Memory Safety Issues in Firefox and Thunderbird Products by Mozilla
CVE-2026-4721

9.8CRITICAL

Key Information:

Vendor

Mozilla
Status
Firefox
Firefox Esr
Thunderbird
Vendor
CVE Published:
24 March 2026

What is CVE-2026-4721?

The identified memory safety vulnerabilities in Firefox and Thunderbird could potentially allow attackers to manipulate memory locations, leading to arbitrary code execution in affected versions. These vulnerabilities were found in versions prior to Firefox 149 and Thunderbird 149, specifically in Firefox ESR 115.33, 140.8, and Thunderbird ESR 140.8. Mozilla has addressed these issues in subsequent updates, underscoring the importance of keeping software up to date to mitigate risks associated with memory corruption.

Affected Version(s)

Firefox < 149

Firefox ESR < 115.34

Firefox ESR < 140.9

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=2013762%2...
https://www.mozilla.org/security/advisories/mfsa2026-20/
https://www.mozilla.org/security/advisories/mfsa2026-21/

CVSS V3.1

Score:
9.8
Severity:
CRITICAL
Confidentiality:
High
Integrity:
High
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Christian Holler, Timothy Nikkel, Tom Schuster and the Mozilla Fuzzing Team
.