CVE-2021-32606: Linux Kernel Privilege Escalation Flaw

CVE-2021-32606 Overview

CVE-2021-32606 is a use-after-free vulnerability in the Linux kernel affecting versions 5.11 through 5.12.2. The flaw exists in the isotp_setsockopt function within net/can/isotp.c, which handles CAN (Controller Area Network) ISOTP (ISO 15765-2) socket options. A local attacker with low privileges can exploit this memory corruption vulnerability to escalate privileges to root, gaining complete control over the affected system.

Critical Impact

Local privilege escalation to root through use-after-free exploitation in the CAN ISOTP subsystem, enabling complete system compromise.

Affected Products

• Linux Kernel versions 5.11 through 5.12.2
• Fedora 32
• Fedora 33
• Fedora 34

Discovery Timeline

• 2021-05-11 - CVE-2021-32606 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-32606

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue where the application references memory after it has been freed. In the context of CVE-2021-32606, the isotp_setsockopt function in the Linux kernel's CAN ISOTP implementation fails to properly manage memory lifecycle during socket option configuration.

The CAN ISOTP protocol implementation was introduced to support ISO 15765-2 transport layer communication, commonly used in automotive diagnostics and ECU communication. When the SF_BROADCAST (Single Frame Broadcast) support was added, it introduced a race condition that allows memory to be accessed after deallocation.

The vulnerability requires local access and low privileges to exploit, but does not require user interaction. An attacker who successfully exploits this vulnerability can achieve full root access, compromising the confidentiality, integrity, and availability of the system.

Root Cause

The root cause is improper memory management in the isotp_setsockopt function when handling CAN ISOTP socket options with SF_BROADCAST support enabled. The function fails to properly synchronize access to memory structures, leading to a use-after-free condition when socket options are modified concurrently or in specific sequences.

The issue specifically affects kernel versions 5.11 and later where CAN ISOTP SF_BROADCAST support was introduced. Earlier kernel versions that lack this feature are not vulnerable.

Attack Vector

The attack requires local access to the system with the ability to create CAN ISOTP sockets. An attacker would:

1. Create a CAN ISOTP socket using the socket() system call with the appropriate protocol family
2. Manipulate socket options through setsockopt() calls targeting the isotp_setsockopt handler
3. Trigger the use-after-free condition through carefully timed or sequenced operations
4. Exploit the freed memory reference to achieve arbitrary code execution with kernel privileges
5. Escalate from the kernel context to obtain root access

The exploitation does not require physical access to CAN bus hardware, as the vulnerability exists in the kernel's socket handling code. Systems with the can_isotp module loaded or compiled into the kernel are potentially vulnerable.

Detection Methods for CVE-2021-32606

Indicators of Compromise

• Unexpected kernel crashes or system instability related to CAN ISOTP operations
• Suspicious processes with elevated privileges that originated from unprivileged users
• Kernel log messages indicating memory corruption or use-after-free conditions in net/can/isotp.c
• Unusual activity involving CAN ISOTP socket creation by non-automotive applications

Detection Strategies

• Monitor kernel version and ensure systems running Linux 5.11 through 5.12.2 are identified for patching
• Implement kernel-level auditing to track socket() and setsockopt() calls related to CAN protocols
• Deploy endpoint detection and response (EDR) solutions capable of detecting privilege escalation attempts
• Use kernel integrity monitoring tools to detect unauthorized kernel memory modifications

Monitoring Recommendations

• Enable kernel audit logging for socket operations involving the AF_CAN protocol family
• Monitor for processes attempting to load or interact with the can_isotp kernel module
• Track privilege escalation events and correlate with CAN-related socket activity
• Implement real-time kernel log analysis for memory corruption indicators

How to Mitigate CVE-2021-32606

Immediate Actions Required

• Update affected Linux kernel installations to version 5.12.3 or later immediately
• If patching is not immediately possible, disable or unload the can_isotp kernel module
• Review systems for evidence of prior exploitation, particularly unexpected privilege escalations
• Prioritize patching for systems where local user access is available

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix is available through the Linux Kernel Commit Update with commit ID 2b17c400aeb44daf041627722581ade527bb3c1d.

Fedora users should apply updates referenced in the Fedora Package Announcements for their respective distribution versions.

Additional security advisories are available from NetApp Security Advisory and OpenWall OSS Security.

Workarounds

• Disable the CAN ISOTP module by adding blacklist can_isotp to /etc/modprobe.d/blacklist.conf
• Remove the can_isotp module if currently loaded using rmmod can_isotp
• Restrict local user access to minimize the attack surface for local privilege escalation
• Implement network namespaces to isolate CAN socket access where applicable

# Disable CAN ISOTP module to mitigate vulnerability
echo "blacklist can_isotp" | sudo tee /etc/modprobe.d/can-isotp-blacklist.conf

# Remove module if currently loaded
sudo rmmod can_isotp 2>/dev/null || echo "Module not loaded"

# Verify module is not loaded
lsmod | grep can_isotp && echo "WARNING: Module still loaded" || echo "Module disabled"