# CVE-2026-4689: Mozilla Firefox and Thunderbird sandbox escape in XPCOM

Source: https://junglewise.ai/threats/cve-2026-4689-mozilla-firefox-and-thunderbird-sandbox-escape-in-xpcom  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-24. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-4689 · Severity: critical · CVSS 10 · Published 2026-03-24

Mozilla Firefox and Thunderbird are popular web browsers and email clients used for accessing the internet and managing communications. A critical vulnerability in these applications could allow an attacker to bypass security 'sandbox' protections that normally isolate the browser from the rest of the computer. If exploited, this could lead to unauthorized access to sensitive user data or the ability to run malicious software on the victim's system.

Technologies: [Mozilla Firefox](https://junglewise.ai/threats/technologies/firefox), [Mozilla Firefox ESR](https://junglewise.ai/threats/technologies/firefox-esr), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2026-4689 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-4689 being exploited in the wild.

**How severe is CVE-2026-4689?** Critical severity, with a CVSS score of 10, published 24 March 2026.

**Which products and versions are affected?** Mozilla Firefox < 149; Mozilla Firefox ESR < 115.34, < 140.9; Mozilla Thunderbird < 149, < 140.9.

**Is there a fix for CVE-2026-4689?** A patch was published on 2026-03-24. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-4689 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-4689?** Check whether you run Mozilla Firefox, Mozilla Firefox ESR or Mozilla Thunderbird 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

A critical vulnerability exists in the XPCOM component of Mozilla Firefox and Thunderbird due to an integer overflow and incorrect boundary conditions. This flaw allows for a sandbox escape, potentially enabling an attacker to execute code outside of the restricted browser environment. The vulnerability can be triggered remotely, and according to some CVSS assessments, may not require user interaction. Patches are available in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, and Thunderbird versions 149 and 140.9.

## Affected products

- Mozilla Firefox < 149
- Mozilla Firefox ESR < 115.34, < 140.9
- Mozilla Thunderbird < 149, < 140.9

## Timeline

- 2026-03-24: disclosed
- 2026-03-24: patched

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2016374](https://bugzilla.mozilla.org/show_bug.cgi?id=2016374)
- [https://www.mozilla.org/security/advisories/mfsa2026-20/](https://www.mozilla.org/security/advisories/mfsa2026-20/)
- [https://www.mozilla.org/security/advisories/mfsa2026-21/](https://www.mozilla.org/security/advisories/mfsa2026-21/)
- [https://www.mozilla.org/security/advisories/mfsa2026-22/](https://www.mozilla.org/security/advisories/mfsa2026-22/)
- [https://www.mozilla.org/security/advisories/mfsa2026-23/](https://www.mozilla.org/security/advisories/mfsa2026-23/)
- [https://www.mozilla.org/security/advisories/mfsa2026-24/](https://www.mozilla.org/security/advisories/mfsa2026-24/)
- [https://access.redhat.com/errata/RHSA-2026:5930](https://access.redhat.com/errata/RHSA-2026:5930)

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