CVE-2025-9187: Memory safety bugs fixed in Firefox 142 and Thunderbird 142

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-08-19 · updated 2026-09-30
Weakness: CWE-119

Description

Memory safety bugs present in Firefox 141 and Thunderbird 141. 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 142 and Thunderbird 142.

Affected products

Mozilla Firefox: before 142.0 — fixed in 142.0
Mozilla Thunderbird: before 142.0 — fixed in 142.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 142.0; Mozilla Thunderbird 142.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=1825621%2C1970079%2C1976736%2C1979072
https://www.mozilla.org/security/advisories/mfsa2025-64/
https://www.mozilla.org/security/advisories/mfsa2025-70/

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