CVE-2016-7943 in libX11
Summary
by MITRE
The XListFonts function in X.org libX11 before 1.6.4 might allow remote X servers to gain privileges via vectors involving length fields, which trigger out-of-bounds write operations.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7943 resides within the XListFonts function of X.org libX11 library versions prior to 1.6.4, representing a critical security flaw that enables remote attackers to execute privilege escalation attacks through carefully crafted malformed length fields. This vulnerability operates within the X Window System ecosystem, which serves as the foundation for graphical user interfaces across numerous Unix-like operating systems including linux and various BSD variants. The X protocol facilitates communication between X server and X client applications, making it a prime target for exploitation due to its widespread deployment and the complex nature of its implementation.
The technical flaw manifests through improper validation of length fields within the XListFonts protocol request handling mechanism, where the library fails to adequately sanitize or bounds-check user-supplied data before processing it. When a remote X server sends a maliciously constructed XListFonts request containing oversized or malformed length parameters, the libX11 library processes these values without sufficient validation, leading to out-of-bounds write operations that can overwrite adjacent memory locations. This memory corruption vulnerability stems from a classic buffer overflow condition that can be exploited to overwrite critical program variables, return addresses, or other memory segments, potentially enabling arbitrary code execution with elevated privileges.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can be leveraged by remote attackers to gain unauthorized access to systems running vulnerable X servers. The attack vector requires the attacker to establish a connection to a vulnerable X server, typically through network-based X11 forwarding or direct server access, making it particularly dangerous in environments where X server services are exposed to untrusted networks. Successful exploitation can result in complete system compromise, allowing attackers to execute arbitrary commands, escalate privileges to root level access, or establish persistent backdoor access to affected systems. This vulnerability particularly affects desktop environments and server configurations that rely on X Window System for graphical interface management, including but not limited to kde, gnome, and xfce desktop environments.
Mitigation strategies for CVE-2016-7943 primarily focus on immediate patch deployment and system hardening measures. Organizations should prioritize upgrading to libX11 version 1.6.4 or later, which includes proper bounds checking and input validation for length fields in the XListFonts function. System administrators should also implement network segmentation to limit access to X server services, disable unnecessary X11 forwarding, and employ firewall rules to restrict X server access to trusted networks only. Additional protective measures include implementing mandatory access controls, monitoring X server connections for suspicious activity, and regularly auditing system configurations to ensure X server services are properly secured. The vulnerability aligns with CWE-121, which describes stack-based buffer overflow conditions, and can be mapped to ATT&CK technique T1068, which covers exploitation for privilege escalation through local and remote attack vectors. Organizations should also consider implementing intrusion detection systems to monitor for patterns consistent with X11 protocol exploitation attempts and maintain comprehensive incident response procedures for handling potential exploitation events.