CVE-2023-42753 - How A Simple Array Index Bug in Linux Netfilter Opens Doors To Local Privilege Escalation

In September 2023, a critical kernel vulnerability identified as CVE-2023-42753 was publicly disclosed. It affects the netfilter subsystem—the backbone of modern firewalls in Linux. At its heart, this bug is a classic: a missing safety check lets local attackers poke memory they shouldn’t touch, potentially leading to system crashes or even gaining root. Here we’ll walk through how it works, where it lives, and how attackers can turn a bug in array math into real-world exploitation.

Attacker can: Locally escalate privileges or crash the kernel.

- Root cause: Use of h->nets buffer without proper boundary checks/macro in some cases.

Impact: All modern Linux kernels using vulnerable netfilter code.

- Patched: Mainline commit

The Vulnerable Code

Inside netfilter, the kernel tracks per-network namespace information with arrays like h->nets[]. Code deep inside net/bridge/netfilter/ebtables.c forgot to use the right index macro—leading to calculations like:

// Vulnerable context
nb = h->nets[ctx->netns_id];

But if ctx->netns_id comes from user input (direct or indirect), and isn’t checked against the array’s real length, attackers can use a negative or out-of-bounds index. With that, memory just outside the h->nets buffer could be incremented/decremented, possibly manipulating sensitive kernel data.

In other words: a well-placed array index is enough to scribble over kernel memory.

Fixing The Flaw

The kernel patch added the missing macro—properly calculating the offset and ensuring that out-of-bounds values can’t sneak by.

Practical Exploit Scenario

This is a local-only bug: an attacker needs to run code on the affected Linux machine. With the right user permissions (usually CAP_NET_ADMIN in a namespace), they can:

1. Trigger Out-Of-Bounds Access: By crafting netfilter rules or net namespaces, set up a negative index to decrement arbitrary memory.
2. Corrupt Adjacent Data: The buffer overrun may target internal counters, pointers, or even parts of the credentials structure (cred) in the kernel. Clever targeting could flip bits that move the attacker’s privilege level to root.
3. Gain Root: After memory corruption, the attack code escalates privileges and drops into a root shell.

Proof-of-Concept (PoC) Code Snippet

This is a *simplified* illustration, not a full ready-to-run exploit. Direct kernel exploitation is dangerous!

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <linux/netlink.h>

int main() {
    // Preparation: create malicious netfilter rule setup with a bad network namespace id
    // This often requires CAP_NET_ADMIN in the current namespace.
    int bad_index = -8; // deliberately out-of-bounds
    // ... set up netlink messages triggering array access ...
    printf("Sent malicious rule with netns_id = %d\n", bad_index);
    return ;
}

Attackers would flesh this out, setting up namespaces and rules to target specific memory locations for a desired privilege escalation or denial of service.

Impact & Mitigation

- Who is affected? Any unpatched Linux kernel with netfilter (iptables/nftables). Most desktop/server systems are at risk if unpatched and where local users can access netfilter APIs.
- How to Fix: Update your kernel. Official patches landed in mainline and backported to supported stable kernels.

References & Further Reading

- CVE-2023-42753 on NVD
- Kernel Patch Commit
- netfilter subsystem documentation
- Exploitation Tracking - oss-security
- LWN coverage (subscription may be required)

Takeaway

CVE-2023-42753 is a reminder that even mature, heavily used kernel subsystems like netfilter can trip over the basics—missing an array bound check means handing attackers a primitive to rewrite kernel memory. For sysadmins: patch fast, restrict local admin rights, and keep a close eye on kernel advisories. For security researchers: this bug’s journey—from a simple macro omission to a potentially game-changing exploit—is a masterclass in kernel exploitation.

*Always test exploits in a safe environment. Kernel bugs can brick your machine!*

Timeline

Published on: 09/25/2023 21:15:00 UTC
Last modified on: 10/16/2023 19:41:00 UTC