CVE-2026-31407: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31407 Overview

A vulnerability has been identified in the Linux kernel's netfilter conntrack subsystem where missing netlink policy validations allow out-of-bounds access in SCTP and ctnetlink components. The vulnerability stems from user-supplied values being assigned directly to kernel structures without proper range validation, enabling attackers to read memory beyond allocated object boundaries.

Critical Impact

Attackers can trigger slab-out-of-bounds reads by supplying malicious netlink attribute values, potentially accessing up to 5600 bytes beyond the intended 320-byte nf_conn object boundaries.

Affected Products

• Linux Kernel (netfilter conntrack subsystem)
• Linux Kernel (SCTP protocol handling)
• Linux Kernel (ctnetlink component)

Discovery Timeline

• 2026-04-06 - CVE CVE-2026-31407 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-31407

Vulnerability Analysis

This vulnerability exists within the Linux kernel's netfilter connection tracking subsystem, specifically affecting the SCTP protocol handler and ctnetlink components. The core issue involves insufficient validation of user-supplied netlink attributes before they are used to access kernel data structures.

The nlattr_to_sctp() function directly assigns the user-controlled CTA_PROTOINFO_SCTP_STATE value to ct->proto.sctp.state without verifying that the value falls within the valid state enumeration range. Similarly, the exp->dir field lacks proper bounds checking, allowing values like 100 to be assigned when the expected range is much smaller.

When an attacker supplies exp->dir = 100, the subsequent access to ct->master->tuplehash[100] results in reading 5600 bytes past the start of a 320-byte nf_conn object. This slab-out-of-bounds read has been confirmed by UBSAN (Undefined Behavior Sanitizer) testing.

Root Cause

The root cause is missing input validation in the netlink policy handling code. User-supplied netlink attributes are passed directly to kernel functions without being checked against valid ranges or bounds. The kernel assumes these values are trustworthy, but unprivileged users with CAP_NET_ADMIN capabilities or access to netlink sockets can supply arbitrary values that exceed expected bounds.

Attack Vector

An attacker with the ability to send netlink messages to the kernel can exploit this vulnerability by crafting malicious netlink attributes containing out-of-range values. By setting attributes like CTA_PROTOINFO_SCTP_STATE to invalid states or exp->dir to large values, the attacker forces the kernel to access memory outside allocated object boundaries.

The attack flow involves:

1. Opening a netlink socket with appropriate permissions
2. Constructing a netlink message with malformed conntrack attributes
3. Setting CTA_PROTOINFO_SCTP_STATE to an out-of-range value or exp->dir to an index exceeding the tuplehash array size
4. Sending the message to trigger the out-of-bounds memory access

The vulnerability allows reading sensitive kernel memory contents, potentially exposing cryptographic keys, credentials, or other security-sensitive data residing in adjacent kernel memory regions.

Detection Methods for CVE-2026-31407

Indicators of Compromise

• Kernel log entries showing UBSAN violations related to nf_conn structure access
• Unexpected slab corruption warnings in kernel logs referencing netfilter or conntrack
• Anomalous netlink traffic patterns targeting conntrack subsystem

Detection Strategies

• Enable kernel address sanitizer (KASAN) or UBSAN to detect out-of-bounds memory accesses in real-time
• Monitor for netlink socket connections from unprivileged processes attempting to interact with conntrack
• Deploy kernel runtime monitoring tools that can detect invalid array index access patterns

Monitoring Recommendations

• Configure audit rules to log netlink socket operations targeting the NETFILTER_NETLINK protocol family
• Implement kernel tracing on nlattr_to_sctp() and related ctnetlink functions to detect suspicious attribute values
• Review and correlate system logs for patterns indicating memory corruption attempts

How to Mitigate CVE-2026-31407

Immediate Actions Required

• Apply the available kernel patches from the stable kernel tree immediately
• Restrict access to netlink sockets by limiting CAP_NET_ADMIN capabilities to essential processes only
• Consider disabling connection tracking if not required for your network configuration

Patch Information

Security patches have been released to the Linux kernel stable tree. The fix extends the netlink policies to properly validate user-supplied attribute values before they are used to access kernel data structures.

Patches are available at the following kernel git commits:

• Kernel Git Commit 0fbae1e74493
• Kernel Git Commit f900e1d77ee0

Workarounds

• Restrict CAP_NET_ADMIN capability distribution using Linux security modules (SELinux, AppArmor)
• Implement namespace isolation to limit netlink access for containerized workloads
• Disable SCTP connection tracking if not required using module parameters

# Configuration example - Restrict conntrack netlink access
# Add to /etc/sysctl.d/99-netfilter-hardening.conf

# Limit conntrack table size to reduce attack surface
net.netfilter.nf_conntrack_max = 65536

# Disable SCTP conntrack helper if not needed
# Unload the nf_conntrack_proto_sctp module
modprobe -r nf_conntrack_proto_sctp

# Alternatively, blacklist the module
echo "blacklist nf_conntrack_proto_sctp" >> /etc/modprobe.d/blacklist-sctp-conntrack.conf