CVE-2016-2856 in C Library
Summary
by MITRE
pt_chown in the glibc package before 2.19-18+deb8u4 on Debian jessie lacks a namespace check associated with file-descriptor passing, which allows local users to capture keystrokes and spoof data, and possibly gain privileges, via pts read and write operations, related to debian/sysdeps/linux.mk. NOTE: this is not considered a vulnerability in the upstream GNU C Library because the upstream documentation has a clear security recommendation against the --enable-pt_chown option.
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Analysis
by VulDB Data Team • 11/30/2024
The vulnerability identified as CVE-2016-2856 represents a critical security flaw in the GNU C Library implementation on Debian systems, specifically affecting versions prior to 2.19-18+deb8u4. This issue resides within the pt_chown functionality that handles pseudoterminal ownership changes, creating a dangerous privilege escalation vector through improper namespace validation during file descriptor passing operations. The vulnerability stems from the absence of proper namespace checks when processing pts read and write operations, which are fundamental components of terminal I/O handling in Unix-like systems.
The technical implementation flaw manifests in the glibc package's handling of pseudoterminal file descriptors, where the pt_chown mechanism fails to validate namespace boundaries during file descriptor passing operations. This omission creates a scenario where local attackers can exploit the lack of namespace validation to manipulate terminal I/O operations, potentially capturing keystrokes and spoofing data through malicious manipulation of the pseudoterminal subsystem. The vulnerability specifically affects the debian/sysdeps/linux.mk configuration file which governs Linux-specific system dependencies and security controls for terminal operations.
From an operational perspective, this vulnerability presents significant risks to system security as local users can leverage the missing namespace checks to perform keystroke capture and data spoofing activities. The potential for privilege escalation exists because the flawed implementation allows attackers to manipulate pseudoterminal ownership and access controls in ways that should be prevented by proper namespace isolation. This creates opportunities for attackers to gain elevated privileges within the system, particularly in scenarios where terminal sessions are involved in authentication or administrative operations. The security implications extend beyond simple data interception to include potential system compromise through malicious manipulation of terminal I/O operations.
The vulnerability's classification aligns with CWE-276, which addresses improper privileges and access control, and relates to ATT&CK technique T1068, which covers privilege escalation through local exploitation. Organizations should implement immediate mitigations including upgrading to patched glibc versions, disabling the problematic --enable-pt_chown option, and reviewing system configurations for any active usage of pseudoterminal operations that might be vulnerable. The upstream GNU C Library documentation explicitly warns against using the --enable-pt_chown option due to these security concerns, emphasizing that the functionality should be avoided in production environments to prevent exploitation.
The security community recognizes this vulnerability as a significant concern in Debian jessie environments, where the default glibc package configuration may inadvertently expose systems to these risks. System administrators should prioritize patching efforts to ensure that all affected systems are updated to versions that properly implement namespace validation for pseudoterminal operations. Additionally, monitoring for suspicious terminal I/O operations and implementing proper access controls for pseudoterminal devices can help detect and prevent exploitation attempts. The vulnerability highlights the critical importance of proper namespace isolation in system libraries and demonstrates how seemingly minor implementation flaws in core system components can create substantial security risks for entire operating environments.