CVE-2025-1942: Disclosure of uninitialized memory when .toUpperCase() causes string to get longer

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: 2025-03-04 · updated 2026-10-05
Weakness: CWE-908

Description

When String.toUpperCase() caused a string to get longer it was possible for uninitialized memory to be incorporated into the result string. This vulnerability was fixed in Firefox 136 and Thunderbird 136.

Affected products

Mozilla Firefox: before 136.0 — fixed in 136.0
Mozilla Thunderbird: before 136.0 — fixed in 136.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 136.0; Mozilla Thunderbird 136.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1947139
https://www.mozilla.org/security/advisories/mfsa2025-14/
https://www.mozilla.org/security/advisories/mfsa2025-17/

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