13 | Firefox ESR 140.13 |
| mozilla | thunderbird | < Thunderbird 140.13 | Thunderbird 140.13 |
| mozilla | thunderbird | < Thunderbird 153 | Thunderbird 153 |
| mozilla | thunderbird | < 140.13.0 | 140.13.0 |
| mozilla | thunderbird | — | — |
| mozilla | thunderbird | >= 141.0 < 153.0 | 153.0 |
| rhel10 | firefox-flatpak | — | — |
| rhel10 | thunderbird-flatpak | — | — |
CVSS provenance
nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
vendor_redhat9.8CRITICAL
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Red Hat
firefox: thunderbird: Site isolation issue in the Networking component
vendor_redhat·2026-07-21·CVSS 9.8
CVE-2026-16387 [CRITICAL] CWE-501 firefox: thunderbird: Site isolation issue in the Networking component
firefox: thunderbird: Site isolation issue in the Networking component
A flaw was found in Firefox and Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue:
Site isolation issue in the Networking component
Statement: Red Hat Product Security rates the severity of this flaw as determined by the Mozilla Foundation Security Advisory.
Package: firefox (Red Hat Enterprise Linux 10) - Affected
Package: rhel10/firefox-flatpak (Red Hat Enterprise Linux 10) - Affected
Package: rhel10/thunderbird-flatpak (Red Hat Enterprise Linux 10) - Affected
Package: thunderbird (Red Hat Enterprise Linux 10) - Affected
Package: firefox (Red Hat Enterprise Linux 6) - Out of support scope
Package: thunderbird (Red Hat Enterprise Linux 6) - Out of support scope
Package: fire
Mozilla
Mozilla Foundation Security Advisory 2026-70: CVE-2026-16387
vendor_mozilla
CVE-2026-16387 Mozilla Foundation Security Advisory 2026-70: CVE-2026-16387
Mozilla Foundation Security Advisory 2026-70
CVE: CVE-2026-16387
Product: Firefox ESR
Impact: high
Fixed in: Firefox ESR 140.13
Mozilla
Mozilla Foundation Security Advisory 2026-72: CVE-2026-16387
vendor_mozilla
CVE-2026-16387 Mozilla Foundation Security Advisory 2026-72: CVE-2026-16387
Mozilla Foundation Security Advisory 2026-72
CVE: CVE-2026-16387
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 140.13
Mozilla
Mozilla Foundation Security Advisory 2026-68: CVE-2026-16387
vendor_mozilla
CVE-2026-16387 Mozilla Foundation Security Advisory 2026-68: CVE-2026-16387
Mozilla Foundation Security Advisory 2026-68
CVE: CVE-2026-16387
Product: Firefox
Impact: high
Fixed in: Firefox 153
Mozilla
Mozilla Foundation Security Advisory 2026-71: CVE-2026-16387
vendor_mozilla
CVE-2026-16387 Mozilla Foundation Security Advisory 2026-71: CVE-2026-16387
Mozilla Foundation Security Advisory 2026-71
CVE: CVE-2026-16387
Product: Thunderbird
Impact: high
Fixed in: Thunderbird 153
GHSA
Site isolation issue in the Networking component.
ghsa_unreviewed·2026-07-21
CVE-2026-16387 [CRITICAL] CWE-200 Site isolation issue in the Networking component.
Site isolation issue in the Networking component. This vulnerability was fixed in Firefox 153 and Firefox ESR 140.13.
No detection rules found.
No public exploits indexed.
2026-07-21
Published