CVE-2021-35939 in RPM
Summary
by MITRE • 08/26/2022
It was found that the fix for CVE-2017-7500 and CVE-2017-7501 was incomplete: the check was only implemented for the parent directory of the file to be created. A local unprivileged user who owns another ancestor directory could potentially use this flaw to gain root privileges. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.
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Analysis
by VulDB Data Team • 10/02/2022
This vulnerability represents a critical flaw in privilege escalation mechanisms that stems from an incomplete security fix implemented for earlier vulnerabilities CVE-2017-7500 and CVE-2017-7501. The original issue involved inadequate directory traversal checks during file creation operations, which were subsequently addressed but left a significant gap in the security model. The flaw specifically resides in the validation logic that only examines the immediate parent directory of the target file rather than implementing comprehensive path validation across the entire directory hierarchy. This design oversight creates a fundamental weakness that can be exploited by local unprivileged users who possess ownership rights over ancestor directories within the file path.
The technical implementation of this vulnerability exploits the principle of least privilege by leveraging the incomplete access control checks that were introduced as part of the original remediation efforts. When a process attempts to create a file, the system performs validation only against the immediate parent directory of the target file location, failing to validate the complete path structure and the permissions of all intermediate directories. This gap allows an attacker with ownership of any ancestor directory in the path to manipulate the file creation process and potentially bypass the intended security restrictions. The vulnerability operates at the operating system level where file system access controls and privilege management intersect, making it particularly dangerous as it can be exploited to create or modify files in locations where the user would normally not have write permissions.
The operational impact of CVE-2021-35939 extends beyond simple privilege escalation to encompass serious threats to data confidentiality, integrity, and system availability. An attacker can leverage this vulnerability to gain root privileges, which would provide complete control over the affected system, allowing for data exfiltration, system modification, and persistent access. The ability to manipulate file creation processes in this manner can lead to the installation of malicious software, modification of critical system files, or the creation of backdoors that persist across system reboots. The vulnerability's exploitation potential is particularly concerning in multi-user environments where different users may have varying levels of directory ownership and access rights, creating numerous attack vectors for malicious actors.
Security mitigations for this vulnerability should focus on implementing comprehensive path validation that examines the entire directory hierarchy rather than just the immediate parent directory. System administrators should ensure that all directory creation operations perform recursive permission checks across the complete path, validating not only the parent directory but also all intermediate directories in the file path. The fix should align with established security principles such as those outlined in the CWE-22 category for Improper Limitation of a Pathname to a Restricted Directory, which emphasizes the importance of preventing directory traversal attacks through complete path validation. Additionally, implementing mandatory access controls and privilege separation mechanisms can help reduce the impact of such vulnerabilities. Organizations should also consider implementing monitoring solutions that can detect anomalous file creation patterns and unauthorized privilege escalation attempts, as described in the ATT&CK framework's privilege escalation techniques. Regular security audits and vulnerability assessments should be conducted to identify similar incomplete fixes in other security patches, ensuring that all access control mechanisms are properly implemented across the entire system architecture.