CVE-2016-7942 in libX11
Summary
by MITRE
The XGetImage function in X.org libX11 before 1.6.4 might allow remote X servers to gain privileges via vectors involving image type and geometry, which triggers out-of-bounds read operations.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7942 affects the XGetImage function within X.org libX11 library versions prior to 1.6.4. This flaw exists within the X Window System implementation that manages graphics operations between X servers and clients, creating a potential privilege escalation vector through remote exploitation. The issue manifests when remote X servers interact with client applications that utilize the XGetImage function, which is responsible for retrieving image data from the X server. The vulnerability specifically arises from insufficient validation of image type and geometry parameters, allowing malicious X servers to craft malformed requests that trigger out-of-bounds read operations within the client application's memory space.
The technical exploitation of this vulnerability involves the manipulation of image type and geometry parameters passed to the XGetImage function, which are processed without adequate bounds checking. When an X server sends crafted image data with invalid dimensions or unsupported image types, the libX11 library fails to validate these parameters properly before attempting to read memory locations. This leads to out-of-bounds memory access patterns that can result in information disclosure, application crashes, or potentially more severe privilege escalation depending on the execution context. The vulnerability is classified under CWE-125 as an out-of-bounds read, which represents a fundamental memory safety issue where the application accesses memory beyond its allocated boundaries.
From an operational perspective, this vulnerability poses significant risks to systems that rely on X Window System for graphical user interfaces, particularly in environments where untrusted X servers might be present. The impact extends across various attack vectors including remote desktop connections, virtualized environments, and networked applications that utilize X11 forwarding. The vulnerability can be exploited by remote attackers who have access to an X server that can communicate with vulnerable X11 clients, potentially leading to information leakage from the client process memory. This could expose sensitive data including cryptographic keys, user credentials, or application state information that might be stored in memory. The vulnerability affects systems running X.org libX11 versions earlier than 1.6.4, making it a critical concern for organizations maintaining legacy X11 implementations.
The mitigation strategy for CVE-2016-7942 requires immediate patching of all affected libX11 installations to version 1.6.4 or later, which includes the necessary input validation fixes to prevent malformed image parameters from causing out-of-bounds memory operations. Organizations should also implement network segmentation and access controls to limit exposure of X11 services to trusted networks only, reducing the attack surface for remote exploitation attempts. Additional defensive measures include monitoring X11 traffic for suspicious image parameter patterns and implementing application-level sandboxing for X11 client applications to contain potential exploitation impacts. Security teams should also consider disabling X11 forwarding in SSH configurations when it is not required, as this reduces the attack vectors available to remote adversaries. The vulnerability demonstrates the importance of proper input validation in graphics libraries and aligns with ATT&CK technique T1059.007 for command and scripting interpreter usage, as exploitation often involves crafting malicious X11 protocol sequences that bypass normal validation checks.