CVE-2021-20239 in Linux
Summary
by MITRE • 05/28/2021
A flaw was found in the Linux kernel in versions before 5.4.92 in the BPF protocol. This flaw allows an attacker with a local account to leak information about kernel internal addresses. The highest threat from this vulnerability is to confidentiality.
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Analysis
by VulDB Data Team • 08/09/2025
This vulnerability exists within the Linux kernel's Berkeley Packet Filter implementation and represents a significant information disclosure flaw that affects systems running kernel versions prior to 5.4.92. The BPF subsystem serves as a powerful networking and tracing framework that allows userspace applications to execute sandboxed programs within the kernel, making it a critical component for system monitoring and network packet processing. The flaw specifically enables local attackers to extract kernel virtual memory addresses through improper access controls within the BPF protocol handling mechanisms.
The technical implementation of this vulnerability stems from insufficient validation of BPF program loading operations and inadequate memory access restrictions within the kernel's BPF subsystem. Attackers with local user privileges can exploit this weakness by crafting specific BPF programs that trigger memory disclosure mechanisms, effectively leaking kernel address space information. This type of vulnerability falls under the CWE-200 category of "Information Exposure" and represents a classic case of insufficient access control where unauthorized memory regions are accessible through improper privilege separation. The vulnerability specifically affects the BPF verifier and program loading infrastructure, where kernel addresses are inadvertently exposed during program validation and execution phases.
The operational impact of this vulnerability extends beyond simple information disclosure, as leaked kernel addresses can significantly aid attackers in planning more sophisticated attacks. When an attacker can obtain kernel memory addresses, they gain valuable information for exploitation techniques such as return-oriented programming attacks, kernel address space layout randomization bypasses, and other advanced exploitation methods. The confidentiality threat level is elevated because this information can be used to circumvent kernel security mitigations, particularly those that rely on address space randomization. This vulnerability aligns with ATT&CK technique T1068 which covers "Local Privilege Escalation" and demonstrates how information disclosure can serve as a prerequisite for more serious exploitation vectors.
Mitigation strategies for this vulnerability primarily focus on kernel version updates, as the flaw was resolved in kernel version 5.4.92 and subsequent releases. System administrators should prioritize patching affected systems and implementing proper kernel security configurations. Additional protective measures include restricting local user access to BPF functionality where possible, implementing proper privilege separation, and monitoring for unusual BPF program loading activities. Organizations should also consider implementing kernel hardening techniques such as kernel address space layout randomization and enabling security modules like SELinux or AppArmor to limit the potential impact of successful exploitation attempts. The vulnerability demonstrates the importance of maintaining up-to-date kernel versions and proper security configurations in protecting against information disclosure attacks that can undermine broader system security posture.