CVE-2026-31495: Linux Kernel Netfilter Validation Flaw

CVE-2026-31495 Overview

A vulnerability has been identified in the Linux kernel's netfilter subsystem, specifically within the ctnetlink code paths. The issue stems from inadequate range and mask validations that were performed manually rather than through netlink policy annotations. This allows the netlink core to potentially accept invalid values, which could lead to undefined behavior when processing certain TCP connection tracking parameters.

The vulnerability affects multiple netlink attribute handlers including TCP state values, TCP window scale parameters, and filter/expect flags. Most notably, the CTA_PROTOINFO_TCP_WSCALE_ORIGINAL/REPLY attributes accepted values from 0-255 through the ctnetlink path, while the normal TCP option parsing path already clamps to the maximum value of 14 (TCP_MAX_WSCALE). This discrepancy can cause undefined behavior when the invalid value is used as a u32 shift count.

Critical Impact

Undefined behavior in kernel netfilter processing when invalid TCP window scale values (0-255) are passed via ctnetlink, potentially causing kernel instability or security issues.

Affected Products

• Linux Kernel (netfilter/ctnetlink subsystem)

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31495 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31495

Vulnerability Analysis

The vulnerability exists in the ctnetlink module, which is responsible for managing connection tracking entries via netlink sockets. The root issue is that manual range and mask validations were being performed inconsistently, allowing certain invalid values to pass through to kernel functions where they could cause undefined behavior.

The fix involves replacing these manual checks with netlink policy annotations, enabling the netlink core to reject invalid values early in the processing pipeline and generate proper extended acknowledgment (extack) errors for invalid input. This is a defense-in-depth improvement that moves validation closer to the input boundary.

Four specific areas were addressed:

• CTA_PROTOINFO_TCP_STATE: Values greater than TCP_CONNTRACK_SYN_SENT2 are now rejected at the policy level
• CTA_PROTOINFO_TCP_WSCALE_ORIGINAL/REPLY: Values are now constrained to 0-14 (TCP_MAX_WSCALE)
• CTA_FILTER_ORIG_FLAGS/REPLY_FLAGS: Now use NLA_POLICY_MASK with CTA_FILTER_F_ALL
• CTA_EXPECT_FLAGS: Now use NLA_POLICY_MASK with a new NF_CT_EXPECT_MASK define

Root Cause

The root cause is improper input validation in the ctnetlink subsystem. The code relied on manual range checks that were inconsistent with the constraints enforced elsewhere in the kernel. Specifically, the TCP window scale value validation through ctnetlink allowed values up to 255, while the standard TCP option parsing path correctly clamped values to 14. When a value greater than 14 is used as a shift count for a 32-bit unsigned integer, the behavior is undefined according to the C standard, potentially leading to unpredictable kernel behavior.

Attack Vector

An attacker with the ability to send netlink messages to the kernel's connection tracking subsystem could potentially exploit this vulnerability. This typically requires local access or specific privileges to interact with the netfilter netlink interface. The attacker could craft malformed netlink messages containing out-of-range values for TCP window scale parameters, TCP states, or filter/expect flags.

The attack surface is primarily local, as netlink sockets are not directly accessible from remote networks. However, in containerized environments or systems where unprivileged users can access netlink sockets, the risk may be elevated.

The vulnerability manifests when invalid attribute values bypass the manual validation checks and reach kernel functions that perform arithmetic operations (such as bit shifts) with these values. For detailed technical information, refer to the kernel stable commit 8f15b50.

Detection Methods for CVE-2026-31495

Indicators of Compromise

• Kernel log messages indicating netfilter or ctnetlink errors or warnings
• Unexpected kernel crashes or instability related to connection tracking operations
• Anomalous netlink socket activity from unprivileged processes
• System logs showing rejected netlink messages with extack errors after patching

Detection Strategies

• Monitor kernel logs for netfilter-related error messages using dmesg or syslog aggregation
• Implement audit rules to track netlink socket operations, particularly those targeting NETLINK_NETFILTER
• Deploy endpoint detection solutions capable of monitoring kernel subsystem interactions
• Use SentinelOne's behavioral AI engine to detect anomalous kernel-level activities

Monitoring Recommendations

• Enable kernel auditing for netlink socket system calls (socket, sendmsg, recvmsg with netlink family)
• Configure centralized logging to capture kernel messages related to netfilter and connection tracking
• Implement alerting for any kernel panics or oops messages that reference ctnetlink or netfilter code paths
• Monitor for unusual patterns in connection tracking table modifications

How to Mitigate CVE-2026-31495

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the netlink policy range checks
• Review and restrict access to netlink sockets where possible, limiting which users and processes can interact with NETLINK_NETFILTER
• Monitor systems for any unusual behavior related to connection tracking operations
• Consider temporarily disabling ctnetlink functionality if not required for operations

Patch Information

Multiple stable kernel patches have been released to address this vulnerability. The fix implements proper netlink policy annotations to enforce range checks at the policy level rather than through manual validation:

• Kernel Stable Commit 2ef7130
• Kernel Stable Commit 435b576
• Kernel Stable Commit 45c33e7
• Kernel Stable Commit 4f7d25f
• Kernel Stable Commit 675c913
• Kernel Stable Commit 8f15b50
• Kernel Stable Commit c6cb41e
• Kernel Stable Commit fcec5ce

Apply the appropriate patch for your kernel version following your distribution's kernel update procedures.

Workarounds

• Restrict access to the NETLINK_NETFILTER protocol family using network namespaces or security policies
• Use kernel module parameters or compile-time options to disable ctnetlink if connection tracking via netlink is not required
• Implement SELinux or AppArmor policies to limit which processes can interact with netfilter netlink sockets
• Consider using network namespaces to isolate connection tracking functionality

# Check current kernel version
uname -r

# List available kernel updates (Debian/Ubuntu)
apt list --upgradable | grep linux-image

# List available kernel updates (RHEL/CentOS)
yum check-update kernel

# View loaded netfilter modules
lsmod | grep nf_conntrack

# Check if ctnetlink is loaded
lsmod | grep nf_conntrack_netlink