CVE-2022-32250: Linux Kernel Privilege Escalation Flaw

CVE-2022-32250 Overview

CVE-2022-32250 is a use-after-free vulnerability in the Linux kernel's netfilter subsystem, specifically within net/netfilter/nf_tables_api.c. This vulnerability allows a local user with the ability to create user/net namespaces to escalate privileges to root. The flaw stems from an incorrect NFT_STATEFUL_EXPR check that leads to a use-after-free condition, enabling attackers to manipulate kernel memory and achieve full system compromise.

Critical Impact

Local privilege escalation to root on Linux systems through 5.18.1, affecting enterprise Linux distributions, cloud infrastructure, and embedded devices running vulnerable kernel versions.

Affected Products

• Linux Kernel through version 5.18.1
• Fedora 35 and 36
• Debian Linux 9.0 and 10.0
• NetApp H300S, H500S, H700S, H410S, and H410C firmware

Discovery Timeline

• June 2, 2022 - CVE-2022-32250 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-32250

Vulnerability Analysis

This use-after-free vulnerability exists in the nf_tables component of the Linux kernel's netfilter framework. The nf_tables subsystem provides packet filtering, NAT, and packet classification functionality. The vulnerability is triggered when processing netfilter table expressions, where an incorrect check for NFT_STATEFUL_EXPR allows a freed memory object to be accessed again.

When a local attacker creates and manipulates user or network namespaces, they can trigger a specific sequence of operations that causes the kernel to free a memory structure while still maintaining a reference to it. Subsequent access to this freed memory can be exploited to corrupt kernel data structures, ultimately leading to arbitrary code execution with kernel privileges.

The attack requires local access and the ability to create unprivileged user namespaces, which is enabled by default on many modern Linux distributions. This makes the vulnerability particularly dangerous in containerized environments and multi-tenant systems where namespace isolation is a primary security boundary.

Root Cause

The root cause lies in the nf_tables_api.c file where the NFT_STATEFUL_EXPR flag check is improperly implemented. When destroying netfilter expressions, the code fails to properly validate whether an expression should be treated as stateful. This leads to a scenario where an expression object is freed prematurely while still being referenced elsewhere in the kernel, creating a classic use-after-free condition (CWE-416).

The vulnerable code path does not correctly handle the lifecycle of expression objects, particularly when expressions are bound to sets or chains. When these objects are deleted in a specific order, the reference counting becomes inconsistent, allowing freed memory to be accessed.

Attack Vector

The attack vector is local, requiring an attacker to have user-level access to the target system. The exploitation process involves:

1. Creating an unprivileged user namespace with network capabilities
2. Setting up netfilter tables and chains with specially crafted expressions
3. Triggering the use-after-free by manipulating the destruction order of these objects
4. Using heap spray techniques to place controlled data in the freed memory region
5. Achieving kernel code execution through corrupted function pointers or data structures

The vulnerability mechanism exploits the improper handling of NFT_STATEFUL_EXPR during expression destruction. When an nf_tables expression is created and bound to a set element, the kernel maintains references to this expression. The vulnerable code path allows an attacker to trigger destruction of the expression while the set element still holds a reference, leaving a dangling pointer. Subsequent operations on the set element access freed memory, which can be reallocated and controlled by the attacker. For detailed technical analysis, see the Theori Blog CVE-2022-32250 Analysis.

Detection Methods for CVE-2022-32250

Indicators of Compromise

• Unexpected processes running with root privileges that originated from unprivileged users
• Suspicious netfilter table manipulation operations in audit logs
• Kernel crash dumps or oops messages referencing nf_tables_api or netfilter components
• Unusual namespace creation patterns from non-administrative users

Detection Strategies

• Monitor for creation of user namespaces by non-privileged users using auditd rules for clone() syscalls with CLONE_NEWUSER flag
• Implement kernel-level monitoring using eBPF probes to detect suspicious nf_tables operations
• Deploy SentinelOne's kernel-level behavioral detection to identify privilege escalation attempts
• Analyze system call patterns for sequences indicative of namespace-based exploitation

Monitoring Recommendations

• Enable kernel auditing for netfilter-related operations and namespace creation events
• Configure real-time alerting on unexpected privilege changes for user processes
• Monitor for processes accessing /proc/self/ns/ and creating new network namespaces
• Implement continuous monitoring for kernel module loads and unexpected kernel memory allocations

How to Mitigate CVE-2022-32250

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit 520778042ccca019f3ffa136dd0ca565c486cedd
• Apply distribution-specific security updates from Debian, Fedora, or your Linux vendor
• Restrict unprivileged user namespace creation by setting kernel.unprivileged_userns_clone=0
• Audit systems for signs of exploitation before and after patching

Patch Information

The vulnerability has been addressed in the Linux Kernel Commit 5207780. Distribution-specific patches are available through:

• Debian Security DSA-5161 and DSA-5173
• Fedora Package Announcements
• NetApp Security Advisory NTAP-20220715-0005
• Red Hat Bugzilla Report 2092427

Workarounds

• Disable unprivileged user namespaces system-wide as a temporary mitigation
• Implement network namespace restrictions using security modules like AppArmor or SELinux
• Restrict access to netfilter operations using capability-based access controls
• Consider deploying SentinelOne endpoint protection for runtime exploit prevention

# Disable unprivileged user namespaces (temporary mitigation)
echo 0 > /proc/sys/kernel/unprivileged_userns_clone

# Make persistent across reboots
echo "kernel.unprivileged_userns_clone = 0" >> /etc/sysctl.conf
sysctl -p

# Verify the setting is applied
sysctl kernel.unprivileged_userns_clone