CVE-2025-38618: Linux Kernel Use-After-Free Vulnerability

CVE-2025-38618 Overview

CVE-2025-38618 is a use-after-free vulnerability [CWE-416] in the Linux kernel's virtual socket (vsock) subsystem. The flaw exists because a vsock can autobind to VMADDR_PORT_ANY, and a subsequent connection to the bound socket triggers memory corruption. The socket returned by accept() retains port VMADDR_PORT_ANY but is absent from the unbound sockets list. Binding it causes an extra refcount decrement, mirroring the issue previously addressed in commit fcdd2242c023. Local authenticated attackers can leverage this condition to escalate privileges or destabilize the kernel on affected Linux and Debian systems.

Critical Impact
Local attackers with low privileges can trigger a use-after-free in the kernel vsock code path, potentially leading to privilege escalation, kernel memory corruption, or denial of service.

Affected Products

Linux Kernel (multiple stable branches, including 6.17-rc1)
Debian Linux 11.0
Distributions shipping vulnerable kernel builds with vsock enabled

Discovery Timeline

2025-08-22 - CVE-2025-38618 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-38618

Vulnerability Analysis

The vulnerability resides in the __vsock_bind_connectible() function within the Linux kernel's vsock subsystem. Vsock provides communication between virtual machines and their host, commonly used in virtualization environments such as KVM and VMware. The flaw allows a socket to autobind to the special address VMADDR_PORT_ANY, a state that should represent an unbound socket. When a connection is subsequently established to such a socket, the kernel's refcount tracking becomes inconsistent. The socket returned by accept() inherits port VMADDR_PORT_ANY but is never added to the list of unbound sockets, breaking the invariant relied upon during socket teardown. This inconsistency produces a use-after-free condition when the socket is destroyed, allowing freed memory to be referenced by other kernel paths.

Root Cause

The root cause is missing validation in __vsock_bind_connectible() that permits binding to VMADDR_PORT_ANY. Because VMADDR_PORT_ANY is treated as a sentinel for unbound sockets, allowing it as a valid bind target corrupts the bookkeeping of bound versus unbound socket lists. Rebinding such a socket triggers an extra refcount decrement similar to the issue previously fixed by commit fcdd2242c023 ("vsock: Keep the binding until socket destruction"). The fix extends the port validation check to reject VMADDR_PORT_ANY explicitly.

Attack Vector

Exploitation requires local access with the ability to create AF_VSOCK sockets. An attacker crafts a sequence that autobinds a vsock to VMADDR_PORT_ANY, invokes accept() on the incoming connection, then binds the returned socket to induce the refcount imbalance. When the freed socket memory is later reallocated, the dangling reference enables kernel memory corruption. Successful exploitation of a vsock use-after-free may permit arbitrary kernel code execution or privilege escalation from an unprivileged process to root. Refer to the kernel commit fix for the authoritative patch.

Detection Methods for CVE-2025-38618

Indicators of Compromise

Unexpected kernel oops or panic messages referencing vsock_bind, __vsock_bind_connectible, or vsock_release in dmesg or /var/log/kern.log.
KASAN (Kernel Address Sanitizer) reports flagging use-after-free within the vsock subsystem.
Unprivileged processes spawning root shells or performing privileged operations after opening AF_VSOCK sockets.

Detection Strategies

Audit process telemetry for unusual socket(AF_VSOCK, ...) calls followed by rapid bind(), accept(), and rebind sequences from unprivileged users.
Monitor kernel crash dumps and system reboots correlated with vsock activity, particularly on hypervisor hosts and guests.
Compare running kernel versions against the fixed commits listed in the vendor advisories to identify unpatched hosts at scale.

Monitoring Recommendations

Forward kernel logs and crash reports to a central SIEM for anomaly correlation across the fleet.
Enable auditd rules for AF_VSOCK socket syscalls on multi-tenant and virtualization hosts where local users have shell access.
Track package versions of linux-image-* on Debian systems and align updates with the Debian LTS Announcement.

How to Mitigate CVE-2025-38618

Immediate Actions Required

Apply the upstream kernel patches from git.kernel.org to the appropriate stable branch and reboot affected systems.
On Debian 11 systems, install the updated linux packages published in the October 2025 Debian LTS advisories.
Restrict local shell access on multi-tenant Linux hosts until patched kernels are deployed.

Patch Information

The Linux kernel maintainers have merged the fix across multiple stable branches. Relevant commits include 32950b19, 44bd006d, 8f010936, aba0c94f, c04a2c1c, cf867047, d1a5b196, d73960f0, and f138be5d. Debian LTS has released corresponding package updates documented in Debian LTS Announcement #7 and Debian LTS Announcement #8. Administrators should verify the running kernel version against fixed builds after applying updates.

Workarounds

Disable or blacklist the vsock and vmw_vsock_* kernel modules on systems that do not require host-guest socket communication.
Use kernel module load restrictions (for example, install vsock /bin/true in /etc/modprobe.d/) to prevent runtime loading.
Limit AF_VSOCK socket creation through seccomp profiles for untrusted workloads and containers.

bash

# Configuration example: blacklist vsock modules until patched kernel is deployed
echo 'blacklist vsock' | sudo tee /etc/modprobe.d/blacklist-vsock.conf
echo 'blacklist vmw_vsock_virtio_transport' | sudo tee -a /etc/modprobe.d/blacklist-vsock.conf
echo 'blacklist vmw_vsock_vmci_transport' | sudo tee -a /etc/modprobe.d/blacklist-vsock.conf
sudo update-initramfs -u
sudo reboot

# Verify kernel version after patch installation
uname -r
dpkg -l | grep linux-image