CVE-2026-23392: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23392 Overview

CVE-2026-23392 is a use-after-free vulnerability in the Linux kernel's netfilter nf_tables subsystem. The vulnerability exists in the flowtable handling code where improper synchronization during error path cleanup can lead to memory being accessed after it has been freed. Specifically, when unregistering hooks from the error path, the code fails to call synchronize_rcu() before releasing the flowtable, allowing already-registered hooks that reference this flowtable to access freed memory through the packet path and nfnetlink_hook control plane.

Critical Impact

This use-after-free vulnerability in the Linux kernel's netfilter subsystem could allow attackers to trigger memory corruption, potentially leading to kernel crashes or arbitrary code execution in kernel context.

Affected Products

• Linux kernel (versions with vulnerable nf_tables flowtable implementation)
• Systems using netfilter/nftables for packet filtering
• Linux distributions with affected kernel versions

Discovery Timeline

• 2026-03-25 - CVE CVE-2026-23392 published to NVD
• 2026-03-25 - Last updated in NVD database

Technical Details for CVE-2026-23392

Vulnerability Analysis

This vulnerability was uncovered by KASAN (Kernel Address Sanitizer), which reported a use-after-free condition in the nfnetlink_hook path when dumping hooks. The issue arises in the error handling code path of the nf_tables flowtable implementation.

When a flowtable registration fails—which can occur when the maximum number of hooks is reached or when hardware offload setup fails—the error path unregisters the hooks but releases the flowtable memory without first calling synchronize_rcu(). This creates a race condition where another hook that already references this flowtable may still be registered and actively processing packets, leading to use-after-free access.

The error path is triggered under rare conditions, specifically:

• When reaching the maximum number of hooks
• When failing to set up hardware offload
• When a device hook check detects an already-used device hook by a different flowtable, resulting in EEXIST at this late stage

Root Cause

The root cause is missing RCU (Read-Copy-Update) synchronization in the error path of flowtable hook registration. The code unregisters hooks but immediately proceeds to free the flowtable memory without waiting for the RCU grace period to complete. This violates the RCU memory reclamation model where readers may still hold references to the object being freed.

The fix adds a synchronize_rcu() call after unregistering the hooks in the error path, ensuring all RCU-protected readers have completed their critical sections before the flowtable memory is released.

Attack Vector

An attacker with local access and sufficient privileges to manipulate netfilter/nftables configuration could potentially trigger this vulnerability by:

1. Creating flowtable configurations that exercise the error paths (e.g., exceeding hook limits or forcing hardware offload failures)
2. Racing the flowtable teardown with packet processing or hook dumping operations via nfnetlink_hook
3. Exploiting the resulting use-after-free to corrupt kernel memory or achieve code execution

The vulnerability requires local access and typically root or CAP_NET_ADMIN capabilities to configure nftables flowtables. The technical complexity of reliably triggering and exploiting this race condition is significant, but successful exploitation could result in privilege escalation or kernel code execution.

Detection Methods for CVE-2026-23392

Indicators of Compromise

• Kernel crashes or panics with stack traces involving nfnetlink_hook, nf_tables, or flowtable-related functions
• KASAN reports indicating use-after-free in netfilter subsystem code paths
• Unexpected system instability when manipulating nftables flowtable configurations

Detection Strategies

• Enable KASAN (Kernel Address Sanitizer) on development and testing systems to detect use-after-free conditions
• Monitor kernel logs for netfilter-related errors, particularly those involving flowtable registration failures
• Deploy kernel live patching solutions to detect and protect against exploitation attempts

Monitoring Recommendations

• Configure kernel crash dump collection to capture and analyze kernel panics for forensic investigation
• Implement audit logging for nftables configuration changes using auditd rules for nft commands
• Use SentinelOne's kernel-level monitoring to detect anomalous netfilter subsystem behavior

How to Mitigate CVE-2026-23392

Immediate Actions Required

• Update to a patched Linux kernel version that includes the fix
• Review and audit nftables flowtable configurations on affected systems
• Restrict CAP_NET_ADMIN capabilities to only trusted users and processes
• Consider temporarily disabling flowtable functionality if not required until patching is complete

Patch Information

The Linux kernel development team has released patches to address this vulnerability. The fix ensures that synchronize_rcu() is called after unregistering hooks in the error path before releasing the flowtable memory.

Multiple patch commits are available for different kernel branches:

• Kernel Patch Change 7e3955b
• Kernel Patch Change adee3436
• Kernel Patch Change c8092edb
• Kernel Patch Change d2632de
• Kernel Patch Change d73f4b5
• Kernel Patch Change e78a2dcc

Workarounds

• Limit access to nftables configuration to only essential administrative users
• Avoid using flowtable features in production environments until the kernel is patched
• Implement network namespace isolation to reduce the attack surface for netfilter manipulation
• Use SELinux or AppArmor policies to restrict processes that can modify netfilter rules

# Restrict nftables access to root only
chmod 700 /usr/sbin/nft

# Audit nftables configuration changes
auditctl -w /usr/sbin/nft -p x -k nftables_changes