CVE-2016-2779 in util-linux
Summary
by MITRE
runuser in util-linux allows local users to escape to the parent session via a crafted TIOCSTI ioctl call, which pushes characters to the terminal's input buffer.
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Analysis
by VulDB Data Team • 09/01/2020
The vulnerability identified as CVE-2016-2779 affects the runuser utility within the util-linux package, presenting a significant local privilege escalation risk. This flaw resides in how runuser handles terminal input operations, specifically through the TIOCSTI ioctl command that allows manipulation of the terminal's input buffer. The vulnerability enables malicious local users to inject arbitrary commands into the parent session by crafting specially formatted ioctl calls that push characters into the terminal input queue. This represents a critical security weakness in Unix-like operating systems where proper privilege separation is essential for maintaining system integrity. The issue stems from insufficient input validation and improper handling of terminal control operations within the runuser utility, creating an unintended execution path that bypasses normal security boundaries. The vulnerability is particularly concerning because it allows attackers to escalate privileges from a regular user account to a higher privilege level, potentially gaining root access or administrative control over system resources.
The technical exploitation of this vulnerability occurs through the manipulation of the TIOCSTI ioctl command which is designed to inject input characters into a terminal's input buffer. When runuser processes user input, it fails to properly sanitize or validate terminal control operations, allowing an attacker to craft malicious ioctl calls that push sequences of characters into the terminal's input queue. These injected characters can contain commands that, when processed by the parent session, execute with elevated privileges. The flaw exists because the system does not properly isolate the terminal input handling between the user session and the parent process, creating a pathway for command injection attacks. This vulnerability specifically targets the terminal session management within the runuser utility, which is designed to execute commands as a different user, but fails to properly secure the terminal input operations during this process. The attack vector leverages the fact that TIOCSTI is a legitimate terminal control function that should be accessible to authorized processes, but the vulnerability allows unauthorized access through improper privilege checks.
The operational impact of CVE-2016-2779 extends beyond simple privilege escalation, as it can lead to complete system compromise when exploited effectively. Local attackers can leverage this vulnerability to execute arbitrary commands with elevated privileges, potentially gaining root access or administrative control over critical system resources. The attack requires local system access and knowledge of the specific terminal control mechanisms, but once exploited, it can provide attackers with persistent access to the system. This vulnerability affects all systems running util-linux versions containing the vulnerable runuser utility, making it a widespread concern across Unix-like operating systems. The exploitation process involves crafting specific ioctl calls that manipulate the terminal input buffer to inject commands that will execute in the parent session with higher privileges. The vulnerability is particularly dangerous in multi-user environments where local access might be obtained through various means, including social engineering, physical access, or other initial compromise vectors that could lead to exploitation of this privilege escalation flaw.
Mitigation strategies for CVE-2016-2779 should focus on both immediate patching and operational security measures to prevent exploitation. The most effective immediate solution is to update the util-linux package to a version that contains the security fix, which addresses the improper handling of TIOCSTI ioctl calls in the runuser utility. System administrators should also implement strict access controls and monitor for unusual terminal input operations that might indicate exploitation attempts. The vulnerability aligns with CWE-200, which covers information exposure, and CWE-78, which addresses command injection vulnerabilities, highlighting the dual nature of the flaw. From an attack framework perspective, this vulnerability maps to ATT&CK technique T1068, which covers local privilege escalation, and T1059, which involves command and scripting interpreters. Organizations should also consider implementing additional security controls such as terminal input monitoring, privilege separation measures, and regular security audits of system utilities to identify similar vulnerabilities. The fix typically involves proper validation of ioctl parameters and ensuring that terminal control operations are properly restricted based on the user's privileges and the session context.