CVE-2023-39192: Linux Kernel Privilege Escalation Flaw

CVE-2023-39192 Overview

A flaw was found in the Netfilter subsystem in the Linux kernel. The xt_u32 module did not validate the fields in the xt_u32 structure. This flaw allows a local privileged attacker to trigger an out-of-bounds read by setting the size fields with a value beyond the array boundaries, leading to a crash or information disclosure.

Critical Impact

Local privileged attackers can exploit improper validation in the Netfilter xt_u32 module to read kernel memory beyond allocated boundaries, potentially causing system crashes or leaking sensitive kernel information.

Affected Products

• Linux Linux Kernel
• Red Hat Enterprise Linux 8.0
• Fedora 38

Discovery Timeline

• 2023-10-09 - CVE CVE-2023-39192 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-39192

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-bounds Read), affecting the Netfilter subsystem within the Linux kernel. The flaw exists in the xt_u32 module, which provides packet matching capabilities based on arbitrary 32-bit values extracted from packets. The module fails to properly validate the size fields within the xt_u32 structure before processing, allowing memory access beyond the intended array boundaries.

The vulnerability requires local access to the system with elevated privileges to exploit. An attacker who successfully exploits this flaw can either cause a denial of service condition through a kernel crash or potentially extract sensitive information from kernel memory through the out-of-bounds read operation.

Root Cause

The root cause of CVE-2023-39192 lies in insufficient input validation within the xt_u32 kernel module. Specifically, the module does not adequately verify that the size fields specified in the xt_u32 structure fall within the valid bounds of the associated arrays. When processing Netfilter rules that utilize the u32 match extension, the kernel trusts the size values provided without boundary checking, creating an opportunity for memory access violations.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have already obtained privileged access to the target system. The exploitation scenario involves a local privileged user manipulating the Netfilter xt_u32 match configuration by crafting malicious values for the size fields that exceed the legitimate array boundaries.

When the kernel processes these malformed values, it performs read operations beyond the allocated memory region. This can result in:

1. Information Disclosure: Reading kernel memory contents that may contain sensitive data such as kernel addresses, cryptographic keys, or other privileged information
2. System Crash: Accessing invalid memory regions that trigger kernel panics or system instability

The vulnerability mechanism involves crafting Netfilter rules with specially constructed xt_u32 match parameters. When the kernel evaluates these rules during packet processing, the out-of-bounds read occurs. Technical details regarding the specific exploitation method can be found in the Zero Day Initiative Advisory ZDI-CAN-18408.

Detection Methods for CVE-2023-39192

Indicators of Compromise

• Unexpected kernel crashes or panics related to the Netfilter subsystem or xt_u32 module
• Anomalous Netfilter rule configurations containing unusual u32 match parameters
• Kernel log entries indicating memory access violations in the xt_u32 code path
• Evidence of privileged users creating or modifying iptables/nftables rules with u32 matches

Detection Strategies

• Monitor kernel logs for out-of-bounds access warnings or errors associated with Netfilter modules
• Implement system auditing to track changes to firewall rules, particularly those using u32 match extensions
• Deploy kernel integrity monitoring tools to detect unexpected memory access patterns
• Use SentinelOne's behavioral AI to detect anomalous kernel module activity and privilege escalation attempts

Monitoring Recommendations

• Enable kernel auditing for iptables and nftables command executions with elevated privileges
• Configure alerting on kernel panic events, especially those referencing Netfilter subsystem components
• Implement centralized log collection to correlate Netfilter-related events across multiple systems
• Monitor for unauthorized attempts to load or configure the xt_u32 kernel module

How to Mitigate CVE-2023-39192

Immediate Actions Required

• Update Linux kernel to the latest patched version available for your distribution
• Apply Red Hat security patches RHSA-2024:2950 and RHSA-2024:3138 for Enterprise Linux systems
• Review and restrict access to users with CAP_NET_ADMIN capability or root privileges
• Audit existing Netfilter rules for any suspicious u32 match configurations

Patch Information

Security patches addressing this vulnerability have been released by multiple Linux distributions. Red Hat has published RHSA-2024:2950 and RHSA-2024:3138 for Enterprise Linux systems. Debian has addressed this in their LTS release as documented in the Debian LTS Announcement January 2024. Additional vendor-specific information is available through the Red Hat Bug Report #2226784.

Workarounds

• Disable or unload the xt_u32 kernel module if not required using modprobe -r xt_u32
• Blacklist the xt_u32 module to prevent automatic loading by adding it to /etc/modprobe.d/blacklist.conf
• Restrict privileged access to minimize the attack surface for local privilege exploitation
• Implement strict access controls on systems where kernel updates cannot be immediately applied

# Disable xt_u32 module if not in use
sudo modprobe -r xt_u32

# Blacklist the module to prevent automatic loading
echo "blacklist xt_u32" | sudo tee /etc/modprobe.d/blacklist-xt_u32.conf

# Verify the module is not loaded
lsmod | grep xt_u32