CVE-2021-23134: Linux Kernel Privilege Escalation Flaw

CVE-2021-23134 Overview

A Use After Free vulnerability exists in the NFC (Near Field Communication) sockets implementation within the Linux Kernel before version 5.12.4. This memory corruption flaw allows local attackers to elevate their privileges on affected systems. In typical configurations, exploitation requires a privileged local user with the CAP_NET_RAW capability, which limits the attack surface but still presents a significant risk in multi-user environments or containerized workloads where privilege boundaries are critical.

Critical Impact

Local privilege escalation through Use After Free in Linux kernel NFC socket handling, potentially allowing attackers to gain root access from a limited privilege account with CAP_NET_RAW capability.

Affected Products

• Linux Kernel (versions before 5.12.4)
• Fedora 33 and Fedora 34
• Debian Linux 9.0

Discovery Timeline

• May 12, 2021 - CVE-2021-23134 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-23134

Vulnerability Analysis

CVE-2021-23134 is classified as CWE-416 (Use After Free), a memory corruption vulnerability class that occurs when a program continues to use a pointer after the memory it references has been freed. In this case, the vulnerability resides in the NFC socket implementation within the Linux kernel's networking subsystem.

The flaw enables local attackers to achieve privilege escalation by exploiting improper memory management in NFC socket operations. When an NFC socket is closed or released, certain memory structures may be freed while references to them still exist elsewhere in the kernel. A subsequent operation using these dangling pointers can lead to arbitrary memory access or code execution in kernel context.

The local attack vector requires the attacker to already have some level of access to the system. The requirement for CAP_NET_RAW capability in typical configurations provides some mitigation, as this capability is not granted to ordinary users by default. However, in container environments or systems with relaxed capability configurations, this prerequisite may be more easily satisfied.

Root Cause

The root cause of this vulnerability lies in improper lifecycle management of NFC socket structures within the Linux kernel. When NFC sockets are created and associated with various kernel objects, the reference counting or synchronization between socket closure and pending operations is insufficient. This leads to a race condition where memory can be freed while still being referenced by active kernel code paths.

The fix involves proper synchronization and reference counting to ensure that socket-related memory structures are not freed while still in use by any kernel component.

Attack Vector

The attack vector requires local access to the vulnerable system with the CAP_NET_RAW capability. An attacker would need to:

1. Create an NFC socket using the affected kernel interfaces
2. Trigger specific socket operations that create dangling references
3. Time the exploitation to access freed memory before it's reallocated
4. Use the corrupted memory state to achieve privilege escalation

The vulnerability exploitation requires precise timing and understanding of kernel memory allocation patterns. The attacker's payload would execute in kernel context, allowing complete system compromise including root privilege acquisition.

For detailed technical information about the fix, refer to the Linux Kernel Commit Change which addresses the improper memory handling in NFC socket operations.

Detection Methods for CVE-2021-23134

Indicators of Compromise

• Unexpected kernel crashes or oops messages related to NFC subsystem or socket operations
• Suspicious processes with elevated privileges that originated from limited user accounts
• Audit logs showing unusual CAP_NET_RAW capability usage or NFC socket operations
• System instability following NFC-related activity on systems that don't typically use NFC functionality

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for Use After Free or memory corruption errors in the NFC subsystem
• Implement capability auditing to track processes using CAP_NET_RAW
• Deploy kernel integrity monitoring solutions to detect unauthorized kernel memory modifications
• Use Linux Security Modules (LSM) such as SELinux or AppArmor to restrict NFC socket access

Monitoring Recommendations

• Enable kernel auditing for socket operations, particularly in the AF_NFC address family
• Configure automated alerting for kernel panic or oops events that reference NFC-related code
• Monitor for privilege escalation patterns where unprivileged processes suddenly acquire root capabilities
• Regularly review system capability configurations to ensure CAP_NET_RAW is only granted where necessary

How to Mitigate CVE-2021-23134

Immediate Actions Required

• Update the Linux kernel to version 5.12.4 or later immediately
• Review and restrict CAP_NET_RAW capability grants to only essential services and users
• Disable NFC functionality in the kernel if not required (blacklist nfc in modprobe configuration)
• Apply vendor-specific patches from Debian, Fedora, or other Linux distributions

Patch Information

The vulnerability has been addressed in Linux kernel version 5.12.4 and later. The fix is available through the official kernel git repository at commit c61760e6940d. Multiple Linux distributions have released security advisories and patches:

• Debian: Security updates available per Debian LTS Advisory June 2021 #19 and Debian LTS Advisory June 2021 #20
• Fedora: Updates for Fedora 33 and 34 available via Fedora Package Announcements
• NetApp: Consult NetApp Security Advisory NTAP-20210625-0007 for affected products

Workarounds

• Blacklist NFC kernel modules if NFC functionality is not required on the system
• Remove CAP_NET_RAW capability from containers and unprivileged processes
• Use seccomp profiles to restrict access to NFC-related system calls
• Implement network namespace isolation to limit socket access scope

# Configuration example
# Disable NFC kernel modules by blacklisting
echo "blacklist nfc" >> /etc/modprobe.d/blacklist-nfc.conf
echo "blacklist nfc_hci" >> /etc/modprobe.d/blacklist-nfc.conf
echo "blacklist nfc_llcp" >> /etc/modprobe.d/blacklist-nfc.conf

# Unload NFC modules if currently loaded
modprobe -r nfc_llcp nfc_hci nfc 2>/dev/null

# Verify modules are not loaded
lsmod | grep nfc

# Remove CAP_NET_RAW from a specific binary
setcap -r /path/to/binary

# Update kernel packages (Debian/Ubuntu)
apt update && apt upgrade linux-image-$(uname -r)

# Update kernel packages (Fedora/RHEL)
dnf update kernel