CVE-2017-5397 in Firefox
Summary
by MITRE
The cache directory on the local file system is set to be world writable. Firefox defaults to extracting libraries from this cache. This allows for the possibility of an installed malicious application or tools with write access to the file system to replace files used by Firefox with their own versions. This vulnerability affects Firefox < 51.0.3.
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Analysis
by VulDB Data Team • 02/17/2020
The vulnerability described in CVE-2017-5397 represents a critical security flaw in Mozilla Firefox versions prior to 51.0.3 where the application's cache directory is configured with world-writable permissions. This configuration creates a fundamental weakness in the browser's security architecture by allowing any user or process with write access to the file system to modify critical components that Firefox relies upon during operation. The cache directory serves as a temporary storage location for downloaded libraries and other resources that Firefox extracts and uses during runtime, making it a prime target for malicious actors seeking to compromise the browser environment.
The technical implementation of this vulnerability stems from Firefox's default behavior of setting cache directory permissions to allow world write access, typically 0777 or equivalent permissions that permit any user to create, modify, or delete files within this directory. When Firefox starts up, it automatically extracts necessary libraries and components from this cache location into memory for execution, creating a window of opportunity for attackers who have already gained write privileges to the file system. This flaw directly maps to CWE-732, which describes improper permission assignment for critical resources, and represents a classic case of insufficient privilege separation between application components and system-level file access controls.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it provides attackers with a persistent means of injecting malicious code into the browser execution environment. An attacker with write access to the file system can replace legitimate libraries with malicious versions, effectively creating a backdoor that persists across browser sessions and system reboots. This allows for the execution of arbitrary code with the privileges of the Firefox process, potentially leading to full system compromise. The vulnerability is particularly dangerous because it leverages existing file system permissions rather than requiring additional attack vectors, making it accessible to attackers who may have limited privileges but can still write to system directories.
Mitigation strategies for CVE-2017-5397 focus on both immediate remediation and long-term architectural improvements. The most direct solution is upgrading to Firefox version 51.0.3 or later, which addressed the issue by implementing proper cache directory permissions. System administrators should also consider implementing additional security controls such as mandatory access controls, file system monitoring, and regular permission audits to prevent unauthorized modifications to critical directories. This vulnerability aligns with ATT&CK technique T1059, which covers command and script injection, as attackers can leverage this weakness to inject malicious code through compromised library files. Organizations should also implement principle of least privilege practices and consider containerization or sandboxing approaches to limit the potential impact of such vulnerabilities in compromised environments.