CVE-2026-31637: Linux Kernel Ticket Decryption Vulnerability

CVE-2026-31637 Overview

CVE-2026-31637 is a vulnerability in the Linux kernel's rxrpc subsystem affecting the RXKAD authentication mechanism. The flaw resides in rxkad_decrypt_ticket(), which decrypts RXKAD response tickets and parses the buffer as plaintext without verifying that crypto_skcipher_decrypt() succeeded. A malformed RESPONSE packet can supply a non-block-aligned ticket length, force the decrypt operation to fail, and still drive the ticket parser with attacker-controlled bytes. The vulnerability is reachable over the network without authentication or user interaction.

Critical Impact
Remote unauthenticated attackers can feed attacker-controlled bytes into the RXKAD ticket parser, putting confidentiality, integrity, and availability of vulnerable Linux systems at risk.

Affected Products

Linux Kernel (mainline branches including 2.6.22 and the 7.0 release candidates rc1 through rc7)
Distributions shipping the rxrpc networking module with RXKAD support enabled
Systems exposing AFS or other Rx-based services that rely on RXKAD authentication

Discovery Timeline

2026-04-24 - CVE-2026-31637 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31637

Vulnerability Analysis

The rxrpc kernel module implements the Rx remote procedure call protocol used by Andrew File System (AFS) clients. RXKAD is the Kerberos-based authentication scheme layered on top of Rx. During session establishment, the responder sends a RESPONSE packet containing an encrypted ticket that the kernel decrypts using crypto_skcipher_decrypt().

The vulnerable code in rxkad_decrypt_ticket() invokes the symmetric cipher decryption routine but does not check its return value. When a peer transmits a RESPONSE with a non-block-aligned ticket length, the cipher operation fails and leaves the destination buffer in an undefined state. The function then proceeds to parse this buffer as plaintext ticket data.

Because an attacker controls the ciphertext bytes within the malformed RESPONSE, they influence what the parser interprets as decrypted ticket fields. The fix aborts the connection with RXKADBADTICKET whenever decryption fails, ensuring the parser never receives undecrypted attacker-supplied data.

Root Cause

The root cause is a missing return-value check after a cryptographic operation, classified under [NVD-CWE-noinfo] but consistent with improper input validation on decryption results. The kernel trusts the contents of a buffer that the cipher never successfully populated.

Attack Vector

An unauthenticated remote attacker sends a crafted RXKAD RESPONSE packet to a Linux host running an Rx service. The malformed packet carries a ticket length that is not a multiple of the cipher block size. The decrypt call fails silently, and the parser consumes attacker-controlled bytes from the unwritten buffer, enabling further exploitation paths within the ticket handling logic.

// No public exploitation code is available for CVE-2026-31637.
// The fix adds a return-value check after crypto_skcipher_decrypt()
// in rxkad_decrypt_ticket() and aborts with RXKADBADTICKET on failure.

Detection Methods for CVE-2026-31637

Indicators of Compromise

Unexpected rxrpc connection abort events referencing RXKAD authentication failures in kernel logs
Inbound traffic to UDP services using the Rx protocol from untrusted networks
Crashes or anomalous behavior in kernel threads associated with AFS or rxrpc workloads

Detection Strategies

Inventory Linux hosts running kernel versions matching the affected CPE ranges and confirm whether the rxrpc module is loaded
Monitor network telemetry for malformed Rx RESPONSE packets directed at AFS clients or servers
Correlate kernel ring buffer messages with network flow data to identify repeated authentication failures from a single source

Monitoring Recommendations

Enable kernel auditing for module load events and unusual rxrpc socket activity
Log and alert on volumetric increases in Rx protocol traffic to internal hosts
Track patch compliance for the kernel commits referenced in the vendor advisory across all Linux fleets

How to Mitigate CVE-2026-31637

Immediate Actions Required

Apply the upstream kernel patches referenced in the Kernel Git Commit Update 1 and the additional fix commits listed by kernel.org
Prioritize patching on hosts that expose AFS or other Rx-based services to untrusted networks
Unload the rxrpc kernel module on systems that do not require AFS connectivity

Patch Information

The Linux kernel maintainers committed the fix across multiple stable branches. The patch adds a check on the result of crypto_skcipher_decrypt() inside rxkad_decrypt_ticket() and aborts the connection with RXKADBADTICKET on failure. Refer to the kernel.org commits 22f6258e7b31, 47073aab8a3a, 58fcd1b15615, a149dcae2330, and fe4447cd9562 for the exact changes.

Workarounds

Block inbound traffic to AFS and other Rx-based services at the network perimeter for hosts that do not require external access
Restrict the rxrpc module from loading using modprobe blacklists where AFS is not in use
Segment AFS clients and servers onto dedicated VLANs accessible only to authenticated peers

bash

# Prevent the rxrpc module from loading on hosts that do not need AFS
echo "blacklist rxrpc" | sudo tee /etc/modprobe.d/blacklist-rxrpc.conf
sudo update-initramfs -u

# Verify the module is not currently loaded
lsmod | grep rxrpc