CVE-2026-16387 Site isolation issue in the Networking component

What is CVE-2026-16387?

CVE-2026-16387 is a critical vulnerability in Mozilla Firefox, published on 21 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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

Weakness: CWE-346, CWE-284, CWE-200

Which versions are affected by CVE-2026-16387?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-16387?

Upgrade to a fixed version.

Is CVE-2026-16387 being exploited?

There is no known exploitation of CVE-2026-16387 in the wild and no public exploit code.

Other vulnerabilities in Mozilla Firefox

• CVE-2019-11708 Mozilla Firefox and Thunderbird Sandbox Escape Exploited Critical 10.0
• CVE-2010-3765 Mozilla Multiple Products Remote Code Execution Exploited Critical 9.8
• CVE-2022-26486 Mozilla Firefox Use-After-Free Exploited Critical 9.6
• CVE-2022-26485 Mozilla Firefox Use-After-Free Exploited High 8.8
• CVE-2019-11707 Mozilla Firefox and Thunderbird Type Confusion Exploited High 8.8

All Mozilla Firefox vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2026-16387, GHSA-24w5-cqc3-h379, UBUNTU-CVE-2026-16387.