CVE-2026-53013

What it actually does

This is a Linux kernel macvlan netlink message sizing defect. When a macvlan interface has the broadcast cutoff attribute set to a non-default value, the kernel may under-allocate space while preparing detailed rtnetlink output for that interface. The subsequent attempt to include IFLA_MACVLAN_BC_CUTOFF can fail with -EMSGSIZE, trigger a kernel WARN_ON in the rtnetlink path, and prevent the macvlan interface from being dumped or displayed through netlink-based tooling such as `ip -d link show`. The primary impact is local denial of service or operational disruption of network-interface introspection rather than remote code execution or privilege escalation.

Why it matters to the business

Systems using macvlan networking, including container, virtualization, or network-segmentation deployments, may experience failed network inventory, troubleshooting, or automation workflows when querying affected macvlan interfaces. The issue can generate kernel warnings and break tooling that depends on complete netlink interface dumps, potentially delaying incident response or network operations. Exploitation likelihood appears low based on the provided EPSS score, absence from the Known Exploited Vulnerabilities catalog, and no reported mature exploit, but the issue should still be remediated during normal kernel maintenance windows for environments relying on macvlan.

How an attack unfolds

An attacker or local user with sufficient privileges to create or modify network interfaces configures a macvlan interface and sets its broadcast cutoff value to a non-default value, such as zero. When a local process later requests detailed link information through rtnetlink, for example using `ip -d link show`, the kernel attempts to serialize the macvlan attributes into a netlink message. Because the size calculation omitted space for the broadcast cutoff attribute, the buffer can be too small. The attribute write fails with `-EMSGSIZE`, a kernel warning is emitted in the rtnetlink path, and the requested interface dump fails. In practice, this can be used to disrupt local network-management tooling or monitoring that depends on netlink interface enumeration.

How to fix it

Upgrade to a Linux kernel version that includes the macvlan fix adding the missing size reservation for `IFLA_MACVLAN_BC_CUTOFF` in `macvlan_get_size()`. If distribution-specific fixed package versions are not available, track the relevant vendor kernel advisory and apply the first kernel update that references this macvlan/rtnetlink fix. As a temporary mitigation, avoid setting non-default macvlan `bc_cutoff` values on affected systems, restrict the ability to create or modify macvlan interfaces to trusted administrators only, and limit container or workload capabilities such as `CAP_NET_ADMIN` where not strictly required. Reboot into the fixed kernel after patching unless the vendor provides and supports a live-patching mechanism for this specific defect.

How to confirm the fix worked

Confirm the running kernel includes the fix from the operating-system or kernel vendor advisory. On a non-production test system, validate that creating a macvlan interface, setting `bc_cutoff` to a non-default value, and running `ip -d link show` succeeds without `-EMSGSIZE` failures or kernel warnings. Review kernel logs after validation to ensure no rtnetlink WARN_ON messages are produced. Also verify that routine monitoring and network inventory jobs can enumerate macvlan interfaces successfully after the update.

Typical attack path

1. Obtain local access with privileges sufficient to create or alter macvlan interfaces, typically administrative or network-administration capabilities.
2. Create a macvlan interface linked to an existing network interface.
3. Set the macvlan `bc_cutoff` attribute to a non-default value, such as `0`.
4. Trigger a detailed rtnetlink interface query using tooling such as `ip -d link show` or by waiting for monitoring/automation to query interface details.
5. Cause the kernel netlink message construction to fail with `-EMSGSIZE`, potentially producing a kernel WARN_ON and preventing successful interface dump output.

How to detect it

To determine whether this issue was triggered, review kernel logs and system journals for WARN_ON reports in rtnetlink or macvlan code paths, netlink message size failures, and `-EMSGSIZE` errors around the time network inventory or interface-detail commands failed. Check command history, audit logs, sudo logs, and orchestration logs for creation or modification of macvlan interfaces with `bc_cutoff` set to a non-default value. On systems running containers or network namespaces, review container runtime and audit telemetry for workloads granted `CAP_NET_ADMIN` that executed `ip link add`, `ip link set`, or similar netlink operations. Because this flaw does not inherently create files, network beacons, persistence, or remote command execution artifacts, detection should focus on local kernel warnings, failed interface dumps, and suspicious or unexpected macvlan configuration changes.

• ip link set macvlan0 type macvlan bc_cutoff 0
• ip -d link show macvlan0
• RTNETLINK answers: Message too long
• EMSGSIZE
• nla_put_s32
• IFLA_MACVLAN_BC_CUTOFF
• macvlan_get_size
• macvlan_fill_info
• rtnetlink: WARN_ON
• WARNING: CPU:
• CAP_NET_ADMIN
• ip link add macvlan0 link eth0 type macvlan mode bridge

What your security team should do

• Inventory Linux systems that use macvlan networking, especially container hosts, virtualization platforms, and systems with custom network namespace automation.
• Prioritize patching systems where untrusted workloads or users may have `CAP_NET_ADMIN` or equivalent ability to create or modify macvlan interfaces.
• Apply vendor kernel updates that include the macvlan `IFLA_MACVLAN_BC_CUTOFF` size-calculation fix, then reboot or live-patch as supported.
• Until patched, avoid non-default macvlan `bc_cutoff` settings where feasible and restrict macvlan configuration changes to trusted administrators.
• Monitor kernel logs for rtnetlink WARN_ON events, `-EMSGSIZE` errors, and failed `ip -d link show` or netlink inventory operations.
• Review container and orchestration policies to ensure workloads do not receive unnecessary network-administration capabilities.