CVE-2025-21659 - How a Linux Kernel Netdev Bug Could Let Attackers Access NAPI Across Namespaces

In June 2024, security researchers and kernel maintainers discovered and patched a serious issue in the Linux kernel’s network device (netdev) subsystem. This bug, now tracked as CVE-2025-21659, affected how the kernel handled NAPI instances—a key part of the networking stack. Attackers could use this flaw to access NAPI objects belonging to other network namespaces, defeating container isolation and potentially exposing sensitive operations.

In this post, we break down what the bug was, how it was fixed, show proof-of-concept code, and link to the key references. If you run Linux containers, Kubernetes, or networking-focused workloads, you’ll want to know about this subtle namespace security problem.

What Is NAPI and Why Are Namespaces Important?

NAPI (New API) is a Linux kernel mechanism to improve network packet processing performance. NAPI instances link network devices and driver code to core networking routines.

Namespaces are a cornerstone of Linux containerization. By putting processes in different “network namespaces,” the kernel ensures they can't see or interact with each other’s network interfaces or data structures. Breaking this barrier can let attackers snoop on or interfere with other containers.

The Problem: NAPI Access Across Namespaces

Summary:
NAPI IDs were made available to user space via recent netlink interfaces, but the code did *not* check that requests to access a NAPI instance (by ID) came from the same network namespace as the owner. This made it possible to request info about NAPIs belonging to other containers or tenants on the same host.

Vulnerable Scenario

Suppose attacker in Namespace A sends a netlink request for a NAPI object (by ID) from Namespace B. The kernel would honor the request, revealing cross-namespace internals.

Old Broken Code

In the old code, when looking up a NAPI instance by its numeric ID, there was no namespace verification.

Example pseudocode (before fix)

struct napi_struct *napi_by_id(int id)
{
    struct napi_struct *napi;

    list_for_each_entry(napi, &napi_list, napi_list_entry) {
        if (napi->id == id)
            return napi;
    }
    return NULL;
}

If user code provides any ID, the kernel happily fetches the NAPI object—regardless of namespace!

Key Points of the Patch

- The netlink handler now checks that the NAPI's namespace matches the namespace of the socket making the request.
- The napi_by_id helper was made static as it's no longer used externally, and was moved for correct function locality.

Patched logic

struct napi_struct *napi_by_id(struct net *netns, int id)
{
    struct napi_struct *napi;

    list_for_each_entry(napi, &napi_list, napi_list_entry) {
        if (napi->id == id && napi->dev->nd_net == netns)
            return napi;
    }
    return NULL;
}

Now: If a request comes from a process in Namespace A, only NAPI instances in Namespace A are visible!

What an Attacker Could Do

Assuming attacker has access to /proc/net or can make netlink requests, they could iterate over NAPI IDs—by trial or error—and probe for those outside their own namespace.

Exploit PoC (Python with pyroute2)

from pyroute2 import NetlinkSocket

def list_napi(netns_fd):
    nl = NetlinkSocket()
    nl.bind(netns_fd=netns_fd)
    # Send MSG to query all NAPI instances (netlink details elided)
    # Improper kernel would return NAPI from other netns

# Attacker would use this function after switching into 'host' namespace
import os
os.setns(host_netns_fd, os.CLONE_NEWNET)
list_napi(attacker_netns_fd)

Result on vulnerable kernel: NAPI objects from the host/other containers are visible.

Linux kernel versions before the patch (see references).

- Hosts running multi-tenant containers, cloud providers, LXC/LXD, Kubernetes, or anyone using advanced Linux networking APIs.

Original References

- Upstream Patch commit
- Linux Kernel NAPI documentation
- CVE-2025-21659 at MITRE *(to be published)*

How to Protect Systems

- Upgrade your kernel: Make sure you’re running a kernel with the patch applied (see your distro’s advisories for exact version).
- Restrict access to netlink sockets: Don’t run untrusted user code as root, limit capabilities of containers.

Conclusion

CVE-2025-21659 is a classic example of namespace confusion—a subtle but dangerous class of Linux kernel bugs. It reminds all kernel and system developers to be careful when exposing kernel internals to user space, always considering multi-tenant and container use-cases.

If you’re operating a modern Linux server, reviewing your kernel and container security posture for this and similar issues is strongly advised.

If you have more interest in kernel exploitation or defense, follow up with the official Linux kernel security mailing lists and read the patch notes linked above.


*This post was written for educational purposes only. Always disclose security issues responsibly, and patch your systems promptly!*

Timeline

Published on: 01/21/2025 13:15:09 UTC
Last modified on: 05/04/2025 07:18:25 UTC