CVE-2017-7761 in Firefox
Summary
by MITRE
The Mozilla Maintenance Service "helper.exe" application creates a temporary directory writable by non-privileged users. When this is combined with creation of a junction (a form of symbolic link), protected files in the target directory of the junction can be deleted by the Mozilla Maintenance Service, which has privileged access. Note: This attack requires local system access and only affects Windows. Other operating systems are not affected. This vulnerability affects Firefox ESR < 52.2 and Firefox < 54.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-7761 represents a critical privilege escalation flaw within the Mozilla Maintenance Service on Windows operating systems. This issue stems from the improper handling of temporary directories by the helper.exe application, which is part of Mozilla's update mechanism. The flaw specifically affects Firefox Extended Support Release versions prior to 52.2 and standard Firefox versions prior to 54, creating a significant security risk for affected users. The vulnerability operates through a combination of directory permissions and symbolic link manipulation that allows attackers with local system access to bypass normal file protection mechanisms.
The technical implementation of this vulnerability involves the Mozilla Maintenance Service creating a temporary directory that is writable by non-privileged users, which creates an inherently insecure condition. When combined with the creation of a junction point or symbolic link, the service can manipulate file paths in a way that allows deletion of protected files in the target directory of the junction. This occurs because the service, running with elevated privileges, can traverse the junction point and access files that should normally be protected from modification by regular user accounts. The flaw demonstrates a classic path traversal vulnerability where the service fails to properly validate or sanitize file paths when dealing with junctioned directories. This issue maps to CWE-22 Path Traversal and CWE-787 Out-of-bounds Write, as the service operates outside the intended directory boundaries while maintaining elevated privileges.
The operational impact of this vulnerability is significant for systems running affected Firefox versions, as it provides a method for local attackers to escalate privileges and potentially delete or modify protected system files. The attack requires local system access, which means that an attacker must already have a foothold on the system, but once achieved, the vulnerability allows for dangerous file manipulation. The fact that this affects only Windows systems indicates that the vulnerability is specific to how Windows handles symbolic links and directory junctions compared to other operating systems. This privilege escalation capability could potentially allow attackers to remove critical Firefox components, modify browser configuration files, or even install malicious software that persists across system reboots. The vulnerability affects the core update mechanism of Firefox, making it particularly dangerous as it could be exploited during normal system operations when the maintenance service is active.
Mitigation strategies for this vulnerability should focus on immediate patching of affected Firefox installations to versions 52.2 ESR or 54 and later, which contain the necessary fixes for the temporary directory handling. System administrators should ensure that all Firefox installations are updated to the latest supported versions and that automatic update mechanisms are properly configured. Additional protective measures include implementing least privilege principles for the Mozilla Maintenance Service, monitoring for unauthorized directory creation or modification, and ensuring that system access controls are properly configured to prevent non-privileged users from creating junction points in sensitive areas. Organizations should also consider implementing application whitelisting policies that restrict execution of helper.exe and related maintenance processes to prevent exploitation. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting T1068 Valid Accounts and T1134 Access Token Manipulation, as the exploitation relies on the service's ability to leverage elevated privileges through improper file system handling. Given the nature of the vulnerability, it is recommended that organizations conduct security audits to identify any systems still running affected versions and implement comprehensive monitoring for suspicious file system activities related to the Mozilla Maintenance Service.