CVE-2026-23457: Linux Kernel Integer Truncation Vulnerability

CVE-2026-23457 Overview

A numeric truncation vulnerability has been identified in the Linux kernel's netfilter nf_conntrack_sip module. The sip_help_tcp() function parses the SIP Content-Length header using simple_strtoul(), which returns an unsigned long value, but incorrectly stores the result in an unsigned int variable (clen). On 64-bit systems, Content-Length values exceeding UINT_MAX (4,294,967,295) are silently truncated, causing the SIP message parser to miscalculate message boundaries and potentially process trailing TCP segment data as separate SIP messages.

Critical Impact

Attackers can craft malicious SIP packets with oversized Content-Length values to cause message boundary confusion, potentially leading to parser exploitation and processing of arbitrary data through the SDP parser.

Affected Products

• Linux kernel with nf_conntrack_sip module enabled
• 64-bit Linux systems processing SIP traffic via netfilter connection tracking
• Network appliances and firewalls using Linux kernel SIP ALG functionality

Discovery Timeline

• 2026-04-03 - CVE CVE-2026-23457 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-23457

Vulnerability Analysis

The vulnerability resides in the sip_help_tcp() function within the netfilter SIP connection tracking helper module. When processing SIP messages over TCP, the function extracts the Content-Length header value to determine message boundaries within a TCP segment. The parsing is performed using simple_strtoul(), which on 64-bit systems returns an 8-byte unsigned long value. However, the result is assigned to clen, declared as an unsigned int (4 bytes).

This type mismatch creates a truncation condition where Content-Length values larger than 2^32 are silently reduced to their lower 32 bits. For example, a Content-Length of 4294967328 (which is 2^32 + 32) becomes 32 after truncation. The parser then incorrectly calculates the end of the current SIP message, treating subsequent data in the TCP segment as a new SIP message and routing it through the SDP parser for processing.

Root Cause

The root cause is an integer type mismatch between the return type of simple_strtoul() (unsigned long, 64-bit on LP64 systems) and the storage variable clen (unsigned int, 32-bit). This allows numeric values to be silently truncated without validation, leading to incorrect message boundary calculations in the TCP reassembly logic.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted SIP packets over TCP with Content-Length header values exceeding UINT_MAX. The attack flow involves:

The malicious SIP packet contains a Content-Length header set to a value like 4294967328. When processed by the kernel's SIP connection tracking helper, the value truncates to 32. The parser then believes the SIP message body is only 32 bytes, causing it to interpret any subsequent data in the TCP segment as a new SIP message. This trailing data is then processed through the SDP parser, potentially allowing an attacker to influence connection tracking state or bypass security policies.

Detection Methods for CVE-2026-23457

Indicators of Compromise

• SIP traffic containing abnormally large Content-Length header values (exceeding 4,294,967,295)
• Unexpected SDP parsing activity or connection tracking entries for malformed SIP sessions
• Kernel log messages indicating SIP helper parsing errors or anomalies

Detection Strategies

• Monitor network traffic for SIP packets with Content-Length values exceeding reasonable bounds (legitimate SIP messages rarely exceed a few megabytes)
• Implement deep packet inspection rules to flag SIP Content-Length headers with values above UINT_MAX
• Deploy intrusion detection signatures matching the pattern of oversized Content-Length values in SIP/SDP traffic

Monitoring Recommendations

• Enable verbose logging for the nf_conntrack_sip module to capture parsing anomalies
• Monitor connection tracking table for unusual SIP-related entries
• Implement network flow analysis to detect abnormal SIP traffic patterns

How to Mitigate CVE-2026-23457

Immediate Actions Required

• Apply the kernel patches from the stable kernel tree immediately
• If patching is not immediately possible, consider temporarily disabling the SIP connection tracking helper if not required
• Implement network-level filtering to reject SIP packets with excessively large Content-Length values

Patch Information

The Linux kernel maintainers have released patches that change the clen variable type from unsigned int to unsigned long, matching the return type of simple_strtoul(). Additionally, the fix adds validation to reject Content-Length values that exceed the remaining TCP payload length.

Multiple patch commits are available across stable kernel branches:

• Kernel Commit 528b4509c9df
• Kernel Commit 75fcaee5170e
• Kernel Commit 865dba58958c
• Kernel Commit b75209debb9a
• Kernel Commit d4f17256544c
• Kernel Commit fbce58e719a1

Workarounds

• Disable the SIP connection tracking helper if SIP ALG functionality is not required: modprobe -r nf_conntrack_sip
• Implement firewall rules to filter SIP traffic before it reaches the connection tracking module
• Use external SIP-aware proxies or session border controllers that perform proper header validation

# Disable SIP connection tracking helper
modprobe -r nf_conntrack_sip

# Prevent automatic loading of the SIP helper
echo "install nf_conntrack_sip /bin/false" >> /etc/modprobe.d/disable-sip-helper.conf

# Verify the module is not loaded
lsmod | grep nf_conntrack_sip