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If you use Linux, especially for firewalls or servers, you’ve probably heard about netfilter. It’s the framework at the heart of firewalling and network address translation on Linux. In 2023, researchers found a critical vulnerability (CVE-2023-4004) in netfilter’s PIPAPO set type. This flaw can let a normal user crash your machine—and with the right know-how, even get root privileges.

In this article, I’ll walk you through how the bug works, what the exploit looks like, and how you can protect your systems.

What is CVE-2023-4004?

CVE-2023-4004 is an important security bug found in the Linux kernel netfilter code, specifically in the nft_pipapo_remove function. Here, a "use-after-free" bug is triggered if the function tries to remove an element without the expected NFT_SET_EXT_KEY_END flag. This essentially means netfilter might try using memory that’s already been freed.

Here’s the netfilter commit that fixed the bug.

Use-After-Free: In Plain English

A use-after-free bug is simple at its core: Imagine you throw away a toy, but someone else grabs the same toy, thinking it’s still theirs. Using memory after it’s been freed opens the door for attackers to sneak in fake memory (crafted objects) and trick the kernel into running rogue code.

How Does the Bug Happen?

The bug happens in the netfilter code when the NFT_SET_EXT_KEY_END flag is missing. The kernel’s netfilter PIPAPO (PI-PAPO) set backend assumes the flag is in place during deletion. If it’s not there, it releases some memory but keeps trying to use it, which can crash the kernel or let users run malicious code.

Here’s a simplified version of the offending code (in C)

int nft_pipapo_remove(struct nft_set *set, const struct nft_set_elem *elem) {
    ... 
    if (!nft_set_ext_exists(ext, NFT_SET_EXT_KEY_END))
        return -EINVAL; // We're missing the key end!
    ...
    // further operations that may reference 'ext'
}

But in the vulnerable version, the code might still go ahead and use that ext struct even after freeing it, leading to chaos.

Kernel frees some memory but then uses it anyway.

4. Attacker can use heap spraying (flooding the memory allocator with controlled data) to fill the hole with malicious structures.

Here’s a pseudo-PoC showing the relevant nftables commands

# Requires CAP_NET_ADMIN or unprivileged user with user namespaces

# Create a new set with pipapo type
nft add table t
nft add set t s { type ipv4_addr\; flags dynamic\; }

# Add and remove element in a way that triggers the bug
nft add element t s { 1.2.3.4 }
nft delete element t s { 1.2.3.4 }
# Now, try to delete the same element again, but force it to miss the NFT_SET_EXT_KEY_END
nft delete element t s { 1.2.3.4 }

(Note: To properly exploit, you’ll need low-level manipulation, potentially via custom C or Python programs using libmnl or pyroute2.)

Crash the system: Immediately, this is a local Denial-of-Service (DoS).

- Elevate privileges: By careful heap spraying, the attacker could place a fake object in the freed space. If the kernel executes it, it might lead to arbitrary code execution as root.

This is especially dangerous on systems that allow unprivileged users to create or manipulate netfilter rules (e.g., unprivileged containers, certain sandboxed apps).

Original References

- Red Hat Security Advisory - CVE-2023-4004
- Kernel.org Git Patch
- Mitre CVE Database

How Can You Fix It?

1. Update your kernel!  
The only safe way to fix this is to update to a version where this patch is applied. Most major distributions have backported the fix.

2. Limit netfilter access:  
Restrict who can create/modify firewall rules. Don’t give untrusted users CAP_NET_ADMIN or similar capabilities.

3. Harden user namespaces:  
Since some exploits rely on user namespaces, disable them if you don’t need them.

Conclusion

CVE-2023-4004 is a textbook example of how a small code oversight can lead to serious system risk. For sysadmins and power users, keeping kernels up-to-date is a must, as these bugs tend to get weaponized quickly after public disclosure.

*Feel free to share this post with your fellow Linux friends or security teams. Stay patched, stay secure!*


Are you interested in technical deep dives like this? Let me know what other CVEs or Linux security topics you want to see covered!

Timeline

Published on: 07/31/2023 17:15:00 UTC
Last modified on: 08/19/2023 18:17:00 UTC