# CVE-2026-4721: Mozilla Firefox and Thunderbird multiple memory safety bugs

Source: https://junglewise.ai/threats/cve-2026-4721-mozilla-firefox-and-thunderbird-multiple-memory-safety-bugs  
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-2026-4721 · Severity: critical · CVSS 9.8 · Published 2026-03-24

Mozilla Firefox and Thunderbird are affected by multiple memory safety vulnerabilities that could allow an attacker to corrupt the application's memory. In practice, a successful exploit could allow an attacker to take control of the affected system or execute unauthorized commands. Users should update to the latest versions of these applications to protect their data and systems from potential compromise.

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

## Quick answers

**Is CVE-2026-4721 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-4721 being exploited in the wild.

**How severe is CVE-2026-4721?** Critical severity, with a CVSS score of 9.8, published 24 March 2026.

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

**Is there a fix for CVE-2026-4721?** 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-2026-4721 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-4721?** 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

Mozilla developers and community members reported multiple memory safety bugs across various versions of Firefox and Thunderbird. These vulnerabilities include issues such as buffer overflows (CWE-120) and expired pointer dereferences (CWE-825). An attacker could potentially exploit these flaws by tricking a user into visiting a malicious website or processing specially crafted content, leading to memory corruption. Mozilla presumes that with sufficient effort, some of these bugs could be leveraged to achieve arbitrary code execution. The issues are resolved in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

## Affected products

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

## Timeline

- 2026-03-24: advisory
- 2026-03-24: patched

## References

- [https://bugzilla.mozilla.org/buglist.cgi?bug_id=2013762%2C2015291%2C2016591%2C2016661%2C2016664%2C2017303%2C2017894%2C2018090%2C2018196%2C2018379%2C2019112%2C2022090%2C2022243%2C2022351%2C2022478%2C2022676](https://bugzilla.mozilla.org/buglist.cgi?bug_id=2013762%2C2015291%2C2016591%2C2016661%2C2016664%2C2017303%2C2017894%2C2018090%2C2018196%2C2018379%2C2019112%2C2022090%2C2022243%2C2022351%2C2022478%2C2022676)
- [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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