The heart of Linux’s networking stack, netfilter, is always a high-value target for hackers. In February 2024, the kernel maintainers patched CVE-2024-27065, a netfilter "nf_tables" vulnerability involving how internal table flags were handled when updating rulesets. This post breaks down the bug, how it was fixed, and what it means for sysadmins and researchers.

What is Netfilter and nf_tables?

Netfilter is the packet filtering framework of the Linux kernel — it’s the engine behind iptables, nftables, and more. nf_tables is the modern replacement, offering flexible and powerful network traffic filtering.

The Bug at a Glance

CVE-2024-27065 was about mishandling "table flags" (tiny bits that control how tables are treated) during update operations. A change introduced in kernel code accidentally compared *internal* flags, not just the user-visible ones. This confused logic around whether a transaction should skip certain steps.

The Real-World Impact

If a table update _didn’t_ actually modify its flags, but only affected internal, invisible flags, netfilter could treat the table as changed. This would wrongly cause a full update instead of a simple no-operation (NOP), introducing risk for unexpected behavior or, worse, security holes if a malicious user crafted an update to trigger this confusion.

Here's a simplified look at the problematic code (pseudocode for clarity)

// PRE-FIX: Checking ALL flags - including internal ones
if (new_table->flags != old_table->flags) {
    // Assume something changed, so do the update
    do_update();
} else {
    // No change detected, skip update
    return;
}

The problem was that new_table->flags included both user-visible flags and internal kernel-only flags. Comparing them would return *false differences* for benign/internal reasons.

The Fix: Compare the Right Flags

The fix was to restore skipping transaction if the update does not modify user-visible flags only. The code now checks only the external, user-controlled flags:

// POST-FIX: Only comparing user-visible flags, ignoring internal bits
if ((new_table->flags & USER_FLAGS_MASK) != (old_table->flags & USER_FLAGS_MASK)) {
    do_update();
} else {
    return; // No effective change, skip update
}

This prevents kernel logic from being tripped up by harmless internal changes.

Reference to the actual patch:
👉 Fix commit: netfilter: nf_tables: do not compare internal table flags on updates on kernel.org

All major kernels before the patch landed in

* v6.8-rc6 (mainline)
* v6.6.19 (LTS)
* v6.7.7

If you’re running an affected kernel and use nftables, you (_were_) at risk. Definitely make sure your distribution has backported this fix.

Can This Vulnerability Be Exploited?

Direct remote code execution is very unlikely (you’d need CAP_NET_ADMIN permissions, i.e., root-level privilege or container breakout).

But:
* In multi-tenant setups, malicious users with limited admin rights could abuse this unexpected update logic.
* It could interfere with ruleset updates in complex environments, leading to firewall misconfigurations, information leaks, or DoS.

No public exploits are known, but this kind of confusion can sometimes be chained with other bugs.

Exploit Example Demonstration (Conceptual)

Suppose an attacker can trigger repeated table updates like below, aiming to cause unexpected side effects:

# Attacker repeatedly sends updates that only change internal flags
sudo nft add table inet testtable
sudo nft flush table inet testtable

Before the fix, even if no external flags changed, the kernel thought there was a change, processed a full update, and could introduce unexpected state resets or performance drops.

What Should You Do?

- Patch your kernel. Make sure to get at least into mainline v6.8-rc6, or the latest stable for your distro.
- If using nftables in production, review update logs for weird/large update cycles.

References and Further Reading

- Linux kernel commit: netfilter: nf_tables: do not compare internal table flags on updates
- CVE record (Mitre, as assigned)
- Linux kernel netfilter docs
- nf_tables introduction

Summary

CVE-2024-27065 reminds us how even subtle kernel logic bugs, like comparing the wrong set of flags, can have real impact in complex systems like netfilter. Reviewing and patching frequently — and understanding what’s happening under the hood — is the best defense.

Timeline

Published on: 05/01/2024 13:15:50 UTC
Last modified on: 05/04/2025 09:03:27 UTC