CVE-2020-25671: Linux Kernel Use-After-Free Vulnerability

CVE-2020-25671 Overview

A use-after-free vulnerability was discovered in the Linux Kernel's NFC (Near Field Communication) subsystem, specifically within the llcp_sock_connect() function. This flaw stems from a reference count leak that occurs during LLCP (Logical Link Control Protocol) socket connection handling. When exploited, this vulnerability can lead to privilege escalation, allowing a local attacker with limited privileges to potentially gain elevated access on the affected system.

Critical Impact

Local privilege escalation vulnerability in the Linux Kernel NFC subsystem that could allow attackers to gain root-level access through improper memory management in LLCP socket connections.

Affected Products

• Linux Kernel (multiple versions)
• Fedora 32, 33, and 34
• Debian Linux 9.0
• NetApp Active IQ Unified Manager for vSphere
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller Firmware
• NetApp H-Series (H410C, H300S, H500S, H700S, H300E, H500E, H700E, H410S) Firmware

Discovery Timeline

• 2020-11-01 - Vulnerability disclosed on Openwall security mailing list
• 2021-05-26 - CVE-2020-25671 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25671

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption issue that occurs when a program continues to use a memory pointer after it has been freed. In this specific case, the flaw exists in the Linux Kernel's NFC LLCP implementation within the llcp_sock_connect() function.

The vulnerability requires local access to exploit, meaning an attacker must have some level of user access to the target system. Once exploited successfully, the attacker can potentially achieve complete compromise of confidentiality, integrity, and availability of the affected system. This makes it particularly dangerous in multi-user environments or systems where untrusted users have local access.

Root Cause

The root cause of this vulnerability is a reference count (refcount) leak in the llcp_sock_connect() function within the NFC LLCP subsystem. Reference counting is a memory management technique used in the Linux Kernel to track how many references exist to a particular object. When the reference count is not properly decremented upon connection failure or during certain error paths, the kernel loses track of the object's lifecycle.

This refcount leak creates a condition where memory can be freed while still being referenced, or conversely, where an object is accessed after its memory has been reclaimed by the system. The inconsistency between the actual memory state and the kernel's reference tracking enables the use-after-free condition.

Attack Vector

The attack vector for CVE-2020-25671 is local, requiring an attacker to have authenticated access to the vulnerable system. The exploitation scenario involves manipulating NFC LLCP socket connections to trigger the reference count leak. The attack does not require user interaction beyond the attacker's own actions.

An attacker would typically:

1. Create an LLCP socket connection through the NFC subsystem
2. Manipulate the connection state to trigger the refcount leak condition
3. Force the kernel to free the memory while maintaining a dangling reference
4. Reallocate the freed memory with attacker-controlled content
5. Trigger use of the dangling reference to achieve code execution or privilege escalation

The vulnerability is particularly concerning on systems where NFC functionality is enabled and local users have access to NFC-related system calls.

Detection Methods for CVE-2020-25671

Indicators of Compromise

• Unexpected kernel panics or crashes related to NFC or LLCP subsystems
• Suspicious processes attempting to access NFC socket interfaces
• Unusual privilege escalation events from low-privileged user accounts
• Memory corruption warnings in kernel logs referencing llcp_sock_connect or related functions

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for LLCP-related errors or use-after-free warnings
• Implement system call auditing for NFC socket operations using auditd
• Deploy kernel-level monitoring tools to detect abnormal memory access patterns in the NFC subsystem
• Use SentinelOne's behavioral AI to detect privilege escalation attempts following NFC socket manipulation

Monitoring Recommendations

• Enable kernel address sanitizer (KASAN) on development and test systems to detect memory corruption
• Configure system logging to capture NFC subsystem events at debug level during security investigations
• Implement file integrity monitoring on critical system binaries to detect post-exploitation modifications
• Monitor for processes attempting to load NFC-related kernel modules unexpectedly

How to Mitigate CVE-2020-25671

Immediate Actions Required

• Update the Linux Kernel to a patched version as provided by your distribution vendor
• Review and apply relevant security advisories from Debian, Fedora, and NetApp
• Audit systems for signs of exploitation if NFC functionality has been enabled
• Restrict local access to systems where possible to reduce attack surface

Patch Information

Security patches addressing this vulnerability have been released by multiple vendors. Organizations should consult their Linux distribution's security advisory channels for specific patch versions:

• Debian: Security updates available via Debian LTS Advisory
• Fedora: Patches released for Fedora 32, 33, and 34 via standard package updates
• NetApp: Advisory NTAP-20210702-0008 provides guidance for affected NetApp products

For the upstream Linux Kernel, patches have been merged to fix the refcount handling in the LLCP socket connection code. Users building custom kernels should ensure they include all relevant NFC subsystem fixes.

Workarounds

• Disable NFC functionality in the kernel if not required by blacklisting the relevant modules (nfc, llcp)
• Restrict access to NFC-related device nodes using appropriate file permissions
• Implement mandatory access control (SELinux, AppArmor) policies to limit which processes can interact with NFC subsystems
• Consider removing NFC kernel modules entirely on systems where this functionality is not needed

# Disable NFC kernel modules as a temporary workaround
echo "blacklist nfc" | sudo tee /etc/modprobe.d/disable-nfc.conf
echo "blacklist llcp" | sudo tee -a /etc/modprobe.d/disable-nfc.conf
sudo update-initramfs -u
# Reboot required for changes to take effect