CVE-2025-38403: Linux Kernel Memory Initialization Flaw

CVE-2025-38403 Overview

CVE-2025-38403 is an uninitialized memory use vulnerability in the Linux kernel's vsock/vmci transport layer. The flaw resides in the vmci_transport_packet_init function, which populates a vmci_transport_packet structure without first zeroing its memory. Residual stack or heap data may leak into transport packets sent across VMware Communication Interface (VMCI) sockets. The issue affects multiple stable Linux kernel branches, including 6.16 release candidates, and Debian Linux 11. A local, authenticated attacker can trigger the code path through standard socket APIs, potentially exposing kernel memory contents to userspace or peer virtual machines.

Critical Impact
Local low-privileged users on affected kernels can trigger disclosure of uninitialized kernel memory through vsock/vmci transport packets, with potential impact to confidentiality, integrity, and availability.

Affected Products

Linux Kernel (multiple stable branches, including 6.16-rc1 through 6.16-rc4)
Debian Linux 11
Systems using the vsock subsystem with the VMCI transport (typically VMware guests and hosts)

Discovery Timeline

2025-07-25 - CVE-2025-38403 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-38403

Vulnerability Analysis

The vulnerability is classified as uninitialized memory use in the kernel's virtual socket implementation for VMware. The vsock/vmci transport is used to exchange datagrams and streams between a hypervisor and its guests, or between processes on the same host. When the kernel constructs a control packet with vmci_transport_packet_init, it allocates a vmci_transport_packet structure and directly assigns fields such as source and destination context IDs, packet type, size, and flags.

Because the allocation is not preceded by a memset to zero, any structure member that the initializer does not explicitly write retains whatever bytes were previously present in that memory region. Padding bytes and reserved fields fall into this category. When the packet is transmitted, those stale bytes travel with it, exposing fragments of prior kernel state to the receiver.

The fix, applied across multiple stable branches, prepends a memset call to zero the entire vmci_transport_packet before field population. Patch commits include 0a01021317375b8d1895152f544421ce49299eb1 and 1c1bcb0e78230f533b4103e8cf271d17c3f469f0.

Root Cause

The root cause is missing initialization of a stack- or heap-allocated packet structure prior to selective field assignment. Structure padding introduced by the compiler for alignment purposes was never cleared, leaving uninitialized bytes adjacent to legitimate fields. This class of defect maps to [CWE-908: Use of Uninitialized Resource] behavior even though NVD tags it as NVD-CWE-noinfo.

Attack Vector

Exploitation requires local access and low privileges on a system where the VMCI transport is loaded. An attacker opens an AF_VSOCK socket that binds to the VMCI transport and drives the kernel through packet construction paths, such as connect, shutdown, or control message operations. Each emitted packet may contain a small quantity of leaked kernel bytes. Repeated invocations allow the attacker to accumulate memory samples, potentially exposing pointers, credentials, or other sensitive kernel data useful for further exploitation. No user interaction is required.

No verified public proof-of-concept has been published for this issue. Refer to the upstream kernel patches for the exact code paths involved.

Detection Methods for CVE-2025-38403

Indicators of Compromise

No public indicators of compromise or exploit artifacts are currently associated with this CVE.
Unexpected loading of the vmw_vsock_vmci_transport kernel module on hosts that do not run VMware workloads may warrant review.
Anomalous processes opening AF_VSOCK sockets and issuing high volumes of control packets can indicate probing.

Detection Strategies

Inventory running kernel versions across Linux hosts and compare them against the fixed versions listed in the kernel.org stable patches.
Use auditd rules to log socket(AF_VSOCK, ...) system calls and correlate unusual callers.
Track kernel module load events for vmw_vsock_vmci_transport on systems where the module is not required.

Monitoring Recommendations

Monitor for unpatched Debian 11 systems referenced in the Debian LTS advisory.
Alert on non-standard processes creating AF_VSOCK sockets in production environments.
Ingest kernel package inventory into a centralized log platform to support version drift detection over time.

How to Mitigate CVE-2025-38403

Immediate Actions Required

Apply the latest stable Linux kernel updates that include the vmci_transport_packet_initmemset fix.
On Debian 11, install kernel packages referenced in the Debian LTS Announcement.
Reboot affected hosts after patching to activate the fixed kernel image.
Restrict local user access on multi-tenant systems, particularly VMware guests and hosts running untrusted workloads.

Patch Information

The fix is committed across multiple stable Linux kernel branches. Reference commits include 0a01021317375b8d1895152f544421ce49299eb1, 19c2cc01ff9a8031398a802676ffb0f4692dd95d, 1c1bcb0e78230f533b4103e8cf271d17c3f469f0, 223e2288f4b8c262a864e2c03964ffac91744cd5, 2d44723a091bc853272e1a51a488a3d22b80be5e, 75705b44e0b9aaa74f4c163d93d388bcba9e386a, 94d0c326cb3ee6b0f8bd00e209550b93fcc5c839, and e9a673153d578fd439919a24e99851b2f87ecbce. Debian users should follow the Debian LTS advisories for backported packages.

Workarounds

Blacklist the vmw_vsock_vmci_transport module on systems that do not require VMCI sockets.
Deny local user access to AF_VSOCK sockets via seccomp filters where feasible.
Reduce the local attack surface by removing shell access for untrusted users on affected hosts.

bash

# Blacklist the VMCI vsock transport module on hosts that do not need it
echo "blacklist vmw_vsock_vmci_transport" | sudo tee /etc/modprobe.d/blacklist-vmci-vsock.conf
sudo update-initramfs -u

# Verify installed kernel version after patching (Debian example)
dpkg -l | grep linux-image
uname -r