CVE-2025-38500: Linux Kernel Use-After-Free Vulnerability

CVE-2025-38500 Overview

CVE-2025-38500 is a use-after-free vulnerability in the Linux kernel's xfrm interface subsystem [CWE-416]. The flaw resides in the xfrmi_changelink() function, which fails to properly validate changes to collect_md xfrm interfaces. The collect_md property can only be set at device creation, but the enforcement check was placed after a lookup that never returned collect_md interfaces. As a result, a local attacker can trigger a double-free condition when the affected network namespace is torn down, leading to kernel memory corruption and potential local privilege escalation.

Critical Impact
Local attackers with the ability to configure network interfaces can trigger kernel memory corruption, causing denial of service or potential privilege escalation on affected Linux systems.

Affected Products

Linux Kernel (multiple stable branches, including 6.16-rc1 through 6.16-rc7)
Debian Linux 11.0
Distributions shipping affected upstream kernel versions

Discovery Timeline

2025-08-12 - CVE-2025-38500 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-38500

Vulnerability Analysis

The vulnerability exists in the xfrm interface implementation, which supports IPsec transformations on virtual network interfaces. The collect_md mode is a special operating mode where a single xfrm interface can collect metadata for multiple tunnels. This mode is only valid at interface creation.

When xfrmi_changelink() is invoked, it attempts to prevent modification of collect_md interfaces. However, the guard check relied on the pointer returned by xfrmi_locate(), which does not track collect_md interfaces. The check therefore never executed for the target interface type, allowing changelink operations to proceed against forbidden interfaces.

The erroneous operation inserts the special interface into the xfrmi_net->xfrmi hash while it simultaneously remains referenced through the xfrmi_net->collect_md_xfrmi pointer. Both structures release the same object during network namespace teardown, producing a double-free. The resulting kernel BUG in unregister_netdevice_many_notify+0x101/0xab0 (net/core/dev.c:12029) crashes the kernel and creates an exploitable use-after-free window.

Root Cause

The root cause is an incorrect placement of the collect_md validation check in xfrmi_changelink(). The check operated on the wrong interface pointer, using the result of xfrmi_locate() rather than the netdev_priv-derived pointer available earlier in the function. The upstream fix moves validation to use netdev_priv, ensuring the special interface is properly detected and rejected before any hash manipulation occurs.

Attack Vector

Exploitation requires local access with the CAP_NET_ADMIN capability, typically held by root or privileged network configuration processes within a user namespace. An attacker creates an xfrm interface with the collect_md property, then invokes ip link set (or an equivalent netlink RTM_NEWLINK message) to trigger xfrmi_changelink(). When the affected network namespace is subsequently destroyed, the double-free triggers, corrupting kernel memory allocator structures. Skilled attackers can leverage this primitive to achieve arbitrary kernel memory writes and escalate privileges from an unprivileged user namespace context.

Detailed exploitation mechanics are documented in the upstream commit references. See the Linux Kernel Commit bfebdb85 for the source-level fix and additional context.

Detection Methods for CVE-2025-38500

Indicators of Compromise

Kernel oops or BUG messages referencing unregister_netdevice_many_notify in net/core/dev.c
Unexpected kernel panics tied to network namespace cleanup (cleanup_net workqueue)
Presence of xfrm interfaces created with the collect_md property followed by changelink netlink operations
Unexpected process activity invoking RTM_NEWLINK messages targeting xfrm interfaces

Detection Strategies

Monitor kernel ring buffer (dmesg) and /var/log/kern.log for BUG or Oops entries referencing xfrmi_changelink, unregister_netdevice, or cleanup_net
Audit netlink activity using auditd rules on the socket and sendmsg syscalls where the netlink family is NETLINK_ROUTE
Inventory running kernel versions against the fixed commits to identify unpatched hosts
Correlate abnormal user-namespace creation with subsequent ip link invocations against xfrm devices

Monitoring Recommendations

Enable kernel crash reporting (kdump) to capture forensic evidence when the double-free is triggered
Track processes that hold CAP_NET_ADMIN and interact with the xfrm subsystem
Alert on repeated network namespace teardown failures, which frequently accompany exploitation attempts

How to Mitigate CVE-2025-38500

Immediate Actions Required

Apply the upstream kernel patches referenced in the vendor advisories to all affected hosts
For Debian systems, install the updated linux package announced in the Debian LTS Announcement
Restrict CAP_NET_ADMIN and disable unprivileged user namespaces where operationally feasible
Reboot systems after patching to activate the fixed kernel image

Patch Information

The fix is available in multiple stable branches through the following upstream commits: 5918c3f, 69a31f7a, a8d4748b, a90b2a1a, and bfebdb85. The patch relocates the collect_md validation to use netdev_priv, ensuring xfrmi_changelink() correctly rejects modifications to special collect_md interfaces.

Workarounds

Disable unprivileged user namespaces by setting kernel.unprivileged_userns_clone=0 via sysctl to reduce the local attack surface
Blocklist the xfrm_interface kernel module on systems that do not require IPsec virtual interfaces
Apply mandatory access control policies (SELinux, AppArmor) to restrict processes that can perform netlink RTM_NEWLINK operations against xfrm devices

bash

# Configuration example: reduce local attack surface pending patch deployment
sudo sysctl -w kernel.unprivileged_userns_clone=0
echo "kernel.unprivileged_userns_clone=0" | sudo tee /etc/sysctl.d/99-hardening.conf

# Blocklist the xfrm_interface module if unused
echo "blacklist xfrm_interface" | sudo tee /etc/modprobe.d/blacklist-xfrmi.conf
sudo update-initramfs -u