CVE-2016-2781 in Coreutils
Summary
by MITRE
chroot in GNU coreutils, when used with --userspec, 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 • 06/14/2025
The vulnerability identified as CVE-2016-2781 affects the chroot command in GNU coreutils version 8.25 and earlier, presenting a significant privilege escalation risk for local attackers. This flaw occurs when chroot is invoked with the --userspec option, which specifies user and group ownership for the new root directory. The vulnerability exploits a race condition in how chroot handles terminal input handling during the privilege transition process, allowing malicious actors to manipulate the terminal's input buffer through a carefully crafted TIOCSTI ioctl system call.
The technical mechanism behind this vulnerability involves the interaction between chroot's privilege escalation process and terminal input handling. When chroot executes with --userspec, it temporarily drops privileges to the specified user context while maintaining access to terminal control functions. The TIOCSTI ioctl call, designed for injecting characters into a terminal's input buffer, becomes exploitable when combined with chroot's privilege transition sequence. An attacker can craft a malicious input sequence that gets injected into the terminal buffer, effectively allowing them to execute commands in the parent session with elevated privileges. This represents a classic case of privilege escalation through improper input validation and terminal handling.
The operational impact of CVE-2016-2781 extends beyond simple local privilege escalation, as it can be leveraged to bypass security controls in environments where chroot is used for sandboxing purposes. Systems using chroot for containerization, virtualization, or application isolation become vulnerable to this attack vector, potentially allowing attackers to escape from confined environments and gain access to parent system resources. The vulnerability is particularly concerning in multi-user environments where chroot is used to limit user access to specific directories while maintaining system integrity. This flaw aligns with CWE-284, which addresses improper access control, and can be categorized under ATT&CK technique T1068, which covers exploit for privilege escalation.
Mitigation strategies for CVE-2016-2781 focus on both immediate patching and operational security improvements. The primary solution involves upgrading to GNU coreutils version 8.26 or later, where the vulnerability has been addressed through proper input validation and terminal handling. Organizations should also implement monitoring for suspicious chroot usage patterns and terminal input manipulation. System administrators should review and restrict the use of chroot with --userspec in environments where privilege escalation risks are significant. Additionally, implementing proper access controls and limiting the ability to execute chroot commands with elevated privileges can reduce the attack surface. The vulnerability demonstrates the importance of considering terminal input handling in privilege escalation scenarios and highlights the need for comprehensive security reviews of system utilities that handle privilege transitions.