CVE-2021-4008 in X11
Summary
by MITRE • 12/17/2021
A flaw was found in xorg-x11-server in versions before 21.1.2 and before 1.20.14. An out-of-bounds access can occur in the SProcRenderCompositeGlyphs function. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.
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Analysis
by VulDB Data Team • 12/24/2021
The vulnerability identified as CVE-2021-4008 resides within the xorg-x11-server component of Linux graphical systems, specifically affecting versions prior to 21.1.2 and 1.20.14. This flaw manifests as an out-of-bounds memory access condition that occurs within the SProcRenderCompositeGlyphs function, which is responsible for handling rendering operations in the X Window System. The issue represents a critical weakness in the server's input validation mechanisms, where improper bounds checking allows malicious actors to manipulate memory access patterns that could lead to system instability or unauthorized data exposure. The vulnerability falls under the category of memory safety issues and is classified as CWE-129, which addresses insufficient input validation leading to out-of-bounds accesses. The impact extends across multiple security domains as the flaw can compromise data confidentiality through potential information leakage, integrity through possible data corruption, and availability by potentially causing system crashes or denial of service conditions.
The technical exploitation of this vulnerability requires an attacker to craft specific X protocol requests that trigger the problematic code path within the SProcRenderCompositeGlyphs function. When the server processes these malformed requests, it fails to properly validate array indices or buffer boundaries, leading to memory access violations that can result in arbitrary code execution or system crashes. The attack surface is particularly significant in environments where the X server processes untrusted input from remote clients, making network-based exploitation a realistic threat vector. This vulnerability demonstrates a classic buffer overflow pattern where insufficient bounds checking allows access beyond allocated memory regions, potentially enabling attackers to read sensitive data from adjacent memory locations or write to unauthorized memory addresses. The ATT&CK framework categorizes this as a privilege escalation technique through memory corruption, specifically mapping to T1068 which involves exploiting vulnerabilities in legitimate programs to gain elevated privileges.
The operational impact of CVE-2021-4008 extends beyond immediate system stability concerns to encompass broader security implications for graphical environments and desktop systems. Organizations relying on X11-based graphical interfaces face potential risks including unauthorized access to graphical sessions, data corruption in rendering operations, and complete system service disruption. The vulnerability affects systems where X server processes are accessible to untrusted users or networks, making it particularly dangerous in multi-user environments or shared computing platforms. System administrators must consider the implications for both local and remote access scenarios, as the flaw can be exploited through network connections to X server instances or through local privilege escalation if an attacker can establish a session with the graphical server. The threat landscape for this vulnerability includes both automated scanning tools and targeted attacks by sophisticated threat actors seeking to exploit the memory safety issue for broader system compromise.
Mitigation strategies for CVE-2021-4008 primarily focus on updating affected xorg-x11-server components to versions 21.1.2 or 1.20.14, which contain the necessary patches to address the out-of-bounds access condition. System administrators should prioritize patching across all affected systems, particularly those with network-accessible X server instances or those handling untrusted graphical input. Additional protective measures include implementing network segmentation to limit access to X server processes, configuring proper access controls through X server configuration files, and monitoring for suspicious X protocol activity that might indicate exploitation attempts. The implementation of address space layout randomization and stack canaries can provide additional defense-in-depth measures, though these are secondary to the primary patching approach. Organizations should also consider disabling unnecessary X server features and implementing strict user access controls to minimize the attack surface. Security monitoring solutions should be configured to detect anomalous X protocol behavior that could indicate exploitation attempts, with particular attention to malformed requests targeting the SProcRenderCompositeGlyphs function. Regular vulnerability assessments and penetration testing should be conducted to ensure comprehensive protection against this and similar memory safety vulnerabilities in graphical server implementations.