CVE-2025-38566: Linux Kernel TLS Alert Handling Flaw

CVE-2025-38566 Overview

CVE-2025-38566 is a security vulnerability in the Linux kernel's sunrpc subsystem that affects the handling of server-side TLS alerts in NFS over TLS implementations. The vulnerability exists in the tls_alert_recv() function, which incorrectly assumes it can read data from the message iterator's kvec structure. This flaw in how kTLS (kernel TLS) control messages are processed can be exploited to cause a denial of service condition on affected systems.

The kTLS implementation splits TLS non-data record payloads between the control message buffer (containing the type, such as TLS alert or TLS cipher change) and the remaining payload (including TLS alert level and description) which goes into the msg payload buffer. The vulnerability arises from improper handling of these control messages during sock_recvmsg() operations.

Critical Impact

Network-accessible vulnerability in Linux kernel NFS over TLS implementation allows remote attackers to trigger denial of service conditions without requiring authentication or user interaction.

Affected Products

• Linux Kernel (multiple versions)
• Linux Kernel 6.17-rc1
• Systems utilizing NFS over TLS with kTLS enabled

Discovery Timeline

• 2025-08-19 - CVE CVE-2025-38566 published to NVD
• 2025-11-26 - Last updated in NVD database

Technical Details for CVE-2025-38566

Vulnerability Analysis

This vulnerability is classified as CWE-754 (Improper Check for Unusual or Exceptional Conditions). The flaw resides in the kernel's sunrpc TLS handling code, specifically in how the tls_alert_recv() function processes incoming TLS control messages. The function makes unsafe assumptions about the availability and structure of data within the message iterator's kvec, leading to improper exception handling when processing TLS alert records.

The kTLS layer processes TLS data record types normally when no control message structure is set up. However, when it encounters a TLS control message, it returns an error. The vulnerability manifests because the msg iterator can advance the kvec pointer during the copy process, but the code fails to properly revert the iterator state before calling into tls_alert_recv().

Root Cause

The root cause is the improper handling of TLS control message setup and consumption by sock_recvmsg(). When kTLS splits TLS non-data record payloads between the control message buffer and the msg payload buffer, the tls_alert_recv() function incorrectly assumes direct access to kvec data. The iterator advancement during the copy process creates a state inconsistency that the code fails to account for, resulting in improper processing of TLS alerts.

Attack Vector

An attacker can exploit this vulnerability over the network without requiring authentication or user interaction. By sending specially crafted TLS alert messages to an NFS server with TLS enabled, an attacker can trigger the vulnerable code path in tls_alert_recv(). The attack exploits the improper state management of the message iterator, causing the kernel to improperly handle the TLS control message and potentially crash or become unresponsive.

The vulnerability affects NFS servers configured with TLS encryption (NFS over TLS), which is increasingly common in enterprise environments requiring secure file sharing.

Detection Methods for CVE-2025-38566

Indicators of Compromise

• Unexpected kernel panics or crashes in systems running NFS over TLS services
• Abnormal TLS alert messages in network traffic targeting NFS ports
• Kernel log entries indicating sunrpc or kTLS subsystem errors
• Unusual denial of service conditions affecting NFS server availability

Detection Strategies

• Monitor kernel logs for sunrpc-related errors or stack traces involving tls_alert_recv()
• Implement network traffic analysis to detect malformed or unusual TLS alert messages targeting NFS services
• Deploy kernel-level monitoring for unexpected crashes in the sunrpc or kTLS subsystems
• Configure alerting for NFS service availability degradation

Monitoring Recommendations

• Enable detailed logging for NFS and TLS subsystems on affected servers
• Implement network intrusion detection rules for anomalous TLS traffic patterns
• Monitor system stability metrics for NFS servers with TLS enabled
• Review kernel crash dumps for evidence of sunrpc or kTLS-related failures

How to Mitigate CVE-2025-38566

Immediate Actions Required

• Review current Linux kernel version and determine if affected
• Apply available kernel patches from the Linux kernel stable tree
• Consider temporarily disabling NFS over TLS if immediate patching is not possible
• Implement network segmentation to limit exposure of NFS services

Patch Information

The Linux kernel maintainers have released fixes for this vulnerability. The following commits address the issue:

• Kernel commit 25bb3647d30a
• Kernel commit 3b549da87541
• Kernel commit 6b33c31cc788
• Kernel commit b1df39462171
• Kernel commit bee47cb026e7

The fix reworks how control messages are set up and used by sock_recvmsg(), ensuring proper iterator state management before calling tls_alert_recv().

Workarounds

• Temporarily disable NFS over TLS and use alternative network security measures such as VPNs or IPsec
• Implement firewall rules to restrict access to NFS services to trusted networks only
• Monitor for and block suspicious TLS traffic patterns at the network perimeter
• Consider using application-layer load balancers with TLS termination to offload TLS processing from affected servers

# Check current kernel version
uname -r

# Verify if NFS over TLS is in use
grep -r "xprtsec" /etc/nfs.conf /etc/nfsmount.conf 2>/dev/null

# Check for sunrpc module status
lsmod | grep sunrpc

# Review kernel logs for related errors
dmesg | grep -i "sunrpc\|ktls\|tls_alert"