CVE-2021-40084 in opensysusers
Summary
by MITRE • 08/25/2021
opensysusers through 0.6 does not safely use eval on files in sysusers.d that may contain shell metacharacters. For example, it allows command execution via a crafted GECOS field whereas systemd-sysusers (a program with the same specification) does not do that.
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Analysis
by VulDB Data Team • 08/29/2021
The vulnerability identified as CVE-2021-40084 affects the opensysusers package version 0.6 and earlier, presenting a critical security risk through improper handling of user configuration files within the sysusers.d directory structure. This flaw stems from the package's unsafe use of the eval function when processing configuration files, creating a path for arbitrary command execution through carefully crafted input data. The vulnerability specifically manifests in the GECOS field of user accounts, where shell metacharacters can be exploited to execute malicious commands with the privileges of the affected system. Unlike systemd-sysusers which properly sanitizes input and prevents such execution scenarios, opensysusers fails to implement adequate protection mechanisms, making it susceptible to privilege escalation and remote code execution attacks.
The technical implementation of this vulnerability resides in the improper parsing and evaluation of configuration data within the sysusers.d directory. When opensysusers processes user account definitions, it uses eval to interpret values from configuration files, including the GECOS field that typically contains user information such as full names, phone numbers, and other descriptive data. This approach creates a direct execution path when malicious shell metacharacters are present in the GECOS field, allowing attackers to inject and execute arbitrary commands. The flaw represents a classic command injection vulnerability that operates at the system configuration level, where legitimate system management tools are being used to process untrusted input without proper sanitization. This vulnerability maps directly to CWE-78, which identifies improper neutralization of special elements used in shell commands, and CWE-94, which covers the execution of arbitrary code or commands.
The operational impact of CVE-2021-40084 extends beyond simple privilege escalation, as it provides attackers with potential access to critical system resources and user accounts. Since the vulnerability affects system-level user management functionality, successful exploitation could result in unauthorized account creation, modification of existing user permissions, or complete system compromise depending on the execution context. The attack vector is particularly concerning because it operates through legitimate system configuration files, making detection more difficult and potentially allowing attackers to maintain persistence within the system. System administrators may not immediately recognize that malicious code has been executed through seemingly benign user account configuration changes, creating a stealthy attack method that aligns with ATT&CK technique T1068, which involves local privilege escalation through exploitation of system configuration flaws.
Mitigation strategies for CVE-2021-40084 require immediate action to upgrade to a patched version of opensysusers, as the vulnerability cannot be effectively addressed through configuration changes alone. Organizations should implement strict file validation and sanitization policies for all sysusers.d configuration files, ensuring that no shell metacharacters are present in user data fields. System administrators should also consider implementing monitoring solutions that can detect unusual patterns in user account modifications or command execution attempts. The vulnerability highlights the importance of secure coding practices in system management tools, particularly regarding input validation and the avoidance of dangerous functions like eval when processing untrusted data. Regular security assessments of system configuration management tools should be conducted to identify similar vulnerabilities that could provide attackers with unauthorized access to critical system resources and user accounts.