CVE-2021-33033: Linux Kernel Use-After-Free Vulnerability

CVE-2021-33033 Overview

CVE-2021-33033 is a use-after-free vulnerability in the Linux kernel affecting versions prior to 5.11.14. The flaw exists in the cipso_v4_genopt function within net/ipv4/cipso_ipv4.c, where improper reference counting for CIPSO (Common IP Security Option) and CALIPSO DOI (Domain of Interpretation) definitions can lead to memory corruption and arbitrary value writes.

Critical Impact

Local attackers with low privileges can exploit this use-after-free condition to achieve high impact on system confidentiality, integrity, and availability, potentially leading to privilege escalation or arbitrary code execution in kernel context.

Affected Products

• Linux Kernel versions before 5.11.14
• Linux Kernel versions before 5.11.7 (partial fix)
• Systems utilizing CIPSO/CALIPSO network labeling features

Discovery Timeline

• May 14, 2021 - CVE-2021-33033 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-33033

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) stems from improper reference counting management in the Linux kernel's CIPSO and CALIPSO implementation. CIPSO (Common IP Security Option) is a network labeling protocol used for implementing mandatory access control in IP networks. The vulnerability occurs when DOI (Domain of Interpretation) definition objects are freed while still being referenced by other kernel components.

When the reference count for DOI definitions is mishandled, an attacker can trigger a condition where memory is freed prematurely while pointers to that memory still exist. Subsequent operations on these dangling pointers result in a use-after-free condition, allowing attackers to write arbitrary values to memory locations.

The vulnerability requires local access and low privileges to exploit, meaning an attacker must have some level of authenticated access to the system. However, no user interaction is required to trigger the vulnerability, making it a significant threat for multi-user systems or containerized environments where privilege escalation could lead to container escape scenarios.

Root Cause

The root cause of CVE-2021-33033 lies in the mishandled refcounting logic for DOI definitions in the CIPSO and CALIPSO subsystems. When processing network packets with CIPSO options, the kernel maintains reference counts to track the lifetime of DOI definition structures. A race condition or logic error in the reference counting mechanism allows the DOI definition to be freed while the cipso_v4_genopt function still holds a reference to it.

The specific issue was tracked as CID-ad5d07f4a9cd in the kernel's commit tracking system. The fix involves properly incrementing and decrementing reference counts at appropriate points in the code path to ensure DOI definitions remain valid throughout their use.

Attack Vector

The attack vector for this vulnerability is local, requiring the attacker to have access to the target system with low-privilege credentials. The exploitation typically involves:

1. Triggering allocation of a DOI definition structure through CIPSO network operations
2. Manipulating the reference counting to cause premature deallocation
3. Triggering a code path that accesses the freed memory via cipso_v4_genopt
4. Using the resulting memory corruption to write arbitrary values

The Syzkaller KASAN Use-After-Free analysis provides detailed information about the memory corruption behavior. Technical details about the bug reproduction can be found in the Syzkaller Bug Report #96e7d.

Detection Methods for CVE-2021-33033

Indicators of Compromise

• KASAN (Kernel Address Sanitizer) reports indicating use-after-free reads in cipso_v4_genopt
• Kernel panic or unexpected crashes related to CIPSO/CALIPSO subsystem operations
• Suspicious local privilege escalation attempts following network label operations
• Memory corruption indicators in kernel logs referencing net/ipv4/cipso_ipv4.c

Detection Strategies

• Deploy kernel-level monitoring to detect abnormal memory access patterns in the CIPSO subsystem
• Enable KASAN in development/testing environments to catch use-after-free conditions
• Monitor for unusual local privilege escalation attempts on systems using network labeling
• Implement audit logging for CIPSO/CALIPSO configuration changes and network operations

Monitoring Recommendations

• Configure system logging to capture kernel warnings and errors related to network subsystems
• Use security monitoring tools to detect memory corruption signatures in kernel space
• Regularly review system logs for crashes or panics involving cipso_v4_genopt or related functions
• Deploy endpoint detection solutions capable of identifying kernel-level exploitation attempts

How to Mitigate CVE-2021-33033

Immediate Actions Required

• Update the Linux kernel to version 5.11.14 or later immediately
• If immediate patching is not possible, consider disabling CIPSO functionality if not required
• Restrict local access to sensitive systems until patches can be applied
• Monitor affected systems for signs of exploitation attempts

Patch Information

The vulnerability was addressed in Linux kernel version 5.11.14. Two relevant commits fix this issue:

• Linux Commit Details #ad5d07f - Primary fix for the refcounting issue
• Linux Commit Details #1165aff - Additional hardening

Full changelog details are available in the Linux Kernel ChangeLog 5.11.14 and Linux Kernel ChangeLog 5.11.7.

Workarounds

• Disable CIPSO and CALIPSO network labeling if not required for operations
• Limit local access to trusted users only until patches are applied
• Use SELinux or AppArmor to restrict access to network labeling interfaces
• Consider deploying runtime kernel protection mechanisms where available

# Check current kernel version
uname -r

# Verify CIPSO module status
lsmod | grep cipso

# If CIPSO is not required, it can be blacklisted
echo "blacklist cipso_ipv4" >> /etc/modprobe.d/blacklist-cipso.conf

# Update kernel on Debian/Ubuntu systems
apt update && apt upgrade linux-image-generic

# Update kernel on RHEL/CentOS systems
yum update kernel