CVE-2019-13233 in Linux
Summary
by MITRE
In arch/x86/lib/insn-eval.c in the Linux kernel before 5.1.9, there is a use-after-free for access to an LDT entry because of a race condition between modify_ldt() and a #BR exception for an MPX bounds violation.
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Analysis
by VulDB Data Team • 08/11/2025
The vulnerability identified as CVE-2019-13233 represents a critical use-after-free condition within the Linux kernel's x86 architecture implementation, specifically affecting the insn-eval.c file in kernel versions prior to 5.1.9. This flaw manifests as a race condition between the modify_ldt() system call and a #BR exception handler for MPX bounds violations, creating a window where memory access patterns can lead to unauthorized code execution or system instability. The issue occurs within the kernel's handling of Local Descriptor Table entries, which are fundamental components for memory management and privilege level enforcement in x86 systems. The race condition allows malicious actors to exploit timing dependencies between these two kernel operations, potentially leading to arbitrary code execution with elevated privileges.
The technical implementation of this vulnerability stems from improper synchronization mechanisms within the kernel's memory management subsystem. When modify_ldt() is invoked to modify Local Descriptor Table entries, concurrent MPX bounds violation exceptions can trigger a scenario where freed memory structures are accessed after deallocation. This use-after-free condition falls under CWE-416, specifically the "Use After Free" vulnerability category, where memory that has been freed is still accessed by the system. The MPX (Memory Protection Extensions) technology, designed to prevent buffer overflows by enforcing bounds checking, paradoxically becomes a vector for exploitation when combined with the flawed race condition in LDT handling. The vulnerability exploits the timing gap between when an LDT entry is freed during a modify_ldt() operation and when the #BR exception handler processes the MPX violation, creating a window of opportunity for attackers to manipulate memory contents.
The operational impact of CVE-2019-13233 extends beyond simple system crashes or denial of service conditions, as it provides a potential path for privilege escalation attacks. Attackers can leverage this vulnerability to execute arbitrary code with kernel-level privileges, effectively compromising the entire system's security posture. The exploitability of this flaw is enhanced by its potential to be combined with other kernel vulnerabilities, as the use-after-free condition can be chained with existing exploitation techniques to bypass modern security mitigations such as stack canaries, ASLR, and SMEP/SMAP protections. The vulnerability affects systems running Linux kernel versions 5.1.8 and earlier, making it particularly concerning for production environments that have not yet been patched, as it can be exploited to gain root access and establish persistent backdoors within affected systems.
Mitigation strategies for CVE-2019-13233 primarily focus on applying the official kernel patches released in version 5.1.9 and subsequent releases, which address the race condition through improved synchronization mechanisms between modify_ldt() and MPX exception handling. System administrators should prioritize patching affected systems, particularly those running older kernel versions, as the vulnerability can be exploited remotely in certain configurations. Additional defensive measures include disabling MPX functionality if not required by the system, implementing kernel module signing to prevent unauthorized code loading, and monitoring for suspicious system behavior that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper race condition handling in kernel code and reinforces the need for comprehensive testing of concurrent operations in security-critical subsystems. Organizations should also consider implementing runtime protections and anomaly detection systems to identify potential exploitation attempts, as the use-after-free condition can sometimes be leveraged in combination with other attack vectors to create more sophisticated exploitation chains that align with ATT&CK framework techniques for privilege escalation and persistence.