CVE-2023-4015

What it actually does

Use-after-free in Linux kernel netfilter nf_tables when an error occurs building an nftables rule: nft_immediate_deactivate() unbinds chains and deactivates objects that are later reused, enabling local privilege escalation.

Why it matters to the business

Local attackers can obtain full root-level control on affected Linux systems, increasing risk of data theft, persistence, or lateral movement inside the environment; the issue affects the widely used Linux kernel and has been public for over three years with no evidence of in-the-wild exploitation.

How an attack unfolds

An attacker with local shell access crafts a malformed nftables rule that triggers an error path during rule construction; the resulting use-after-free in nft_immediate_deactivate allows the attacker to reclaim the freed memory with controlled data and overwrite kernel structures to escalate privileges.

How to fix it

Upgrade the Linux kernel to a version containing commit 0a771f7b266b02d262900c75f1e175c7fe76fec2 or later; rebuild and reboot with the patched kernel; apply vendor-supplied updates as soon as they become available.

How to confirm the fix worked

Verify the running kernel contains commit 0a771f7b266b02d262900c75f1e175c7fe76fec2 or a later equivalent; confirm nftables rule construction no longer triggers the described deactivation path by reviewing kernel source or applying vendor security tests.

Typical attack path

1. Obtain local unprivileged shell on target system
2. Load or interact with nftables to submit a crafted rule set that forces an error during immediate expression deactivation
3. Trigger the use-after-free to corrupt kernel memory
4. Execute privilege-escalation payload to gain root

How to detect it

Review system logs for unusual nftables rule loading or errors around netfilter; inspect kernel crash dumps or audit logs for nft_immediate_deactivate references; correlate with unexpected privilege changes or new root processes.

What your security team should do

• Prioritize kernel updates on all internet-facing and multi-tenant systems
• Restrict unprivileged users from loading nftables modules where possible via kernel module blacklisting or capabilities
• Monitor for new kernel releases and schedule staged rollouts
• Review systems for custom nftables rules that could trigger error paths