CVE-2026-52937: Linux Kernel TAP Driver Stack Memory Leak Defeats KASLR

Technical summary

The tap_ioctl() function in the Linux kernel tap driver handles the SIOCGIFHWADDR ioctl, which retrieves a network interface's hardware address. The code declares an uninitialized sockaddr_storage structure on the stack and passes it to netif_get_mac_address(). That function writes only the sa_family field and up to dev->addr_len bytes (typically 6 for Ethernet) into the sa_data array, but the structure itself is 16 bytes. This leaves sa_data[6..13]—eight bytes—uninitialized with whatever stack contents were present. The ioctl then copies the entire 16-byte structure to userspace via ifr_hwaddr without zeroing the trailing bytes. The fix initializes the sockaddr_storage structure to zero at declaration, ensuring no uninitialized data reaches userspace.

Business impact

This vulnerability enables information disclosure from the kernel to unprivileged local users, a critical stepping stone in privilege escalation or kernel exploitation chains. The specific leakage of KASLR-defeating pointers on macvtap systems weakens the OS defense posture. Any organization running tap or macvtap devices (common in virtualization, container, and network namespace environments) may have user-accessible tap devices that can be exploited. The threat is primarily relevant in multi-tenant or hostile-local-user threat models; isolated single-user systems face lower risk.

Affected systems

The Linux kernel is affected; the vulnerability exists in the tap driver code present across multiple kernel versions. Tap and macvtap devices are used extensively in KVM-based virtualization, LXC containers, Open vSwitch, and other network virtualization scenarios. Any Linux system exposing tap device management to untrusted local users—including container hosts, hypervisors with unprivileged guests, and systems with unprivileged network namespaces—is in scope. The kernel version cutoff and specific fixed versions depend on the distribution; verify against your kernel source and vendor advisories.

Exploitability

Exploitability is straightforward for a local user with access to a tap or macvtap device. Standard network inspection tools (e.g., ifconfig, ip link) on a tap device trigger the leak; no special capabilities are required beyond ability to open and ioctl a tap chardev. However, this is a local-only vulnerability (CVSS:3.1 AV:L) requiring prior system access. The leak is deterministic and repeatable, but converting the disclosed pointers into a full exploit depends on chaining this with other kernel vulnerabilities. It is not currently tracked in the CISA Known Exploited Vulnerabilities catalog.

Remediation

Apply the kernel patch that initializes the sockaddr_storage structure to zero before use in tap_ioctl(). Check your Linux distribution's security advisories and kernel update schedule for the patched version. Most major distributions (Red Hat, Debian, Ubuntu, SUSE, etc.) will release updates; timelines vary. For systems unable to patch immediately, restrict tap device access (via file permissions, AppArmor, or SELinux) to trusted users only, though this may break legitimate container or virtualization workloads.

Patch guidance

Kernel patches for this issue follow standard Linux development workflows and will be backported to stable and long-term-support branches. Check your distribution's errata pages and kernel changelogs. Typical guidance: update the kernel package to the first available version containing the fix for your kernel branch (verify the fix commit or changelog mentions tap_ioctl and sockaddr_storage initialization). For custom kernels, apply the upstream patch. Test in a staging environment given the invasive nature of kernel updates, particularly on systems with network virtualization dependencies.

Detection guidance

Detection at runtime is difficult because this is a passive information leak with no system-level side effects (no crash, no audit event triggered by the ioctl itself). Kernel-based detection would require live-patching or BPF probes to monitor tap_ioctl calls, but such hooks are not standard. Detection strategies: (1) audit-log and alert on tap device opens and ioctl calls for suspicious users; (2) scan kernel memory or core dumps post-mortem for leaked pointers and correlate timing with tap device access; (3) monitor for exploitation of derived kernel vulnerabilities (which attackers would chain with this leak). The most practical approach is version scanning and patch compliance monitoring.

Why prioritize this

Although classified as MEDIUM severity (CVSS 5.5), this vulnerability merits high priority in environments with unprivileged user access to tap devices because it directly enables KASLR bypass, a foundational step in kernel exploitation. In single-user or air-gapped systems, risk is lower. For container platforms (Kubernetes, Docker on Linux) and multi-tenant hypervisors, treat as high priority and patch promptly. The low barrier to local exploitation and the deterministic nature of the leak justify expedited patching in shared-tenancy or hostile-local-user scenarios.

Risk score, explained

CVSS 5.5 (MEDIUM) reflects local-only access requirement (AV:L) and low complexity (AC:L), but the lack of confidentiality loss in the CVSS vector understates the practical impact: kernel pointers defeat KASLR, a critical mitigation. The score penalizes lack of direct system compromise, yet in real-world kernel exploitation chains, this leak is a high-value information primitive. Organizations should apply business context: elevate priority if running container platforms or multi-tenant systems; de-prioritize only if tap devices are inaccessible to untrusted users.

Frequently asked questions