Is CVE-2026-84140 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-84140 being exploited in the wild.

How severe is CVE-2026-84140?

Critical severity, with a CVSS score of 9.8, published 1 September 2026.

Which products and versions are affected?

Mozilla Firefox before 155; Mozilla Firefox ESR before 153.2; Mozilla Thunderbird before 155; Mozilla Thunderbird ESR before 153.2.

Is there a fix for CVE-2026-84140?

A patch was published on 2026-09-01: Fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-84140 attacked?

It is attacked over the network.

Am I affected by CVE-2026-84140?

Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-84140: Mozilla Firefox site isolation issue in DOM Navigation component

CVE-2026-84140 · Severity: critical · CVSS 9.8 · Published 2026-09-01

Firefox's DOM Navigation component contains a site isolation vulnerability that could allow an attacker to bypass security boundaries between websites. If exploited, this flaw could enable unauthorized access to sensitive data from other websites or actions performed on behalf of a user on another site. Mozilla has released patches in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2 to address this issue.

Technologies: Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird, Mozilla Thunderbird ESR. Vendors: Mozilla.

Quick answers

Is CVE-2026-84140 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-84140 being exploited in the wild.
How severe is CVE-2026-84140?: Critical severity, with a CVSS score of 9.8, published 1 September 2026.
Which products and versions are affected?: Mozilla Firefox before 155; Mozilla Firefox ESR before 153.2; Mozilla Thunderbird before 155; Mozilla Thunderbird ESR before 153.2.
Is there a fix for CVE-2026-84140?: A patch was published on 2026-09-01: Fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-84140 attacked?: It is attacked over the network.
Am I affected by CVE-2026-84140?: Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

CVE-2026-84140 is a site isolation issue in the DOM Navigation component of Firefox and related Mozilla products. Site isolation vulnerabilities undermine the browser's security model that separates web pages from different origins, potentially allowing cross-origin data access or script execution. The vulnerability affects Firefox versions prior to 155, Firefox ESR versions prior to 153.2, Thunderbird prior to 155, and Thunderbird ESR prior to 153.2. The issue was reported by Mohamed Mbarek and requires a network attack vector via malicious web content. Patches are available in the fixed versions listed above.

Affected products

Mozilla Firefox before 155
Mozilla Firefox ESR before 153.2
Mozilla Thunderbird before 155
Mozilla Thunderbird ESR before 153.2

Timeline

2026-09-01: disclosed: Mozilla Foundation Security Advisory MFSA2026-82
2026-09-01: patched: Fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2

References

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