CVE-2026-53219: Linux Kernel Netfilter Pointer Leak – Local Information Disclosure

Technical summary

CVE-2026-53219 affects the netfilter x_tables module's get-entries code paths (native and compat variants for IPv4, IPv6, and ARP). The vulnerability stems from a race-like condition in the copy-to-userspace sequence: the code first copies a fixed rule header containing entry->counters.pcnt (a percpu allocation address on SMP systems), then later overwrites the counter fields with sanitized snapshots. A malicious caller can trigger a page fault during the initial copy after pcnt has been read but before the sanitized counter copy executes, leaving the raw kernel pointer in the userspace buffer and returning -EFAULT. The fix reorders the copy sequence to extract only the fixed prefix before the counter fields, then separately copy the sanitized counter snapshot into place, preventing the window where the percpu pointer can leak.

Business impact

Information disclosure vulnerabilities that leak kernel memory layout undermine address space layout randomization (ASLR) and enable attackers to refine subsequent privilege-escalation or code-execution attacks. While this alone is not a direct code-execution vector, it removes a key defensive layer. Organizations running SMP Linux systems with unprivileged user accounts or containers face increased risk from local attackers who can map kernel memory and use that knowledge to target further vulnerabilities more reliably.

Affected systems

The Linux kernel across multiple versions is affected. The vulnerability is present in the netfilter x_tables subsystem, impacting systems that use iptables, ip6tables, or arptables for network filtering. Affected configurations include all SMP (multi-processor) Linux systems, since single-processor kernels do not use per-CPU counters in the same manner. Any distribution shipping an unpatched Linux kernel with the x_tables subsystem is susceptible.

Exploitability

Exploitation requires local access and is straightforward for an unprivileged user or containerized workload. No special kernel module, debugger, or exotic hardware is needed—only the ability to call the netfilter get-entries ioctl (e.g., via standard iptables listing operations) while triggering a userspace page fault. Reliability is high because the fault window, though small, can be hit repeatedly. Remote exploitation is not possible; this is a local-only vulnerability.

Remediation

Apply kernel patches from your Linux distribution that incorporate the upstream netfilter x_tables fix. Verify that patches address the native and compat code paths for IPv4, IPv6, and ARP variants. Alternatively, disable or restrict unprivileged access to iptables operations via LSM policies or containers with reduced capabilities. Monitor for suspicious iptables queries or unusual kernel memory access patterns in system logs.

Patch guidance

Check your distribution's kernel security advisories for the fix to netfilter x_tables get-entries code. The upstream patch reorders copy operations in the native and compat get-entries functions to sanitize counter data before it is exposed to userspace. Verify the patch applies to your kernel version and architecture (x86_64, ARM, etc.). Test in a staging environment before production rollout, particularly if you rely on iptables for policy enforcement.

Detection guidance

Monitor system call tracing (strace, audit) for repeated calls to netfilter get-entries ioctls (e.g., SO_GET_ENTRIES variants) that return -EFAULT. Detect attempts by unprivileged users to query firewall rules repeatedly in short bursts, which may indicate reconnaissance. Implement mandatory access controls to restrict direct iptables access to authorized administrators. Enable kernel Address Sanitizer (KASAN) or similar kernel debugging to catch pointer leaks in lab environments.

Why prioritize this

Although the CVSS score is MEDIUM (5.5) and the vulnerability requires local access, kernel memory disclosure significantly amplifies the risk of chained local privilege escalation attacks. Organizations with strong privilege boundaries and container isolation may deprioritize this relative to remote vectors, but those permitting unprivileged user accounts on shared systems should treat it as important. Patch promptly within a 30-day window.

Risk score, explained

The CVSS v3.1 score of 5.5 reflects the vulnerability's local-only attack surface (AV:L), low complexity (AC:L), and requirement for low privileges (PR:L). The impact is confidentiality only—no authentication bypass, code execution, or system crash—hence High availability impact is not applicable. The score is conservative; in environments with weak privilege separation, the real-world risk is higher because leaked kernel pointers directly enable exploitation of subsequent vulnerabilities.

Frequently asked questions