CVE-2026-31671: Linux Kernel Information Disclosure Flaw

CVE-2026-31671 Overview

CVE-2026-31671 is an information disclosure vulnerability in the Linux kernel's XFRM (IPsec) subsystem. The vulnerability exists in the build_report() function within xfrm_user, where the struct xfrm_user_report structure contains uninitialized padding bytes that are copied to userspace without being zeroed first. This allows a local attacker with access to XFRM netlink sockets to potentially leak sensitive kernel memory information.

The struct xfrm_user_report begins with a __u8 proto field followed by a struct xfrm_selector, creating three bytes of padding between these fields due to structure alignment requirements. Because this padding is never zeroed before the structure is copied to userspace, whatever data happens to reside in those memory locations could be exposed to user applications.

Critical Impact

Local attackers can potentially extract sensitive kernel memory contents through uninitialized structure padding, which may be leveraged for information gathering or as part of a larger exploit chain.

Affected Products

• Linux Kernel versions from 2.6.19 onward
• Linux Kernel 7.0-rc1 through 7.0-rc7
• Multiple stable kernel branches prior to patching

Discovery Timeline

• April 24, 2026 - CVE-2026-31671 published to NVD
• April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31671

Vulnerability Analysis

This vulnerability represents a classic uninitialized memory disclosure issue that has affected various kernel subsystems over the years. In the XFRM framework, which handles IPsec policy and state management, the build_report() function constructs struct xfrm_user_report messages to be sent to userspace via netlink sockets.

The root of the issue lies in C structure alignment. When the compiler lays out struct xfrm_user_report, it inserts padding bytes after the __u8 proto field to align the subsequent struct xfrm_selector on its required boundary. These three padding bytes are never explicitly initialized, meaning they contain whatever data was previously at those memory addresses.

While the individual member variables are set correctly, the structure as a whole is not zeroed first, allowing stale kernel memory to leak through the padding bytes when the report is copied to userspace via netlink.

Root Cause

The vulnerability stems from improper initialization of kernel structures before copying to userspace. Specifically, the struct xfrm_user_report was being populated by setting individual member fields without first zeroing the entire structure. This left the compiler-inserted padding bytes between the __u8 proto field and struct xfrm_selector uninitialized, containing potentially sensitive kernel memory contents.

The fix involves zeroing the entire structure before setting individual member variables, ensuring no uninitialized memory can leak to userspace.

Attack Vector

Exploitation requires local access to the system with permissions to interact with XFRM netlink sockets. An attacker would need to:

1. Open a netlink socket for the XFRM subsystem
2. Trigger conditions that cause the kernel to generate XFRM user reports
3. Read the returned structures and extract the three bytes of padding data
4. Repeat the process to collect additional kernel memory fragments

While the amount of data leaked per operation is small (three bytes), repeated exploitation could potentially reveal sensitive kernel addresses or other memory contents useful for bypassing security mechanisms like KASLR.

The vulnerability is described in prose form as the fix involves ensuring structure initialization. The patch commits available at the kernel git repository show the structure is now zeroed using memset() or structure initialization before individual fields are populated.

Detection Methods for CVE-2026-31671

Indicators of Compromise

• Unusual netlink socket activity targeting the XFRM subsystem
• Processes repeatedly requesting XFRM user reports without corresponding IPsec operations
• Unprivileged processes attempting to access XFRM netlink interfaces

Detection Strategies

• Monitor for processes opening XFRM netlink sockets, especially from unexpected user contexts
• Implement kernel auditing to track netlink message traffic on the XFRM family
• Use SentinelOne's kernel-level monitoring to detect anomalous XFRM subsystem interactions
• Deploy behavioral analysis to identify information leak exploitation patterns

Monitoring Recommendations

• Enable audit rules for netlink socket operations related to XFRM
• Monitor system calls associated with IPsec configuration from non-administrative processes
• Track processes that repeatedly interact with XFRM interfaces without legitimate IPsec workloads
• Review kernel logs for unusual XFRM activity patterns

How to Mitigate CVE-2026-31671

Immediate Actions Required

• Update Linux kernel to the latest patched version for your distribution
• Review systems for any signs of exploitation or unusual XFRM netlink activity
• Restrict access to XFRM netlink sockets to only necessary processes using namespace isolation or seccomp filters
• Apply vendor-provided kernel updates as soon as they become available

Patch Information

Multiple patches have been released across various stable kernel branches to address this vulnerability. The fix ensures the struct xfrm_user_report is properly zeroed before individual member variables are set.

Relevant patch commits include:

• Kernel Patch Commit ff5ee50
• Kernel Patch Commit e0c8542
• Kernel Patch Commit d27c02e
• Kernel Patch Commit d101199
• Kernel Patch Commit 716c546
• Kernel Patch Commit 6c55714
• Kernel Patch Commit 0a30dce
• Kernel Patch Commit 0616314b

Workarounds

• Restrict access to XFRM netlink sockets using SELinux, AppArmor, or seccomp policies
• Implement network namespaces to isolate IPsec operations from untrusted processes
• Consider disabling IPsec/XFRM functionality on systems that don't require it
• Use SentinelOne endpoint protection to monitor for exploitation attempts

# Example: Audit XFRM netlink socket access
auditctl -a always,exit -F arch=b64 -S socket -F a0=16 -F a2=6 -k xfrm_socket