#VU109401 Out-of-bounds write in Mozilla Firefox and Firefox ESR - CVE-2025-4919

Vulnerability identifier: #VU109401

Vulnerability risk: High

CVSSv4.0: 6.1 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Amber]

CVE-ID: CVE-2025-4919, CVE-2025-4921

CWE-ID: CWE-787

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Mozilla Firefox
Client/Desktop applications / Web browsers
Firefox ESR
Client/Desktop applications / Web browsers

Vendor: Mozilla

Description

The vulnerability allows a remote attacker to compromise vulnerable system.

The vulnerability exists due to a boundary error when manipulating a JavaScript object by confusing array index sizes. A remote attacker can trick the victim into visiting a specially crafted website, trigger an out-of-bounds write and execute arbitrary code on the target system.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Mozilla Firefox: 110.0 - 138.0.3

Firefox ESR: 102.0, 102.0.1, 102.1.0, 102.2.0, 102.3.0, 102.4.0, 102.5.0, 102.6.0, 102.7.0, 102.8.0, 102.9.0, 102.10.0, 102.11.0, 102.12.0, 102.13.0, 102.14.0, 102.15.0, 102.15.1, 115.0, 115.0.1, 115.0.2, 115.0.3, 115.1.0, 115.2.0, 115.2.1, 115.3.0, 115.3.1, 115.4.0, 115.5.0, 115.6.0, 115.7.0, 115.8.0, 115.9.0, 115.9.1, 115.10.0, 115.11.0, 115.12.0, 115.13.0, 115.14.0, 115.15.0, 115.16.0, 115.16.1, 115.17.0, 115.18.0, 115.19.0, 115.20.0, 115.21.0, 115.21.1, 115.22.0, 115.23.0, 128.0, 128.1.0, 128.2.0, 128.3.0, 128.3.1, 128.4.0, 128.5.0, 128.5.1, 128.5.2, 128.6.0, 128.7.0, 128.8.0, 128.8.1, 128.9.0, 128.10.0

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External links
https://bugzilla.mozilla.org/show_bug.cgi?id=1966614
https://www.mozilla.org/security/advisories/mfsa2025-36/
https://www.mozilla.org/security/advisories/mfsa2025-37/
https://www.mozilla.org/en-US/security/advisories/mfsa2025-38/
https://www.zerodayinitiative.com/advisories/ZDI-25-292/
https://www.zerodayinitiative.com/advisories/ZDI-25-291/

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.