CVE-2016-7946 in libXi
Summary
by MITRE
X.org libXi before 1.7.7 allows remote X servers to cause a denial of service (infinite loop) via vectors involving length fields.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7946 affects the X.org libXi library version 1.7.6 and earlier, representing a critical denial of service flaw that can be exploited by remote X servers. This issue stems from improper handling of length fields within the library's processing routines, creating a condition where maliciously crafted X protocol messages can trigger an infinite loop in the affected software. The vulnerability specifically impacts systems that utilize the X Input extension library, which is fundamental to input device handling in X Window System environments.
The technical flaw manifests when remote X servers send specially crafted protocol messages containing malformed length fields that cause the libXi library to enter an infinite loop during processing. This occurs because the library fails to properly validate input parameters before entering iterative processing loops that depend on these length fields. The vulnerability is categorized under CWE-121 as a buffer overflow condition, though it manifests specifically as an infinite loop rather than memory corruption. The flaw exists in the X Input extension processing code where length fields are used to control loop iterations without proper bounds checking, creating a scenario where malicious input can cause indefinite execution.
From an operational perspective, this vulnerability poses significant risk to systems that rely on X Window System for graphical interfaces, particularly in server environments, remote desktop implementations, and virtualization platforms. Attackers can exploit this weakness by connecting to vulnerable X servers and sending crafted protocol messages that trigger the infinite loop, effectively causing the affected X server to become unresponsive and consume excessive CPU resources. The impact extends beyond simple service disruption as the infinite loop can cause cascading failures in dependent services and may require system restarts to recover. This vulnerability affects a wide range of applications including X11 servers, remote desktop solutions, and any system that processes X protocol input through the libXi library.
Mitigation strategies for CVE-2016-7946 primarily involve upgrading to libXi version 1.7.7 or later, which contains the necessary patches to properly validate length fields and prevent the infinite loop condition. System administrators should also implement network segmentation and access controls to limit exposure to untrusted X servers, particularly in environments where remote X connections are permitted. Additionally, monitoring for unusual CPU usage patterns and implementing intrusion detection systems can help identify exploitation attempts. The vulnerability aligns with ATT&CK technique T1499.004 for network denial of service and represents a classic example of improper input validation that can be addressed through proper defensive programming practices and regular security updates. Organizations should also consider implementing X server access controls and authentication mechanisms to reduce the attack surface for this type of remote exploitation.