CVE-2016-5293 in Firefox
Summary
by MITRE
When the Mozilla Updater is run, if the Updater's log file in the working directory points to a hardlink, data can be appended to an arbitrary local file. This vulnerability requires local system access. Note: this issue only affects Windows operating systems. This vulnerability affects Firefox ESR < 45.5 and Firefox < 50.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 11/25/2025
The vulnerability described in CVE-2016-5293 represents a significant local privilege escalation risk within Mozilla Firefox's update mechanism on Windows systems. This flaw exists in the Mozilla Updater component that handles Firefox updates, specifically when the updater executes and attempts to write log data to a designated file in the working directory. The vulnerability stems from improper handling of file system hardlinks during the logging process, creating a scenario where malicious actors can manipulate the updater's behavior to append data to arbitrary local files. The technical implementation of this vulnerability requires an attacker to already possess local system access, making it a local privilege escalation rather than a remote code execution threat. However, the implications remain severe as it allows for persistent modification of system files and potentially enables more sophisticated attacks once initial access is established.
The core technical flaw manifests when the Mozilla Updater creates or appends to log files that are actually hardlinks pointing to sensitive system locations. This behavior violates fundamental security principles of file system operations, as the updater does not properly validate or sanitize the target file paths before performing write operations. The vulnerability specifically affects Windows operating systems due to the way hardlink mechanisms function within the windows file system architecture, where multiple directory entries can point to the same physical file. This flaw is categorized under CWE-59, which deals with improper handling of symbolic or hard links, and represents a classic case of insecure file handling in update mechanisms. The attack vector requires an attacker to have already compromised the local system, but once achieved, the vulnerability enables arbitrary file modification through the updater's logging functionality, potentially allowing for the corruption of critical system files or the injection of malicious code into legitimate processes.
The operational impact of CVE-2016-5293 extends beyond simple file corruption as it provides a mechanism for persistent system compromise through the Firefox update process. Attackers can leverage this vulnerability to modify system files, registry entries, or configuration data that would normally require elevated privileges to alter. The vulnerability affects Firefox Extended Support Release versions prior to 45.5 and standard Firefox versions prior to 50, representing a substantial user base that would have been exposed to this risk. This vulnerability aligns with ATT&CK technique T1059.001, which covers command and scripting interpreter usage, as the attacker could potentially use this flaw to establish persistence through modification of system files that are regularly accessed or executed. The impact is particularly concerning because the updater runs with the privileges of the user who initiated the update process, meaning that even if an attacker cannot directly execute code with system privileges, they can still manipulate the update process to indirectly affect system integrity and security posture.
Mitigation strategies for CVE-2016-5293 primarily focus on immediate patching and system hardening measures. Users should immediately upgrade to Firefox versions 45.5 ESR or 50 and later to eliminate the vulnerability. System administrators should implement file system monitoring to detect unauthorized hardlink creation in directories where update processes operate. The vulnerability highlights the importance of secure file handling practices and proper validation of file system objects before performing write operations. Organizations should consider implementing application whitelisting policies to restrict execution of unauthorized update processes and ensure that the updater component operates in secure, isolated environments. Additionally, the vulnerability underscores the need for proper privilege separation in update mechanisms and highlights the importance of following secure coding practices such as those outlined in the OWASP Secure Coding Practices. The fix implemented by Mozilla addressed the core issue by ensuring proper file path validation and preventing the updater from following hardlinks when creating or appending to log files, thereby eliminating the possibility of arbitrary file modification through this specific attack vector.