CVE-2020-25220: Linux Kernel Use-After-Free Vulnerability

CVE-2020-25220 Overview

CVE-2020-25220 is a use-after-free vulnerability [CWE-416] in the Linux kernel affecting versions 4.9.x before 4.9.233, 4.14.x before 4.14.194, and 4.19.x before 4.19.140. The flaw resides in the cgroups subsystem and was introduced during a backport of the patch for CVE-2020-14356. Stable kernel maintainers failed to account for the skcd->no_refcnt field, leaving a reference counting error that allows freed kernel memory to be reused. A local authenticated attacker can leverage the condition to corrupt kernel memory, leading to privilege escalation or denial of service.

Critical Impact
A local user with low privileges can trigger kernel memory corruption through the cgroups subsystem, enabling escalation to root or kernel-level code execution.

Affected Products

Linux kernel 4.9.x prior to 4.9.233
Linux kernel 4.14.x prior to 4.14.194
Linux kernel 4.19.x prior to 4.19.140

Discovery Timeline

2020-09-10 - CVE-2020-25220 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25220

Vulnerability Analysis

The vulnerability stems from an incomplete backport of the upstream fix for CVE-2020-14356 into the 4.9, 4.14, and 4.19 stable kernel branches. The original mainline fix relied on the skcd->no_refcnt flag inside the socket cgroup data structure (sock_cgroup_data) to determine whether the cgroup reference held by a socket should be released. The stable backport omitted handling for this field, which causes the kernel to release a cgroup reference it never acquired or to skip releasing one it did. Either path desynchronizes the cgroup refcount and produces a use-after-free condition on the cgroup object.

Once the cgroup structure is freed prematurely, subsequent socket operations or cgroup walks dereference dangling pointers. An attacker who can place controlled data into the freed slab object can hijack kernel control flow.

Root Cause

The root cause is improper synchronization of reference counting during the backport of commit context related to CVE-2020-14356. The skcd->no_refcnt field signals whether sock_cgroup_data carries a real reference to its associated cgroup. Without checking this flag, the cleanup path operates on objects whose lifetime it does not own, violating the underlying ownership model of the cgroup v2 socket attachment logic.

Attack Vector

Exploitation requires local code execution with low privileges. An attacker creates and manipulates cgroups along with sockets bound to those cgroups, then triggers the mismatched reference release path. By spraying the kernel slab with controlled objects after the free, the attacker reuses the dangling memory to overwrite function pointers or credential structures. The result is kernel-level code execution or escalation to root. Network access is not required, and no user interaction is involved.

No verified public exploit code is available for this issue. Refer to the Linux Kernel Commit 82fd2138 for the corrective patch and the Red Hat Bug Report #1868453 for vendor analysis.

Detection Methods for CVE-2020-25220

Indicators of Compromise

Unexpected kernel oops or panic messages referencing cgroup_sk_alloc, cgroup_sk_free, or sock_cgroup_ptr in dmesg or /var/log/kern.log.
KASAN reports flagging use-after-free conditions inside cgroup or socket subsystem functions on instrumented kernels.
Unprivileged processes suddenly executing with elevated UID/GID or holding unexpected capabilities.

Detection Strategies

Inventory kernel versions across Linux hosts and flag any system running 4.9.x < 4.9.233, 4.14.x < 4.14.194, or 4.19.x < 4.19.140.
Monitor for anomalous local privilege escalation patterns, including processes transitioning from non-root UIDs to UID 0 without an authorized setuid binary invocation.
Audit syscall telemetry for unusual sequences combining unshare, clone with CLONE_NEWCGROUP, and rapid socket creation/destruction.

Monitoring Recommendations

Enable kernel auditing (auditd) on rules covering execve, setuid, setgid, and namespace-related syscalls.
Collect and centrally analyze kernel ring buffer logs to surface oops traces tied to the cgroup or networking subsystems.
Track user namespace creation events, since they expand the local attack surface for kernel bugs of this class.

How to Mitigate CVE-2020-25220

Immediate Actions Required

Upgrade affected kernels to 4.9.233, 4.14.194, 4.19.140, or any later stable release that includes the corrective patch.
Reboot systems after kernel package installation to load the patched image; live patching can be used where supported.
Restrict unprivileged user namespace creation on hosts that do not require it, reducing exposure to local kernel flaws.

Patch Information

The fix is published in the stable kernel changelogs: Linux Kernel ChangeLog 4.9.233, Linux Kernel ChangeLog 4.14.194, and Linux Kernel ChangeLog 4.19.140. Distribution-level updates are available through Debian LTS Announcement #00032, Debian LTS Announcement #00034, the Red Hat Bug Report #1868453, and the NetApp Security Advisory NTAP-20201001-0004.

Workarounds

Disable unprivileged user namespaces by setting kernel.unprivileged_userns_clone=0 where the workload permits.
Limit interactive shell access on multi-tenant hosts and enforce least-privilege execution for service accounts.
Apply mandatory access controls such as SELinux or AppArmor profiles to constrain processes that interact heavily with cgroups and sockets.

bash

# Verify current kernel version and apply distribution patches
uname -r

# Debian/Ubuntu
sudo apt-get update && sudo apt-get install --only-upgrade linux-image-$(uname -r | cut -d- -f3-)

# RHEL/CentOS
sudo yum update kernel

# Reduce attack surface by disabling unprivileged user namespaces
echo 'kernel.unprivileged_userns_clone=0' | sudo tee /etc/sysctl.d/99-harden-userns.conf
sudo sysctl --system

# Reboot to activate the patched kernel
sudo reboot