CVE-2026-52937: Linux Kernel Info Disclosure Vulnerability

CVE-2026-52937 Overview

CVE-2026-52937 is a kernel stack information disclosure vulnerability in the Linux kernel's tap driver. The flaw resides in the tap_ioctl() handler for the SIOCGIFHWADDR request. The function copies 16 bytes of an uninitialised on-stack struct sockaddr_storage to userspace through ifr_hwaddr. The helper netif_get_mac_address() only populates sa_family and dev->addr_len bytes, leaving eight trailing bytes of sa_data[6..13] uninitialised. Issuing SIOCGIFHWADDR on a macvtap character device returns kernel .text and direct-map pointers, defeating Kernel Address Space Layout Randomization (KASLR).

Critical Impact
Local users with access to a macvtap chardev can leak kernel pointers, undermining KASLR and easing exploitation of other kernel vulnerabilities.

Affected Products

Linux kernel — tap driver (macvtap chardev path)
Distributions shipping vulnerable stable kernels prior to the referenced fix commits
Virtualization hosts exposing /dev/tapN to unprivileged users or containers

Discovery Timeline

2026-06-24 - CVE-2026-52937 published to NVD
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-52937

Vulnerability Analysis

The vulnerability is an Uninitialized Memory Use issue in the tap_ioctl() function within the Linux tap driver. When userspace issues the SIOCGIFHWADDR ioctl on a macvtap character device, the kernel allocates a struct sockaddr_storage on the stack without initialising it. The handler then calls netif_get_mac_address(), which writes only the sa_family field and dev->addr_len bytes of the hardware address into sa_data. For Ethernet, that is six bytes.

The ioctl path subsequently copies the full 16-byte ifr_hwaddr structure to userspace. Bytes sa_data[6..13] are never written by the helper and retain residual contents from the current kernel stack frame. Those eight bytes can contain return addresses, saved registers, and direct-map pointers from prior kernel activity on the same stack.

Root Cause

The root cause is the failure to zero the on-stack struct sockaddr_storage before partially populating it. The kernel relies on the helper to fill the structure, but the helper only writes the address-family-specific portion. The fix initialises the structure at declaration so any trailing bytes are zero before being copied to userspace.

Attack Vector

A local attacker with read access to a macvtap character device opens the device and issues a single SIOCGIFHWADDR ioctl. The returned ifr_hwaddr.sa_data buffer contains eight bytes of leaked kernel stack memory. Repeating the call across different process contexts and CPUs yields varied kernel pointers, allowing the attacker to recover KASLR slide values and direct-map addresses. These leaked pointers feed into follow-on kernel exploits that require defeating address randomization.

No exploitation code is generated here because no verified public proof-of-concept is referenced in the advisory. See the upstream commits 05305e8, 719007c, and bddc092 for the patch source.

Detection Methods for CVE-2026-52937

Indicators of Compromise

Unexpected processes opening /dev/tapN or /dev/macvtap* character devices outside of virtualization management workflows.
Repeated SIOCGIFHWADDR ioctl calls from non-root or container-confined processes against macvtap devices.
Userspace consumers reading hardware address fields longer than six bytes from tap devices.

Detection Strategies

Audit ioctl syscalls with auditd rules targeting SIOCGIFHWADDR (0x8927) issued against tap/macvtap file descriptors.
Deploy eBPF programs that trace tap_ioctl entry and log calling PID, UID, and command name for anomaly review.
Compare kernel build versions against the patched stable releases referenced in the upstream commits to identify exposed hosts.

Monitoring Recommendations

Track access permissions on /dev/tap* and /dev/macvtap* nodes and alert on changes that grant access to unprivileged users or containers.
Monitor for container escape preparation patterns such as kernel pointer leaks correlated with subsequent privileged syscall activity.
Forward auditd and kernel telemetry to a central log platform to retain ioctl traces for post-incident analysis.

How to Mitigate CVE-2026-52937

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in commits 05305e8, 719007c, and bddc092 from your distribution vendor.
Restrict access to macvtap character devices to root and trusted virtualization daemons only.
Inventory hosts running unpatched kernels that expose tap/macvtap interfaces to non-administrative users or container workloads.

Patch Information

The fix initialises struct sockaddr_storage at declaration inside tap_ioctl(), ensuring the trailing sa_data bytes are zeroed before the structure is copied to userspace. Patched commits are available on git.kernel.org: 05305e8, 719007c, and bddc092. Consult your Linux distribution's security advisories for backported package versions.

Workarounds

Remove or tighten permissions on /dev/tap* and /dev/macvtap* device nodes so that only privileged virtualization processes can open them.
Use seccomp or LSM policies to block the ioctl syscall with SIOCGIFHWADDR from untrusted container workloads.
Disable the macvtap kernel module on hosts that do not require it via modprobe blacklist configuration.

bash

# Configuration example: blacklist macvtap on hosts that do not need it
echo 'blacklist macvtap' | sudo tee /etc/modprobe.d/blacklist-macvtap.conf
sudo rmmod macvtap 2>/dev/null || true

# Restrict device node permissions where macvtap must remain loaded
sudo chmod 0600 /dev/tap* /dev/macvtap* 2>/dev/null
sudo chown root:root /dev/tap* /dev/macvtap* 2>/dev/null