CVE-2023-32233: Linux Kernel Privilege Escalation Flaw

CVE-2023-32233 Overview

CVE-2023-32233 is a use-after-free vulnerability in the Linux kernel's Netfilter nf_tables subsystem. The flaw exists in kernels up to and including 6.3.1 and arises from mishandling of anonymous sets when processing batch requests. Attackers with local access can trigger arbitrary read and write operations on kernel memory. Unprivileged local users can leverage the flaw to obtain root privileges on affected systems. The vulnerability is tracked under [CWE-416] (Use After Free) and impacts mainline Linux, Red Hat Enterprise Linux 7/8/9, and several NetApp HCI Baseboard Management Controller products.

Critical Impact
Local unprivileged users can escalate to root by abusing batch request processing in nf_tables, gaining full control of the host kernel.

Affected Products

Linux Kernel (up to and including 6.3.1)
Red Hat Enterprise Linux 7.0, 8.0, and 9.0
NetApp HCI Baseboard Management Controller (H300S, H410C, H410S, H500S, H700S)

Discovery Timeline

2023-05-08 - CVE-2023-32233 published to NVD and disclosed on the OpenWall OSS-Security list
2025-05-05 - Last updated in NVD database

Technical Details for CVE-2023-32233

Vulnerability Analysis

The Netfilter nf_tables subsystem implements the kernel-side machinery for nftables, the modern packet filtering framework on Linux. Configuration changes are applied as transactional batch requests, allowing multiple rule, set, and table operations to be processed atomically. The vulnerability stems from improper lifetime management of anonymous sets during these batched transactions.

Anonymous sets are inline sets created as part of a rule rather than as standalone named objects. When a batch deletes a rule that references an anonymous set and another operation in the same batch re-activates or references that set, the kernel fails to correctly track its activation state. The dangling reference allows an attacker to reclaim freed memory and operate on it through subsequent nf_tables calls.

Because nf_tables netlink operations are reachable from unprivileged user namespaces on many distributions, a local user without privileges can craft batches that produce arbitrary read and write primitives against kernel memory. Attackers chain these primitives to overwrite credential structures and escalate to root.

Root Cause

The root cause is that nf_tables_deactivate_set() did not properly transition anonymous sets to a deactivated state during the NFT_TRANS_PREPARE phase. As a result, the kernel could continue using or re-bind a set whose backing memory was already scheduled for release, producing a use-after-free condition.

Attack Vector

Exploitation requires local code execution as an unprivileged user on a kernel where nf_tables is reachable, typically via user namespaces. An attacker submits a carefully ordered netlink batch that mixes set bind, unbind, and rule replace operations. By steering the kernel allocator to reuse the freed set object, the attacker gains a controlled read/write primitive in kernel space and overwrites process credentials to obtain root.

The fix introduces a dedicated nf_tables_activate_set() helper and explicitly deactivates anonymous sets during the prepare phase:

void nf_tables_activate_set(const struct nft_ctx *ctx, struct nft_set *set)
{
	if (nft_set_is_anonymous(set))
		nft_clear(ctx->net, set);

	set->use++;
}
EXPORT_SYMBOL_GPL(nf_tables_activate_set);

void nf_tables_deactivate_set(const struct nft_ctx *ctx, struct nft_set *set,
			      struct nft_set_binding *binding,
			      enum nft_trans_phase phase)
{
	switch (phase) {
	case NFT_TRANS_PREPARE:
		if (nft_set_is_anonymous(set))
			nft_deactivate_next(ctx->net, set);

		set->use--;
		return;
	case NFT_TRANS_ABORT:

Source: Linux kernel commit c1592a89942e

Detection Methods for CVE-2023-32233

Indicators of Compromise

Unexpected processes running with UID 0 that were spawned from unprivileged shells or service accounts.
New or modified nftables rulesets created by non-administrative users, particularly batches that rapidly bind and unbind anonymous sets.
Kernel oops, panic, or KASAN messages referencing nf_tables_api.c, nft_set, or use-after-free conditions.

Detection Strategies

Audit netlink NFNL_SUBSYS_NFTABLES traffic from unprivileged user namespaces and flag unusual volumes of batched set operations.
Monitor for the creation of user namespaces (unshare, clone with CLONE_NEWUSER) followed by nftables activity from the same process tree.
Correlate sudden privilege transitions (e.g., setuid(0) from non-root processes) with preceding network namespace or nf_tables syscalls.

Monitoring Recommendations

Enable kernel auditing (auditd) for unshare, setns, and clone syscalls and ingest the events into a central analytics platform.
Forward dmesg and kernel ring buffer output to a SIEM and alert on KASAN, BUG, or general protection fault entries.
Track patch state across the fleet and alert on hosts still running vulnerable kernel builds below the fixed version.

How to Mitigate CVE-2023-32233

Immediate Actions Required

Apply the upstream Linux kernel patch from commit c1592a89942e or upgrade to a distribution kernel that includes the fix.
On Red Hat Enterprise Linux, install the kernel update referenced in the Red Hat Bugzilla report and reboot affected hosts.
For NetApp HCI BMC products, apply the firmware update described in the NetApp Security Advisory.
Where immediate patching is not possible, disable unprivileged user namespaces to reduce the attack surface for nf_tables operations.

Patch Information

The upstream fix is the kernel commit c1592a89942e9678f7d9c8030efa777c0d57edab, which introduces nf_tables_activate_set() and ensures anonymous sets are deactivated during the prepare phase of a batch transaction. Distribution-specific updates are available through the Debian Security Advisory DSA-5402, Debian LTS June announcement, and the Ubuntu Kernel Live Patch LSN-0095-1.

Workarounds

Set kernel.unprivileged_userns_clone = 0 (Debian/Ubuntu) or user.max_user_namespaces = 0 to prevent unprivileged users from creating namespaces required to reach nf_tables.
Restrict the CAP_NET_ADMIN capability and limit which users can load or modify nftables rulesets.
Apply Linux kernel live patches such as Ubuntu Livepatch on systems where reboot scheduling is constrained.

bash

# Configuration example: disable unprivileged user namespaces
sysctl -w kernel.unprivileged_userns_clone=0
echo 'kernel.unprivileged_userns_clone=0' | sudo tee /etc/sysctl.d/99-cve-2023-32233.conf
sysctl -w user.max_user_namespaces=0

# Verify the running kernel version after patching
uname -r