CVE-2026-53238: Linux Kernel Netlabel Mask Validation Denial of Service

Technical summary

The netlbl_unlabel_addrinfo_get() function in the kernel's netlabel subsystem uses the address attribute length to determine whether to parse IPv4 or IPv6 address data. However, it does not enforce strict length validation on the corresponding mask attribute. Since NLA_BINARY policy attributes are treated as maximum lengths by default, a Generic Netlink request can supply a valid address attribute paired with an undersized mask attribute. The kernel then attempts to read the mask as a complete struct in_addr (IPv4) or struct in6_addr (IPv6), reading past the provided buffer into uninitialized kernel memory. The fix applies NLA_POLICY_EXACT_LEN() to both address and mask attributes, enforcing strict length validation at the netlink policy layer and rejecting non-compliant requests before kernel processing.

Business impact

This vulnerability has limited direct business impact due to its local-only attack vector and MEDIUM severity rating. It does not enable remote code execution or privilege escalation. However, it can be exploited by unprivileged local users to crash the kernel, resulting in unplanned downtime for affected systems. In environments where untrusted users have local access—such as shared hosting, containers, or multi-tenant cloud deployments—this becomes a practical concern. The denial of service could disrupt services or trigger cascading failures in dependent infrastructure.

Affected systems

The Linux kernel is the sole affected component. This vulnerability impacts the network labeling subsystem, which is commonly built into kernel distributions used in enterprise servers, cloud platforms, and container hosts. The specific attack surface is limited to systems that accept Generic Netlink requests from non-root users—a permission model that varies by deployment. Not all systems have netlink socket access exposed to unprivileged processes by default, so environmental factors determine actual risk.

Exploitability

Exploitation requires local access and unprivileged user-level privileges (PR:L per CVSS vector). No network access is needed. An attacker with a local user account can craft a malicious Generic Netlink request to trigger the flaw. The attack is straightforward to develop once the vulnerability is understood, but requires the attacker to already be on the system. This vulnerability is not being actively exploited in the wild as of the publication date, and it has not been designated for inclusion in CISA's Known Exploited Vulnerabilities (KEV) catalog.

Remediation

Patching is the primary remedy. Apply the kernel security update that introduces NLA_POLICY_EXACT_LEN() validation for the unlabeled address and mask attributes in the netlabel module. Simultaneously, review access controls to the netlink socket interface—restrict unprivileged user access where operationally feasible. In environments with high multi-tenancy risk, consider applying kernel hardening techniques such as restricted netlink access or SELinux/AppArmor policies that limit netlink socket creation.

Patch guidance

Contact your Linux distribution vendor or consult the kernel.org security advisories for patched kernel versions specific to your release. Most major distributions (Red Hat Enterprise Linux, Ubuntu LTS, Debian, SLES, etc.) will issue kernel updates addressing this issue. Verify the fix by confirming that the netlabel policy uses NLA_POLICY_EXACT_LEN() rather than the default NLA_BINARY length semantics. Testing should validate that malformed Generic Netlink requests are properly rejected before kernel processing resumes.

Detection guidance

Monitor kernel logs and audit trails for Generic Netlink requests originating from unprivileged users that might probe the netlabel subsystem. A successful exploitation attempt will manifest as an unexpected kernel panic or oops related to the netlabel module. Implement alerting on kernel crashes, particularly if they correlate with suspicious netlink activity from local user accounts. Security audits should examine which local accounts have access to netlink sockets; in some deployments, this can be restricted via seccomp, LSM policies, or network namespace isolation.

Why prioritize this

This vulnerability merits patching but is not in the highest urgency tier. The MEDIUM CVSS score (5.5) reflects the local-only attack surface, lack of information disclosure or privilege escalation, and requirement for unprivileged user credentials. However, denial of service attacks can significantly disrupt business continuity, especially in multi-tenant or high-availability environments. Organizations with a large number of local user accounts or untrusted container workloads should prioritize patching over those with tightly controlled local access.

Risk score, explained

The CVSS 3.1 score of 5.5 (MEDIUM) is driven by: local attack vector (AV:L), which eliminates remote exploitation; low complexity (AC:L), meaning no special conditions are needed; low privilege level (PR:L), as unprivileged users can trigger it; no user interaction required (UI:N); and ultimately a high impact on availability (A:H) through denial of service. The lack of confidentiality or integrity impact prevents a higher severity rating.

Frequently asked questions