CVE-2026-53002: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-53002 Overview

CVE-2026-53002 affects the Linux kernel netfilter connection tracking subsystem. The vulnerability resides in the SIP (Session Initiation Protocol) application-layer helper, specifically in mangle_content_len() invoked from nf_nat_sdp_session() during SDP (Session Description Protocol) processing. The function used sprintf() without sufficient buffer guarantees, producing a stack out-of-bounds write detected by KASAN (Kernel Address Sanitizer). The fix replaces sprintf() with scnprintf() and increases the destination buffer size in mangle_content_len(). The issue impacts kernels that process SIP/SDP traffic through the conntrack helper during NAT (Network Address Translation) traversal.

Critical Impact
A crafted SIP/SDP message traversing a Linux NAT gateway can trigger a kernel stack out-of-bounds write in the netfilter conntrack SIP helper, leading to memory corruption or denial of service.

Affected Products

Linux kernel netfilter nf_conntrack_sip / nf_nat_sip modules
Linux kernel versions prior to the commits referenced below across multiple stable branches
Systems running conntrack SIP helpers behind NAT for VoIP traffic

Discovery Timeline

2026-06-24 - CVE-2026-53002 published to NVD
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-53002

Vulnerability Analysis

The Linux kernel netfilter conntrack subsystem includes an application-layer gateway (ALG) for SIP traffic. When a SIP INVITE carries an SDP payload, the kernel may need to rewrite the Content-Length field to reflect NAT-modified payloads. The original implementation called sprintf() into a fixed-size stack buffer to format the new content length value. Because sprintf() does not bound its output, a sufficiently large value caused the formatted string to exceed the buffer, producing a stack out-of-bounds write of one byte beyond the buffer boundary.

KASAN reported the bug with the following call chain: vsnprintf → sprintf → mangle_content_len → nf_nat_sdp_session → process_sdp → process_invite_request → process_sip_msg → sip_help_tcp → nf_confirm. The corruption occurs in softirq context during packet processing, making the impact relevant to any host forwarding SIP traffic through the kernel NAT helpers.

Root Cause

The root cause is unsafe use of sprintf() against a stack buffer in mangle_content_len() without verifying the formatted output fits the destination. This is classified as an Out-of-Bounds Write [CWE-787] in kernel space. The patch replaces sprintf() with scnprintf(), which truncates safely and returns the number of bytes actually written, and also increases the destination buffer size as a defense-in-depth measure.

Attack Vector

An attacker capable of sending or routing SIP traffic through a Linux host running the nf_conntrack_sip and nf_nat_sip modules can craft SIP/SDP messages that drive mangle_content_len() to format values exceeding the original buffer. Because conntrack SIP helpers are commonly enabled on edge NAT routers and VoIP gateways, network-reachable adversaries on either side of the NAT can reach the vulnerable code path. The vulnerability is triggered automatically by the kernel during normal SIP ALG processing — no authentication is required.

No public proof-of-concept exploit is available. See the upstream commits for technical details on the fix, including Linux Kernel Commit 1c9fb8a and Linux Kernel Commit 2f793ba.

Detection Methods for CVE-2026-53002

Indicators of Compromise

KASAN reports referencing stack-out-of-bounds in vsnprintf with a call stack including mangle_content_len, nf_nat_sdp_session, and sip_help_tcp
Kernel panics, oopses, or stack traces in dmesg originating from nf_conntrack_sip or nf_nat_sip modules under SIP traffic
Unexpected restarts of network-facing Linux gateways handling VoIP traffic

Detection Strategies

Audit lsmod output for loaded nf_conntrack_sip and nf_nat_sip modules on edge and NAT systems
Correlate kernel ring buffer messages with inbound SIP traffic flows to identify malformed SDP payloads
Inspect SIP Content-Length header values in passing traffic for abnormally large or malformed integers

Monitoring Recommendations

Forward dmesg and /var/log/kern.log to a centralized log store and alert on KASAN, oops, or BUG entries referencing netfilter modules
Monitor SIP/SDP traffic at the network perimeter for anomalous message sizes and malformed headers
Track kernel version inventory across Linux hosts to identify systems still running unpatched builds

How to Mitigate CVE-2026-53002

Immediate Actions Required

Apply the upstream kernel patches referenced in the commit list to all affected Linux systems
Where patching is not immediately possible, unload or blacklist the nf_conntrack_sip and nf_nat_sip modules on hosts that do not require SIP ALG functionality
Identify all Linux NAT gateways, firewalls, and VoIP infrastructure handling SIP traffic and prioritize them for remediation

Patch Information

The fix is distributed across multiple stable branches. Relevant upstream commits include Linux Kernel Commit 1c9fb8a, Linux Kernel Commit 2f793ba, Linux Kernel Commit 6bbf829, Linux Kernel Commit 6e7066b, Linux Kernel Commit 8e3be0d, Linux Kernel Commit a8e0a32, Linux Kernel Commit ab64e61, and Linux Kernel Commit c08ff52. The patch replaces sprintf() with scnprintf() in mangle_content_len() and increases the destination buffer size.

Workarounds

Unload the SIP conntrack helpers if not required: rmmod nf_nat_sip nf_conntrack_sip
Blacklist the modules via /etc/modprobe.d/ to prevent automatic load on reboot
Filter or rate-limit untrusted SIP traffic at the network edge before it reaches the conntrack helper

bash

# Configuration example: disable SIP conntrack helpers until patched
echo 'blacklist nf_conntrack_sip' | sudo tee /etc/modprobe.d/disable-sip-helper.conf
echo 'blacklist nf_nat_sip' | sudo tee -a /etc/modprobe.d/disable-sip-helper.conf
sudo rmmod nf_nat_sip 2>/dev/null
sudo rmmod nf_conntrack_sip 2>/dev/null

# Verify modules are not loaded
lsmod | grep -E 'nf_(nat|conntrack)_sip'