CVE-2023-31248 in Linux
Summary
by MITRE • 07/05/2023
Linux Kernel nftables Use-After-Free Local Privilege Escalation Vulnerability; `nft_chain_lookup_byid()` failed to check whether a chain was active and CAP_NET_ADMIN is in any user or network namespace
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Analysis
by VulDB Data Team • 11/12/2025
The vulnerability identified as CVE-2023-31248 represents a critical use-after-free flaw within the Linux kernel's nftables subsystem that enables local privilege escalation. This issue stems from improper validation within the `nft_chain_lookup_byid()` function which fails to verify whether a chain is actively in use before proceeding with operations. The flaw exists in the kernel's netfilter framework where nftables manages packet filtering and manipulation rules. When a malicious user with access to a network namespace can manipulate chain objects, the absence of proper active state checking creates an exploitable condition where freed memory can be accessed and potentially overwritten.
The technical exploitation of this vulnerability occurs through careful manipulation of nftables chain objects within network namespaces. An attacker can create and destroy chain objects rapidly while simultaneously attempting to access the freed memory locations through the vulnerable lookup function. This particular flaw is classified under CWE-416 as a use-after-free condition, where memory that has been freed is accessed again. The vulnerability specifically affects systems running Linux kernels where the nftables subsystem is enabled and where users have the capability to create and manage network namespaces. The issue is particularly dangerous because it requires only local access and can be exploited by any user who has the CAP_NET_ADMIN capability within a network namespace, which is commonly available in containerized environments and user namespaces.
The operational impact of this vulnerability extends beyond simple privilege escalation as it can be leveraged to gain root access on affected systems. This makes it particularly concerning for containerized environments where multiple users may share the same host system and where network namespaces are commonly used for isolation. The exploit can be automated and does not require specialized knowledge of kernel internals beyond understanding how to manipulate nftables objects and network namespaces. Organizations running Linux systems with nftables enabled, particularly those hosting containerized applications or multi-tenant environments, face significant risk from this vulnerability. The attack vector is relatively simple as it only requires local access and the ability to create network namespaces, making it accessible to a wide range of potential attackers.
Mitigation strategies for CVE-2023-31248 should focus on immediate kernel updates to patched versions that address the use-after-free condition in nftables. System administrators should prioritize applying security patches from their respective Linux distribution vendors as soon as available. Additionally, implementing proper namespace isolation and limiting CAP_NET_ADMIN capabilities to only essential processes can reduce the attack surface. Organizations should consider disabling nftables if it is not actively required for their network filtering needs, as this removes the attack vector entirely. Monitoring for unauthorized network namespace creation and nftables manipulation should be implemented as part of security operations centers' detection capabilities. The vulnerability also highlights the importance of proper input validation and memory management practices within kernel subsystems, reinforcing the need for comprehensive code review processes and formal verification techniques for critical kernel components. This issue demonstrates how seemingly minor validation flaws in kernel subsystems can have significant security implications and underscores the importance of maintaining up-to-date security patches across all system components.