CVE-2020-36694: Linux Kernel Use-After-Free Vulnerability

CVE-2020-36694 Overview

A use-after-free vulnerability has been discovered in the netfilter subsystem of the Linux kernel before version 5.10. The vulnerability exists in the packet processing context due to mishandling of the per-CPU sequence count during concurrent iptables rules replacement. This flaw could be exploited by an attacker with the CAP_NET_ADMIN capability in an unprivileged namespace to potentially achieve local privilege escalation or cause system instability.

Critical Impact

Local attackers with CAP_NET_ADMIN capability in an unprivileged namespace can exploit this use-after-free condition during concurrent iptables rules replacement, potentially leading to privilege escalation, arbitrary code execution, or denial of service.

Affected Products

• Linux Kernel versions prior to 5.10
• Linux Kernel versions where commit cc00bca was reverted (noted in 5.12)
• Systems using netfilter/iptables with namespace capabilities

Discovery Timeline

• 2023-05-21 - CVE-2020-36694 published to NVD
• 2025-01-31 - Last updated in NVD database

Technical Details for CVE-2020-36694

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to reference memory after it has been freed. In this specific case, the vulnerability resides in the Linux kernel's netfilter subsystem, which is responsible for packet filtering, network address translation, and other packet mangling operations.

The root issue lies in the improper handling of per-CPU sequence counts during concurrent iptables rules replacement operations. When multiple rule replacement operations occur simultaneously, a race condition can arise that leads to accessing freed memory structures in the packet processing context.

The vulnerability requires local access and the CAP_NET_ADMIN capability, which can be obtained within an unprivileged user namespace. This makes the vulnerability exploitable even by non-root users who can create user namespaces with network administration capabilities.

Root Cause

The vulnerability stems from incorrect synchronization of per-CPU sequence counters used during iptables rules replacement. When rules are being replaced concurrently across multiple CPUs, the sequence count mechanism fails to properly coordinate access to rule structures. This allows packet processing code to reference rule data structures that have already been freed, resulting in a use-after-free condition.

The fix was implemented in commit cc00bcaa589914096edef7fb87ca5cee4a166b5c, which was included in Linux kernel version 5.10. However, this commit was subsequently reverted in version 5.12, indicating potential regression issues or alternative fixes being applied.

Attack Vector

The attack requires local access to the system with the ability to obtain CAP_NET_ADMIN capability. On systems where unprivileged user namespaces are enabled, an attacker can:

1. Create an unprivileged user namespace with network capabilities
2. Trigger concurrent iptables rule replacement operations
3. Race the packet processing context against the rule replacement
4. Exploit the resulting use-after-free to corrupt memory

The exploitation of this vulnerability could allow an attacker to escalate privileges, execute arbitrary code in kernel context, or cause denial of service through system crashes.

Detection Methods for CVE-2020-36694

Indicators of Compromise

• Unexpected kernel crashes or oops messages related to netfilter or iptables subsystems
• Suspicious user namespace creation activity combined with iptables manipulation
• Memory corruption indicators in kernel logs referencing netfilter structures
• Abnormal frequency of iptables rule modifications from non-root processes

Detection Strategies

• Monitor kernel logs for use-after-free warnings related to netfilter or nf_tables components
• Implement audit rules to track CAP_NET_ADMIN capability usage within user namespaces
• Deploy kernel-level memory sanitizers (KASAN) in development environments to detect exploitation attempts
• Track processes creating user namespaces and subsequently manipulating iptables rules

Monitoring Recommendations

• Enable kernel crash dump collection and analysis for post-incident forensics
• Configure alerts for abnormal iptables rule modification patterns
• Monitor for processes operating within user namespaces with elevated network capabilities
• Implement syslog monitoring for netfilter-related kernel warnings and errors

How to Mitigate CVE-2020-36694

Immediate Actions Required

• Upgrade Linux kernel to version 5.10 or later where the fix was initially applied
• Restrict unprivileged user namespace creation by setting kernel.unprivileged_userns_clone=0
• Audit systems for processes using CAP_NET_ADMIN capability inappropriately
• Review and apply vendor-specific patches from distributions

Patch Information

The vulnerability was addressed in the Linux kernel commit cc00bcaa589914096edef7fb87ca5cee4a166b5c, which was included in kernel version 5.10. Organizations should consult their distribution vendor for specific patched package versions. Additional advisories have been published by NetApp and the issue was originally reported via the Syzkaller automated testing system.

For detailed changelog information, refer to the Linux Kernel ChangeLog 5.10 and Linux Kernel ChangeLog 5.12.

Workarounds

• Disable unprivileged user namespace creation to prevent exploitation path
• Restrict CAP_NET_ADMIN capability to trusted users and processes only
• Implement SELinux or AppArmor policies to limit iptables manipulation
• Consider using network namespace isolation to limit exposure

# Configuration example
# Disable unprivileged user namespace creation to mitigate exploitation
echo 'kernel.unprivileged_userns_clone=0' >> /etc/sysctl.conf
sysctl -p

# Verify the setting is active
sysctl kernel.unprivileged_userns_clone