CVE-2026-23455: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-23455 Overview

A vulnerability has been identified in the Linux kernel's netfilter connection tracking module for H.323 protocol handling. Specifically, the DecodeQ931() function in nf_conntrack_h323 contains an integer underflow flaw that can lead to an out-of-bounds read condition. When processing UserUserIE (User-to-User Information Element) data, the function reads a 16-bit length value from the packet and decrements it by 1 to skip the protocol discriminator byte. If the encoded length is 0, the decrement operation wraps to -1 (interpreted as a very large unsigned value), which is then passed to DecodeH323_UserInformation(), resulting in an out-of-bounds memory read.

Critical Impact

This vulnerability allows attackers to trigger out-of-bounds memory reads in kernel space by sending specially crafted H.323 packets, potentially leading to information disclosure or system instability.

Affected Products

• Linux Kernel (multiple stable branches affected)
• Systems with nf_conntrack_h323 module loaded
• Network devices processing H.323 protocol traffic

Discovery Timeline

• April 3, 2026 - CVE-2026-23455 published to NVD
• April 7, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23455

Vulnerability Analysis

This vulnerability resides in the netfilter connection tracking subsystem responsible for handling H.323 Voice over IP signaling protocol. The flaw occurs within the DecodeQ931() function when processing Q.931 messages, which are part of the H.323 call signaling protocol suite.

The vulnerability stems from insufficient validation of length fields in incoming network packets. When the UserUserIE code path processes a packet, it extracts a 16-bit length value and performs an arithmetic operation (decrement by 1) without first verifying that the value is greater than zero. This creates a classic integer underflow condition where decrementing zero results in a wrap-around to the maximum unsigned integer value.

When this corrupted length value is passed to DecodeH323_UserInformation(), the decoder attempts to read memory far beyond the actual packet buffer boundaries, causing an out-of-bounds read. This can expose sensitive kernel memory contents or trigger a kernel panic depending on what memory is accessed.

Root Cause

The root cause is a missing boundary check in the DecodeQ931() function. The code assumes the length field will always be at least 1 (to account for the protocol discriminator byte), but malformed or malicious packets can contain a zero length. The fix adds a validation check to ensure the length value is positive after the decrement operation before proceeding with decoding.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted H.323 Q.931 signaling packets to a target system that has the nf_conntrack_h323 kernel module loaded. The attack requires network access to send packets that will be processed by the netfilter connection tracking system.

The exploitation does not require authentication or user interaction. The attacker constructs a packet with a UserUserIE information element containing a zero-length field. When the kernel processes this packet through the connection tracking module, the integer underflow triggers the out-of-bounds read condition.

Detection Methods for CVE-2026-23455

Indicators of Compromise

• Kernel panic or system crash events related to netfilter or H.323 processing
• Unusual H.323 signaling traffic patterns with malformed packet structures
• Kernel log entries indicating memory access violations in nf_conntrack_h323 module
• Increased network traffic on H.323 signaling ports (TCP 1720, UDP 1719)

Detection Strategies

• Monitor kernel logs (dmesg) for memory access violations or OOPS messages referencing nf_conntrack_h323 or DecodeQ931
• Implement network intrusion detection rules to identify malformed H.323 packets with zero-length UserUserIE fields
• Deploy packet inspection to detect anomalous Q.931 signaling messages
• Use kernel debugging tools to trace nf_conntrack_h323 module behavior

Monitoring Recommendations

• Enable kernel auditing for netfilter module activities
• Configure network monitoring to alert on unusual H.323 protocol traffic patterns
• Implement system stability monitoring to detect kernel panics or crashes
• Review firewall logs for connection attempts to H.323 related ports from untrusted sources

How to Mitigate CVE-2026-23455

Immediate Actions Required

• Update the Linux kernel to a patched version containing the fix
• If H.323 connection tracking is not required, unload the nf_conntrack_h323 module
• Block H.323 signaling traffic (TCP 1720, UDP 1719) at network perimeter if not needed
• Monitor affected systems for signs of exploitation attempts

Patch Information

The Linux kernel development team has released patches across multiple stable branches to address this vulnerability. The fix adds a validation check to ensure the length field is positive after the decrement operation in DecodeQ931().

Available kernel patches:

• Linux Kernel Commit 495e97a
• Linux Kernel Commit 633e8f8
• Linux Kernel Commit 9d00fe7
• Linux Kernel Commit b652b05
• Linux Kernel Commit f173d0f
• Linux Kernel Commit f5e4f4e

Workarounds

• Disable the H.323 connection tracking helper by unloading the nf_conntrack_h323 module if not required for operations
• Use firewall rules to restrict H.323 traffic to trusted sources only
• Implement network segmentation to isolate systems that must process H.323 protocol traffic
• Deploy network-based intrusion prevention to filter malicious H.323 packets before they reach vulnerable systems

# Configuration example
# Unload the vulnerable module if H.323 tracking is not needed
sudo modprobe -r nf_conntrack_h323

# Blacklist the module to prevent automatic loading
echo "blacklist nf_conntrack_h323" | sudo tee /etc/modprobe.d/blacklist-h323.conf

# Block H.323 signaling ports at the firewall if not required
sudo iptables -A INPUT -p tcp --dport 1720 -j DROP
sudo iptables -A INPUT -p udp --dport 1719 -j DROP