CVE-2026-23456: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23456 Overview

A critical out-of-bounds read vulnerability has been identified in the Linux kernel's netfilter connection tracking module for H.323 protocol. The flaw exists in the decode_int() function within nf_conntrack_h323, where insufficient boundary validation allows attackers to trigger a slab-out-of-bounds read of 1-4 bytes when processing malformed H.323/RAS packets.

Critical Impact

This vulnerability allows attackers to read up to 4 bytes of kernel memory beyond allocated buffer boundaries by sending specially crafted H.323/RAS network packets, potentially leading to information disclosure or system instability.

Affected Products

• Linux kernel with netfilter H.323 connection tracking enabled
• Systems processing H.323/RAS protocol traffic
• Network appliances and firewalls using Linux kernel netfilter subsystem

Discovery Timeline

• 2026-04-03 - CVE-2026-23456 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-23456

Vulnerability Analysis

The vulnerability resides in the decode_int() function within the netfilter H.323 connection tracking module (nf_conntrack_h323). When processing the CONS (constructed) case, the function performs a two-stage read operation that lacks proper boundary validation between stages.

The function first calls get_bits(bs, 2) to read a 2-bit length value from the bitstream. This initial operation is properly validated against the buffer boundaries. However, the subsequent call to get_uint(bs, len) reads 1-4 bytes based on the decoded length value without verifying that sufficient bytes remain in the buffer.

This discrepancy creates a window where an attacker can craft a malicious H.323/RAS packet with a truncated buffer that passes the initial 2-bit boundary check but causes the get_uint() function to read beyond the allocated slab memory. The result is a 1-4 byte slab-out-of-bounds read that can expose sensitive kernel memory contents.

Root Cause

The root cause is a missing boundary check between the get_bits() and get_uint() function calls in the CONS case of decode_int(). The existing validation only accounts for the 2 bits required by get_bits(), not the additional 1-4 bytes that get_uint() subsequently reads based on the decoded length value. This incomplete validation allows controlled out-of-bounds memory access when processing malformed packet data.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted H.323/RAS packets to a target system with netfilter H.323 connection tracking enabled. The attack requires network access to deliver malformed packets that will be processed by the vulnerable nf_conntrack_h323 module.

The exploitation flow involves:

1. Crafting an H.323/RAS packet with a truncated data section
2. Setting the length field to indicate more bytes than actually present in the buffer
3. Sending the packet to a system processing H.323 traffic through netfilter
4. The kernel's connection tracking module processes the packet, triggering the OOB read

The fix adds a boundary check for len bytes after get_bits() returns and before get_uint() is called, ensuring the buffer contains sufficient data for the complete read operation.

Detection Methods for CVE-2026-23456

Indicators of Compromise

• Kernel log messages indicating slab-out-of-bounds access in netfilter modules
• Unexpected system crashes or kernel panics when processing H.323 traffic
• KASAN (Kernel Address Sanitizer) reports showing OOB reads in nf_conntrack_h323

Detection Strategies

• Monitor kernel logs for memory access violations related to netfilter or connection tracking subsystems
• Deploy network intrusion detection rules to identify malformed H.323/RAS packets with inconsistent length fields
• Use kernel memory debugging tools (KASAN, KMSAN) to detect runtime boundary violations

Monitoring Recommendations

• Enable kernel auditing for netfilter-related operations on systems processing H.323 traffic
• Implement network traffic analysis to detect anomalous H.323 protocol patterns
• Configure alerting for kernel oops or memory corruption events in production environments

How to Mitigate CVE-2026-23456

Immediate Actions Required

• Apply the kernel security patches from the official kernel.org repositories
• Disable H.323 connection tracking if not required by unloading the nf_conntrack_h323 module
• Implement network-level filtering to block or inspect H.323 traffic at perimeter devices

Patch Information

The Linux kernel developers have released patches to address this vulnerability by adding proper boundary validation. The fix ensures that len bytes are available in the buffer before calling get_uint(bs, len). Multiple patch commits are available for different kernel branches:

• Kernel Patch Overview
• Kernel Security Fix
• Kernel Code Update
• Kernel Improvement Report
• Kernel Change Notification
• Kernel Release Note

Workarounds

• Disable H.323 connection tracking by blacklisting or unloading the nf_conntrack_h323 kernel module
• Implement firewall rules to drop H.323 traffic at network boundaries if the protocol is not required
• Use application-layer gateways to sanitize H.323 traffic before it reaches vulnerable systems

# Disable nf_conntrack_h323 module
sudo modprobe -r nf_conntrack_h323
echo "blacklist nf_conntrack_h323" | sudo tee /etc/modprobe.d/disable-h323-conntrack.conf
sudo update-initramfs -u