CVE-2026-31631: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31631 Overview

A buffer overread vulnerability has been identified in the Linux kernel's rxrpc subsystem, specifically within the rxgk_do_verify_authenticator() function. The vulnerability occurs due to insufficient buffer size validation before checking the nonce value during GSSAPI authentication processing. This flaw allows a remote attacker to potentially read beyond allocated buffer boundaries, leading to information disclosure and denial of service conditions.

Critical Impact
Remote attackers can exploit this vulnerability over the network without authentication to cause denial of service conditions and potentially leak sensitive kernel memory information.

Affected Products

Linux Kernel versions prior to the security fix
Linux Kernel 6.16
Linux Kernel 7.0 (rc1 through rc7)

Discovery Timeline

2026-04-24 - CVE-2026-31631 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31631

Vulnerability Analysis

The vulnerability resides in the rxrpc (Rx Remote Procedure Call) subsystem of the Linux kernel, which provides network transport and security mechanisms for distributed file systems like AFS (Andrew File System). The rxgk_do_verify_authenticator() function is responsible for verifying GSSAPI-based authenticator tokens during connection establishment.

The core issue is a missing bounds check that allows the function to read beyond the allocated buffer when processing authenticator data. When an attacker sends a specially crafted authenticator token with insufficient data, the function attempts to verify the nonce without first confirming that the buffer contains enough bytes. This results in an out-of-bounds read operation.

The vulnerability can be exploited remotely over the network without requiring authentication, making it particularly concerning for systems running AFS services or other rxrpc-dependent applications. Successful exploitation could lead to kernel memory disclosure, potentially revealing sensitive information, or trigger a kernel panic resulting in system unavailability.

Root Cause

The root cause is improper input validation in the rxgk_do_verify_authenticator() function. The function failed to verify that the buffer contained sufficient data before attempting to access and compare the nonce value. This missing validation allows attackers to provide malformed authenticator tokens that trigger out-of-bounds memory reads. The fix ensures the buffer size is validated before any nonce verification operations are performed.

Attack Vector

The attack can be conducted remotely over the network by sending malformed rxrpc packets to a vulnerable system. The attacker does not require any authentication or user interaction to exploit this vulnerability. The attack targets the rxrpc protocol layer, which handles GSSAPI authentication for secure communications.

An attacker would craft a malicious rxrpc packet containing an authenticator token with an insufficient buffer size. When the kernel processes this packet, the vulnerable function attempts to read beyond the buffer boundaries to verify the nonce, triggering the out-of-bounds read condition.

The vulnerability exists in the kernel's network protocol handling code, meaning exploitation attempts can originate from any network-accessible location that can send packets to the target system's rxrpc service port.

Detection Methods for CVE-2026-31631

Indicators of Compromise

Kernel log messages indicating rxrpc protocol errors or memory access violations
Unexpected system crashes or kernel panics related to rxrpc or network subsystems
Unusual network traffic patterns targeting rxrpc service ports
Core dumps containing references to rxgk_do_verify_authenticator() function

Detection Strategies

Monitor kernel logs for segmentation faults or memory violations in the rxrpc subsystem
Implement network intrusion detection rules for malformed rxrpc authentication packets
Use kernel tracing tools (ftrace, eBPF) to monitor rxgk_do_verify_authenticator() function calls
Deploy endpoint detection solutions capable of identifying kernel-level anomalies

Monitoring Recommendations

Enable kernel auditing for rxrpc-related system calls and events
Configure SIEM alerts for unusual rxrpc protocol activity or authentication failures
Monitor for systems running affected kernel versions that have network-exposed rxrpc services
Track system stability metrics for unexpected reboots or crashes on AFS-enabled systems

How to Mitigate CVE-2026-31631

Immediate Actions Required

Update the Linux kernel to the patched version containing the fix
If immediate patching is not possible, consider disabling rxrpc/AFS services if not required
Implement network-level filtering to restrict access to rxrpc service ports from untrusted networks
Monitor affected systems for signs of exploitation attempts

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix adds proper buffer size validation before the nonce verification check in rxgk_do_verify_authenticator(). The patches are available through the official kernel git repository:

System administrators should apply these patches or update to a kernel version containing these fixes as soon as possible.

Workarounds

Disable the rxrpc kernel module if AFS functionality is not required: modprobe -r rxrpc
Implement firewall rules to block external access to rxrpc ports (typically UDP port 7000-7009)
Use network segmentation to isolate systems running AFS services
Consider using alternative secure communication protocols until patches can be applied

bash

# Disable rxrpc module if not needed
sudo modprobe -r rxrpc

# Block rxrpc ports using iptables
sudo iptables -A INPUT -p udp --dport 7000:7009 -j DROP

# Verify module is not loaded
lsmod | grep rxrpc