CVE-2013-1996 in libFS
Summary
by MITRE
X.org libFS 1.0.4 and earlier allows X servers to trigger allocation of insufficient memory and a buffer overflow via vectors related to an unexpected sign extension in the FSOpenServer function.
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Analysis
by VulDB Data Team • 05/14/2021
The vulnerability identified as CVE-2013-1996 affects X.org libFS version 1.0.4 and earlier, representing a critical memory safety issue within the X Window System's font server implementation. This flaw exists in the FSOpenServer function where improper handling of data types leads to predictable memory allocation failures and subsequent buffer overflow conditions. The vulnerability specifically exploits an unexpected sign extension behavior that occurs during integer processing within the font server communication protocol.
The technical root cause of this vulnerability stems from inadequate input validation and improper type handling within the FSOpenServer function. When X servers communicate with the font server, they transmit data structures containing integer values that undergo sign extension operations. The libFS library fails to properly validate these values before using them to determine memory allocation sizes, creating a scenario where malicious X servers can craft specially crafted requests that result in insufficient memory allocation followed by buffer overflow conditions. This represents a classic case of improper integer handling that can be categorized under CWE-190, which deals with integer overflow and underflow conditions.
From an operational perspective, this vulnerability presents significant security implications for systems utilizing the X Window System environment. Attackers can leverage this flaw to execute arbitrary code on vulnerable systems, potentially leading to complete system compromise. The buffer overflow condition creates opportunities for privilege escalation attacks where malicious actors can overwrite critical memory structures or inject malicious code into the font server process. The vulnerability is particularly concerning in multi-user environments where X servers are actively communicating with font servers, as it can be exploited remotely without requiring authentication. This aligns with ATT&CK technique T1068 which covers 'Exploitation for Privilege Escalation' and T1059 which covers 'Command and Scripting Interpreter' through potential code execution vectors.
The impact of this vulnerability extends beyond immediate exploitation capabilities to encompass broader system stability and security posture concerns. Systems running vulnerable versions of libFS become susceptible to denial of service attacks that can crash the font server process, disrupting graphical user interfaces and potentially affecting other X server components. Additionally, the memory corruption aspects of this vulnerability can lead to unpredictable system behavior, making it particularly dangerous in production environments where system reliability is paramount. Organizations utilizing the X Window System infrastructure should consider this vulnerability as part of their comprehensive security assessment, particularly in environments where untrusted X servers might be present or where network exposure exists. The vulnerability demonstrates the critical importance of proper input validation and memory management practices in system libraries that handle inter-process communications, serving as a reminder of how seemingly minor implementation flaws can create substantial security risks in widely deployed software components.