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

Published Jul 21, 2026
·
Updated

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 Software

10 affected componentsFixes available
Mozilla Firefox>153, =153
Mozilla Firefox ESR 140.13=140.13
Mozilla Firefox<153
153
Mozilla Firefox ESR<140.13
140.13
Mozilla Thunderbird<140.13
140.13
Mozilla Thunderbird<153
153
Mozilla Firefox<140.13.0
Mozilla Firefox<153.0.0
Mozilla Thunderbird<140.13.0
Mozilla Thunderbird>=141.0<153.0

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 153
  2. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 140.13
  3. Upgrade

    Upgrade Mozilla Firefox to a version that resolves this vulnerability.

    Fixed in 153
  4. Upgrade

    Upgrade Firefox ESR to a version that resolves this vulnerability.

    Fixed in 140.13
  5. Upgrade

    Upgrade Mozilla Thunderbird to a version that resolves this vulnerability.

    Fixed in 153
  6. Upgrade

    Upgrade Thunderbird ESR to a version that resolves this vulnerability.

    Fixed in 140.13

Event History

Jul 21, 2026
CVE Published
via Mozilla·12:00 AM
Data Sourced
via Mozilla·12:00 AM
DescriptionSeverityAffected Software
Updated
via Mozilla·12:00 AM
Affected Software
CVE Published
via MITRE·12:38 PM
Data Sourced
via MITRE·12:38 PM
Description
Data Sourced
via NVD·01:17 PM
DescriptionSeverityWeaknessAffected Software

Parent advisories

This vulnerability appears in the following advisories.

Peer vulnerabilities

Found alongside the following vulnerabilities.

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Frequently Asked Questions

1

What is the severity of CVE-2026-16387?

CVE-2026-16387 has a critical severity rating of 9.8 according to the CVSS 3.1 scoring system.

2

How do I fix CVE-2026-16387?

To fix CVE-2026-16387, update to Firefox version 153, Firefox ESR version 140.13, Thunderbird version 153, or Thunderbird ESR version 140.13.

3

What issue does CVE-2026-16387 address?

CVE-2026-16387 addresses a site isolation issue in the Networking component that could lead to an information leak.

4

Which software is affected by CVE-2026-16387?

CVE-2026-16387 affects Mozilla Firefox, Mozilla Thunderbird, and their respective ESR versions.

5

When was CVE-2026-16387 published?

CVE-2026-16387 was published on July 21, 2026.

Company

Resources

Contact

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