CVE-2025-1020: Memory safety bugs fixed in Firefox 135 and Thunderbird 135

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: 2025-02-04 · updated 2026-10-05
Weakness: CWE-787

Description

Memory safety bugs present in Firefox 134 and Thunderbird 134. 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 135 and Thunderbird 135.

Affected products

Mozilla Firefox: before 135.0 — fixed in 135.0
Mozilla Thunderbird: from 131.0 before 135.0 — fixed in 135.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 135.0; Mozilla Thunderbird 135.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=1939063%2C1942169
https://www.mozilla.org/security/advisories/mfsa2025-07/
https://www.mozilla.org/security/advisories/mfsa2025-11/

Get alerted when CVE-2025-1020 or the next vulnerability affects your stack