Security restrictions bypass in Firefox ESR

Published: 2021-05-04
Risk Low
Patch available YES
Number of vulnerabilities 1
CVE ID CVE-2021-29951
Exploitation vector Local
Public exploit N/A
Vulnerable software
Firefox ESR
Client/Desktop applications / Web browsers

Vendor Mozilla

Security Advisory

This security advisory describes one low risk vulnerability.

1) Security restrictions bypass

Risk: Low

CVSSv3.1: 2.9 [CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L/E:U/RL:O/RC:C]

CVE-ID: CVE-2021-29951

CWE-ID: CWE-264 - Permissions, Privileges, and Access Controls

Exploit availability: No


The vulnerability allows a local user to bypass implemented security restrictions.

The vulnerability exists due to the way Mozilla Maintenance Service is installed in the Windows operating system. After installation the Mozilla Maintenance Service is granted SERVICE_START access to BUILTIN|Users which, in a domain network, grants normal remote users access to start or stop the service. A local domain user can spam  the "Stop" command and prevent the browser update service from operating.

The vulnerability affects only Firefox ESR installed on operating system Windows 10 build 1709 and older.


Install updates from vendor's website.

Vulnerable software versions

Firefox ESR: 78.0, 78.0.1, 78.0.2, 78.1.0, 78.2.0, 78.3.0, 78.3.1, 78.4.0, 78.4.1, 78.5.0, 78.6.0, 78.6.1, 78.7.0, 78.7.1, 78.8.0, 78.9.0, 78.10.0

CPE External links

Q & A

Can this vulnerability be exploited remotely?

No. This vulnerability can be exploited locally. The attacker should have authentication credentials and successfully authenticate on the system.

How the attacker can exploit this vulnerability?

The attacker would have to trick the victim to visit a specially crafted website.

The attacker would have to login to the system and perform certain actions in order to exploit this vulnerability.

Is there known malware, which exploits this vulnerability?

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