CVE-2026-31676: Linux Kernel Privilege Escalation Flaw

CVE-2026-31676 Overview

CVE-2026-31676 is a race condition vulnerability in the Linux kernel's rxrpc (Rx Remote Procedure Call) subsystem. The vulnerability exists in the handling of RESPONSE packets during service connection challenges. The issue allows duplicate or late RESPONSE packets to re-run the security setup path when a service connection has already transitioned out of the RXRPC_CONN_SERVICE_CHALLENGING state.

The flaw occurs because the code does not properly check the connection state under state_lock before running response verification and security initialization. This allows attackers to trigger the setup path multiple times through carefully crafted RESPONSE packets, potentially causing denial of service conditions.

Critical Impact
Network-accessible denial of service vulnerability in Linux kernel rxrpc subsystem that can be exploited without authentication to disrupt system availability.

Affected Products

Linux Kernel (rxrpc subsystem)

Discovery Timeline

2026-04-25 - CVE-2026-31676 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31676

Vulnerability Analysis

This vulnerability is a race condition in the Linux kernel's rxrpc protocol implementation. The rxrpc subsystem provides reliable, authenticated communication services within the kernel, commonly used by distributed file systems like AFS (Andrew File System).

The vulnerable code path processes RESPONSE packets during service connection authentication. When a service connection is in the RXRPC_CONN_SERVICE_CHALLENGING state, it expects RESPONSE packets from clients to complete the challenge-response authentication process. However, the state check was performed without proper locking, creating a time-of-check time-of-use (TOCTOU) race condition.

An attacker capable of sending network traffic to affected systems can exploit this race condition by sending duplicate or late RESPONSE packets. These packets can bypass the intended state check and cause the security initialization code to execute multiple times, leading to undefined behavior and potential system instability.

Root Cause

The root cause is an improper synchronization mechanism in the rxrpc connection state management. The code was checking the connection state without holding the state_lock, allowing concurrent packet processing to observe stale state information. After transitioning out of the RXRPC_CONN_SERVICE_CHALLENGING state, subsequent RESPONSE packets could still trigger the response verification and security initialization code paths.

The fix implements proper locking by checking the state under state_lock before running response verification and security initialization. A local secured flag is used to determine whether to queue the secured-connection work after the state transition, ensuring thread-safe state management.

Attack Vector

The vulnerability is exploitable over the network without requiring any authentication or user interaction. An attacker can send crafted RESPONSE packets to an rxrpc service endpoint:

The attacker initiates or observes a legitimate rxrpc service connection entering the challenge state
Multiple RESPONSE packets are sent in quick succession, exploiting the race window
The unlocked state check allows duplicate packets to re-run the security setup path
This can cause resource exhaustion, state corruption, or system instability

The attack can be conducted remotely against any system with the rxrpc module loaded and accepting connections. No special privileges are required on the attacking system.

Detection Methods for CVE-2026-31676

Indicators of Compromise

Unusual volume of rxrpc RESPONSE packets from external sources
Kernel log messages indicating rxrpc connection state errors or security initialization failures
System instability or crashes in the rxrpc or related kernel modules
Abnormal resource consumption associated with rxrpc connection handling

Detection Strategies

Monitor network traffic for high volumes of rxrpc protocol traffic targeting affected systems
Implement kernel auditing to detect unusual rxrpc subsystem activity
Deploy intrusion detection rules to identify suspicious RESPONSE packet patterns
Review kernel logs for rxrpc-related warnings or errors indicating exploitation attempts

Monitoring Recommendations

Enable detailed kernel logging for the rxrpc subsystem to capture connection state transitions
Configure network monitoring to baseline normal rxrpc traffic patterns and alert on anomalies
Implement system resource monitoring to detect potential denial of service conditions
Deploy endpoint detection solutions capable of monitoring kernel-level activity

How to Mitigate CVE-2026-31676

Immediate Actions Required

Apply the kernel security patches from the official kernel.org stable branches immediately
If patching is not immediately possible, consider disabling the rxrpc module if not required
Monitor affected systems for signs of exploitation or unusual rxrpc activity
Review firewall rules to restrict rxrpc traffic to trusted sources only

Patch Information

The vulnerability has been addressed in multiple kernel stable branches. Patches are available from the official kernel.org repository:

The fix ensures state checks are performed under state_lock and uses a local secured flag to safely handle state transitions.

Workarounds

Disable the rxrpc kernel module using modprobe -r rxrpc if the functionality is not required
Implement network-level filtering to block rxrpc traffic from untrusted sources
Use host-based firewalls to restrict access to rxrpc services to authorized clients only
Consider network segmentation to isolate systems running rxrpc services from untrusted networks

bash

# Disable rxrpc module if not required
modprobe -r rxrpc

# Block rxrpc traffic at the firewall (example using iptables)
# Note: rxrpc uses UDP port 7001 by default for AFS
iptables -A INPUT -p udp --dport 7001 -j DROP

# Check if rxrpc module is loaded
lsmod | grep rxrpc