CVE-2025-6436: Memory safety bugs fixed in Firefox 140 and Thunderbird 140

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 4.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-06-24 · updated 2026-09-30
Weakness: CWE-119

Description

Memory safety bugs present in Firefox 139 and Thunderbird 139. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 140 and Thunderbird 140.

Affected products

Mozilla Firefox: before 140.0 — fixed in 140.0
Mozilla Thunderbird: before 140.0 — fixed in 140.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.0; Mozilla Thunderbird 140.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=1941377%2C1960948%2C1966187%2C1966505%2C1970764%2C1942930
https://www.mozilla.org/security/advisories/mfsa2025-51/
https://www.mozilla.org/security/advisories/mfsa2025-54/

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