CVE-2023-52433: Linux Kernel Use-After-Free Vulnerability

CVE-2023-52433 Overview

CVE-2023-52433 is a vulnerability in the Linux kernel's netfilter subsystem, specifically affecting the nft_set_rbtree component. The flaw involves improper handling of synchronous garbage collection (GC) for newly added elements within ongoing transactions. When new elements are added to an nftables set backed by the red-black tree implementation, these elements may expire before the transaction completes. If synchronous GC processes these elements prematurely, the commit path can access already-released memory objects, leading to system instability or denial of service conditions.

Critical Impact

Local attackers with high privileges can trigger a denial of service condition by exploiting the race condition between transaction handling and garbage collection in the netfilter subsystem.

Affected Products

• Linux Kernel (multiple versions)
• NetApp ONTAP Tools 9 for VMware vSphere
• NetApp ONTAP Tools 10 for VMware vSphere

Discovery Timeline

• February 20, 2024 - CVE-2023-52433 published to NVD
• March 25, 2025 - Last updated in NVD database

Technical Details for CVE-2023-52433

Vulnerability Analysis

The vulnerability exists in the netfilter framework's nft_set_rbtree implementation, which provides red-black tree-based set storage for nftables firewall rules. The core issue stems from a race condition between the synchronous garbage collection mechanism and transaction handling for set elements.

When new elements are inserted into an rbtree-backed nftables set, they may have timeout values that cause them to expire before the containing transaction commits. The synchronous GC process, designed to clean up expired elements, does not properly account for elements that are still part of an uncommitted transaction. This can result in the GC freeing memory that the transaction commit path subsequently attempts to access, creating a use-after-free condition.

The fix implements a check to skip synchronous GC for elements that are part of the current transaction. Once the transaction finishes (either through commit or abort), the asynchronous GC mechanism safely handles cleanup of any expired elements. This approach ensures proper synchronization between element lifecycle management and transaction boundaries.

Root Cause

The root cause is improper check for unusual or exceptional conditions (CWE-273) in the garbage collection logic. The synchronous GC routine failed to verify whether elements were part of an active transaction before processing them for removal, leading to premature deallocation of memory still referenced by the transaction commit path.

Attack Vector

Exploitation requires local access to the system with elevated privileges to interact with the netfilter subsystem. An attacker would need to craft specific nftables transactions involving set elements with timeout configurations that trigger the race condition between element expiration and transaction completion. While the attack vector is limited due to the privilege requirements, successful exploitation can cause kernel crashes or system instability, resulting in denial of service.

Detection Methods for CVE-2023-52433

Indicators of Compromise

• Kernel panic or oops messages referencing nft_set_rbtree or netfilter components
• Unexpected system crashes during nftables rule manipulation
• Kernel log entries showing use-after-free warnings in netfilter subsystem
• Core dumps containing stack traces through nft_rbtree_gc functions

Detection Strategies

• Monitor kernel logs for oops or panic messages related to netfilter operations
• Implement kernel memory debugging tools such as KASAN to detect use-after-free conditions
• Track nftables configuration changes and correlate with system stability events
• Deploy endpoint detection solutions capable of monitoring kernel-level anomalies

Monitoring Recommendations

• Enable kernel auditing for netfilter and nftables operations
• Configure alerting for unexpected kernel module crashes or restarts
• Monitor system uptime metrics for anomalous restart patterns
• Implement centralized log collection for kernel messages across affected systems

How to Mitigate CVE-2023-52433

Immediate Actions Required

• Update Linux kernel to a patched version containing the fix for CVE-2023-52433
• Apply vendor patches from NetApp for ONTAP Tools deployments on affected versions
• Review and audit nftables configurations using timeout-based set elements
• Consider temporary removal of timeout configurations on critical firewall rules until patches are applied

Patch Information

Multiple kernel patches have been released to address this vulnerability. The fix modifies the nft_set_rbtree code to skip synchronous garbage collection for elements that are part of an active transaction, delegating cleanup to the asynchronous GC after transaction completion.

Patch commits are available from the kernel git repository:

• Kernel Git Commit 03caf75
• Kernel Git Commit 2ee52ae
• Kernel Git Commit 9a8c544
• Kernel Git Commit 9af7dfb
• Kernel Git Commit 9db9feb
• Kernel Git Commit c323ed6
• Kernel Git Commit e3213ff

For NetApp ONTAP Tools, refer to the NetApp Security Advisory NTAP-20240828-0003 for vendor-specific guidance.

Workarounds

• Avoid using timeout configurations in nftables sets until the kernel is patched
• Limit nftables administrative access to reduce the attack surface
• Consider using alternative set backends (hash, bitmap) that are not affected by this specific issue
• Implement network segmentation to limit exposure of systems where immediate patching is not feasible

# Check current kernel version for vulnerability assessment
uname -r

# List nftables sets with timeout configurations (potential risk areas)
nft list sets

# Verify loaded netfilter modules
lsmod | grep nf_tables