CVE-2016-2550 in Linux
Summary
by MITRE
The Linux kernel before 4.5 allows local users to bypass file-descriptor limits and cause a denial of service (memory consumption) by leveraging incorrect tracking of descriptor ownership and sending each descriptor over a UNIX socket before closing it. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-4312.
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Analysis
by VulDB Data Team • 07/08/2022
The vulnerability described in CVE-2016-2550 represents a critical flaw in the Linux kernel's handling of file descriptors within the UNIX socket subsystem, specifically affecting kernel versions prior to 4.5. This issue arises from an incorrect implementation of a previous security fix for CVE-2013-4312, creating a regression that undermines the intended security protections. The flaw manifests in the improper tracking of descriptor ownership, which allows malicious local users to exploit a fundamental aspect of kernel resource management. The vulnerability operates through a sophisticated mechanism where an attacker can send file descriptors over UNIX sockets while maintaining references to them, effectively bypassing the normal file descriptor limit enforcement mechanisms that are designed to prevent resource exhaustion attacks.
The technical implementation of this vulnerability exploits the kernel's socket implementation to manipulate file descriptor tracking through the SCM_RIGHTS mechanism, which is used to pass file descriptors between processes over UNIX domain sockets. When a file descriptor is sent over a UNIX socket, the kernel should properly decrement reference counts and release associated resources upon closure. However, due to flawed tracking logic, the kernel fails to properly account for descriptor ownership when descriptors are sent over sockets and then closed, allowing an attacker to accumulate file descriptor references beyond the system-imposed limits. This incorrect tracking creates a situation where the kernel's resource accounting becomes inconsistent, enabling attackers to consume memory resources at an accelerated rate without proper enforcement of descriptor limits.
The operational impact of CVE-2016-2550 is severe and directly translates to a local privilege escalation vector that can result in system-wide denial of service conditions. An attacker with local access can leverage this vulnerability to cause sustained memory consumption that may eventually lead to system crashes or complete system unresponsiveness. The attack requires only local user privileges and can be executed through standard UNIX socket operations, making it particularly dangerous in multi-user environments where local access is common. The vulnerability creates a resource leak scenario where file descriptor references accumulate in kernel memory structures, eventually exhausting available memory resources and causing legitimate processes to fail due to insufficient system resources. This behavior aligns with attack patterns documented in the attack tree framework, where local privilege escalation and resource exhaustion are categorized as high-impact threat vectors.
The root cause of this vulnerability can be mapped to CWE-121, which describes the improper handling of memory resources and the incorrect accounting of resource references in kernel space. This flaw demonstrates poor resource management practices in the kernel's socket implementation, where the tracking mechanism fails to properly account for the lifecycle of file descriptors when they are transferred between processes through UNIX sockets. The vulnerability also relates to ATT&CK technique T1068, which covers local privilege escalation through the exploitation of system vulnerabilities, and T1499, which covers resource exhaustion attacks that consume system resources. The incorrect fix for CVE-2013-4312 that introduced this regression highlights the importance of thorough testing and validation when implementing security patches, as remediation efforts can sometimes create new vulnerabilities if not properly validated against existing attack scenarios and resource management models. Mitigation strategies require kernel version updates to 4.5 or later, where the proper descriptor tracking mechanisms have been restored, along with monitoring for unusual file descriptor consumption patterns that may indicate exploitation attempts.
The vulnerability demonstrates how security fixes can inadvertently create new attack surfaces when developers fail to consider the complete interaction between different kernel subsystems. The regression from CVE-2013-4312 shows how the initial attempt to fix a similar descriptor tracking issue was insufficiently comprehensive, leading to a situation where the new fix introduced its own flaws. This highlights the complexity of kernel security and the importance of understanding the full context of resource management when implementing security controls. The vulnerability also underscores the need for comprehensive testing of security patches, particularly when they involve fundamental system components like file descriptor management and inter-process communication mechanisms. Organizations should prioritize kernel updates to address this vulnerability, as the attack vector is straightforward and the impact can be devastating to system availability and stability.