CVE-2026-12304: Same-origin policy bypass in the Networking: Cookies component

Q: How do I fix CVE-2026-12304?

firefox: upgrade to 140.12.0, 152.0.0; thunderbird: upgrade to 140.12.0, 152.0.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-12304 being exploited?

CVE-2026-12304 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-12304 affect?

CVE-2026-12304 affects firefox, thunderbird. Severity is critical (CVSS 9.1). Same-origin policy bypass in the Networking: Cookies component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

Q: How urgent is CVE-2026-12304?

Priority 39/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 9.1REMOTEUPGRADEUPGRADE

CVE-2026-12304 is a critical-severity flaw (CVSS 9.1) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Same-origin policy bypass in the Networking: Cookies component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 92%)
CISA exploited list	No
Weakness	CWE-346
Published	2026-06-16
Last modified	2026-10-26

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-16 13:16 UTCPublished CVE record published
2026-10-26 13:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 140.12.0 · < 152.0.0

Fixed in: 140.12.0, 152.0.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy firefox || rpm -q firefox || firefox --version

2. UPGRADE to 140.12.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade firefox

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade firefox

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

mozilla thunderbird UPGRADE

Affected: < 140.12.0 · < 152.0.0

Fixed in: 140.12.0, 152.0.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version

2. UPGRADE to 140.12.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade thunderbird

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade thunderbird

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-12304?

firefox: upgrade to 140.12.0, 152.0.0; thunderbird: upgrade to 140.12.0, 152.0.0. Exact commands for each OS are on this page.

Is CVE-2026-12304 being exploited?

CVE-2026-12304 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-12304 affect?

CVE-2026-12304 affects firefox, thunderbird. Severity is critical (CVSS 9.1). Same-origin policy bypass in the Networking: Cookies component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

How urgent is CVE-2026-12304?

Priority 39/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

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CVE-2026-12294 CRITICAL 9.6 UPGRADE UPGRADE Sandbox escape in the DOM: Workers component
CVE-2026-12315 CRITICAL 9.1 UPGRADE UPGRADE Mitigation bypass in the DOM: Security component

References

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