CVE-2026-52986: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-52986 Overview

CVE-2026-52986 is a Linux kernel vulnerability in the nf_conntrack_sip netfilter module. The flaw stems from unsafe use of simple_strtoul() to parse Session Initiation Protocol (SIP) port values from sk_buff (skb) data. Because simple_strtoul() assumes NUL-terminated strings, the parser dereferenced pointers without bounds checks after sip_parse_addr(), operating on non-NUL-terminated network buffer data.

The vulnerability affects three functions in the SIP connection tracking helper: epaddr_len(), ct_sip_parse_header_uri(), and ct_sip_parse_request(). Attackers sending malformed SIP packets could trigger out-of-bounds reads during kernel-side packet inspection.

Critical Impact
A remote attacker can send crafted SIP traffic to a Linux host using nf_conntrack_sip to trigger out-of-bounds reads in kernel memory during connection tracking.

Affected Products

Linux kernel versions containing the nf_conntrack_sip netfilter helper prior to the upstream fix
Linux distributions enabling SIP connection tracking (nf_conntrack_sip module) on edge or NAT gateways
Stable kernel branches referenced by the eight patch commits in git.kernel.org

Discovery Timeline

2026-06-24 - CVE-2026-52986 published to the National Vulnerability Database (NVD)
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-52986

Vulnerability Analysis

The nf_conntrack_sip helper parses SIP messages to track related media flows and apply Network Address Translation (NAT). Port values inside SIP URIs and headers were extracted using simple_strtoul(), a kernel helper that expects a NUL-terminated input string. Network packet data in skb buffers is not NUL-terminated, so the parser could read past the end of the valid buffer region.

The upstream fix introduces a new sip_parse_port() helper that validates each digit against an explicit buffer limit. The helper rejects ports that reach the buffer boundary without a terminating character, bails out when the parsed integer exceeds UINT_MAX, and rejects overly long digit sequences such as sip:10.0.0.1:005060 padded with thousands of leading zeros. The fix also forces base-10 parsing in ct_sip_parse_numerical_param() for expires= and rport= values, and restricts accepted ports in nf_nat_sip.c to the 1024–65535 range.

Root Cause

The root cause is improper input validation [CWE-20] combined with an out-of-bounds read [CWE-125]. The code path trusted simple_strtoul() to stop at a non-digit character, but on a non-NUL-terminated skb buffer the function continues reading kernel memory until it encounters a non-digit byte by chance.

Attack Vector

An attacker sends a crafted SIP message — for example a REGISTER or INVITE containing a malformed URI or header — to a Linux system that loads the nf_conntrack_sip helper. The helper processes the packet in softirq context, and the unbounded read occurs before any application-layer validation. The vulnerability is reachable over the network without authentication wherever SIP connection tracking is enabled on a router, firewall, or VoIP gateway.

No public proof-of-concept code is available. See the Linux Kernel Commit 8cf6809 for the canonical patch implementing sip_parse_port().

Detection Methods for CVE-2026-52986

Indicators of Compromise

Unexpected kernel warnings or KASAN out-of-bounds read reports referencing ct_sip_parse_header_uri, ct_sip_parse_request, or epaddr_len in dmesg
SIP packets to UDP/TCP port 5060 containing malformed port fields, excessively long digit runs, or ports outside the 1–65535 range
Conntrack helper failures or sudden drops in established SIP-related conntrack entries on edge gateways

Detection Strategies

Enable kernel address sanitizer (KASAN) on test kernels and replay captured SIP traffic to surface out-of-bounds reads in nf_conntrack_sip
Inspect SIP traffic at the network perimeter for URIs containing non-numeric port characters, leading-zero padded ports, or values greater than 65535
Correlate kernel oops and WARN events with inbound SIP flow records to identify probing attempts

Monitoring Recommendations

Forward dmesg and kern.log to a centralized logging pipeline and alert on stack traces referencing netfilter SIP symbols
Monitor conntrack table churn and helper assignment counts via conntrack -L and /proc/net/nf_conntrack
Track inventory of hosts with the nf_conntrack_sip module loaded using lsmod or kernel auditing tooling

How to Mitigate CVE-2026-52986

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the NVD entry and rebuild or update affected kernels
If patching is not immediately possible, unload the helper with modprobe -r nf_conntrack_sip on systems that do not require SIP connection tracking
Restrict inbound SIP traffic at the network edge to known peers and rate-limit malformed SIP messages

Patch Information

The fix is distributed across multiple stable-tree commits. Refer to the canonical patches: Linux Kernel Commit 523762e, Linux Kernel Commit 7df9863, Linux Kernel Commit 8cd0358, Linux Kernel Commit 8cf6809, Linux Kernel Commit 9c6afcb, Linux Kernel Commit 9f69c32, Linux Kernel Commit b3264c9, and Linux Kernel Commit ea2ecd2.

Workarounds

Disable automatic SIP helper assignment by setting net.netfilter.nf_conntrack_helper=0 and removing explicit CT helper sip rules
Blacklist the module via /etc/modprobe.d/ on systems that do not perform SIP NAT or tracking
Filter SIP traffic upstream with a Session Border Controller (SBC) or application-aware firewall that enforces RFC 3261 compliance

bash

# Configuration example
# Unload the vulnerable helper
modprobe -r nf_conntrack_sip

# Persistently blacklist the module
echo 'blacklist nf_conntrack_sip' | sudo tee /etc/modprobe.d/blacklist-nf_conntrack_sip.conf

# Disable automatic helper assignment
sudo sysctl -w net.netfilter.nf_conntrack_helper=0