CVE-2026-31671 Overview
CVE-2026-31671 is an information disclosure vulnerability in the Linux kernel's xfrm_user subsystem, specifically in the build_report() function. The struct xfrm_user_report contains a __u8 proto field followed by a struct xfrm_selector, creating three bytes of padding between the fields. The kernel never zeroes this padding before copying the structure to userspace, leaking uninitialized kernel stack memory. The flaw is tracked under [CWE-401] and affects multiple Linux kernel versions, including releases derived from the 2.6.19 baseline and 7.0 release candidates. The issue has been resolved by zeroing the structure before populating its members.
Critical Impact
A local user can trigger the xfrm_user reporting path to receive netlink messages containing uninitialized kernel memory, potentially exposing sensitive data such as pointers or secrets useful for further exploitation.
Affected Products
- Linux Kernel (multiple stable branches, see kernel.org commits)
- Linux Kernel 2.6.19
- Linux Kernel 7.0 release candidates (rc1 through rc7)
Discovery Timeline
- 2026-04-24 - CVE-2026-31671 published to NVD
- 2026-04-27 - Last updated in NVD database
Technical Details for CVE-2026-31671
Vulnerability Analysis
The vulnerability resides in build_report() within the xfrm_user netlink interface, which is responsible for emitting IPsec policy and state events to userspace listeners. The function populates a struct xfrm_user_report instance and copies it into a netlink message destined for user processes subscribed to the XFRMNLGRP_REPORT multicast group.
Because struct xfrm_user_report begins with a single-byte __u8 proto field followed by struct xfrm_selector, the C compiler inserts three bytes of alignment padding between the two members. The original implementation set each member individually but never cleared the structure beforehand. As a result, the padding bytes retained whatever values were present on the kernel stack from previous function activations.
When the message reaches userspace, those three bytes carry uninitialized kernel memory contents. Although small in size, this leak is enough to disclose values such as residual pointers, register spills, or fragments of secret material. Such disclosures undermine kernel address space layout randomization (KASLR) and assist attackers in constructing follow-on exploits.
Root Cause
The root cause is missing memory initialization before structure use, classified under [CWE-401]. The fix replaces the implicit reliance on per-member assignment with an explicit zeroing operation, ensuring padding bytes are cleared prior to populating fields and copying the structure to userspace.
Attack Vector
Exploitation requires local access to the system and the ability to interact with the xfrm netlink interface. An unprivileged or low-privileged process that subscribes to XFRM_MSG_REPORT notifications can trigger or observe build_report() output and capture the leaked padding bytes from received netlink messages. No user interaction is required beyond running the listening process.
The vulnerability does not directly grant code execution. Its value lies in defeating kernel hardening protections and supplying memory contents that enable other exploit primitives. The vendor patches are available via the kernel.org stable tree commits listed in the references.
Detection Methods for CVE-2026-31671
Indicators of Compromise
- No known in-the-wild exploitation has been reported, and the CVE is not listed in the CISA Known Exploited Vulnerabilities catalog.
- Unexpected user processes subscribing to the XFRM netlink multicast group XFRMNLGRP_REPORT on systems without IPsec workloads.
- Repeated invocations of xfrm_send_report() or related xfrm_user reporting paths originating from low-privileged users.
Detection Strategies
- Audit running kernel versions against the fixed commits published on git.kernel.org/stable to identify hosts still exposed.
- Monitor netlink socket creation with NETLINK_XFRM family by non-administrative processes using auditd rules or eBPF tracing.
- Correlate kernel build identifiers reported by uname -r with vendor advisories to confirm patch status across the fleet.
Monitoring Recommendations
- Enable kernel audit logging for netlink subscriptions and unusual xfrm policy queries from user space.
- Track package update events for kernel components and flag systems lagging behind distribution security advisories.
- Inspect process behavior for binaries that open AF_NETLINK sockets bound to NETLINK_XFRM outside expected IPsec management tools such as ip xfrm or strongSwan daemons.
How to Mitigate CVE-2026-31671
Immediate Actions Required
- Apply the upstream kernel patches referenced in the kernel.org stable commits and reboot affected hosts.
- Update to a distribution-supplied kernel package that incorporates the build_report() zeroing fix.
- Restrict access to the xfrm netlink interface by limiting which users and containers can create NETLINK_XFRM sockets where feasible.
Patch Information
The fix zeros struct xfrm_user_report before populating individual member variables, ensuring padding bytes do not carry stale kernel stack contents. The patch has been backported across multiple stable branches. Affected operators should consult the upstream commits, including Kernel Commit 0616314b, Kernel Commit 0a30dce, Kernel Commit 6c55714, Kernel Commit 716c546, Kernel Commit d101199, Kernel Commit d27c02e, Kernel Commit e0c8542, and Kernel Commit ff5ee50.
Workarounds
- Disable or unload the xfrm subsystem on hosts that do not require IPsec functionality by avoiding CONFIG_XFRM_USER in custom kernel builds.
- Use seccomp or Linux namespaces to prevent untrusted processes and containers from creating NETLINK_XFRM sockets.
- Apply mandatory access control profiles (SELinux, AppArmor) that restrict which binaries can interact with the xfrm netlink interface until the patched kernel is deployed.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
