CVE-2020-25672: Linux Kernel Information Disclosure Flaw

CVE-2020-25672 Overview

A memory leak vulnerability was discovered in the Linux kernel's NFC (Near Field Communication) subsystem, specifically within the llcp_sock_connect function. This flaw occurs when establishing LLCP (Logical Link Control Protocol) socket connections, where allocated memory is not properly freed under certain error conditions. An attacker capable of triggering repeated connection attempts can cause progressive memory exhaustion, ultimately leading to system instability or denial of service.

Critical Impact

This memory leak vulnerability in the Linux kernel NFC stack can be exploited remotely to exhaust system memory, causing denial of service conditions on affected systems.

Affected Products

• Linux Kernel (all vulnerable versions)
• Fedora 32, 33, and 34
• Debian Linux 9.0
• NetApp Active IQ Unified Manager (vSphere)
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller Firmware
• NetApp H410C, H300S, H500S, H700S, H300E, H500E, H700E, H410S (and associated firmware)

Discovery Timeline

• November 1, 2020 - Vulnerability disclosed on Openwall OSS Security List
• May 25, 2021 - CVE-2020-25672 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25672

Vulnerability Analysis

The vulnerability resides in the llcp_sock_connect function within the Linux kernel's NFC LLCP implementation. When processing LLCP socket connection requests, the function allocates memory for connection state management. However, under specific error paths—particularly when connection establishment fails—the allocated memory is not properly released back to the system.

This memory management flaw falls under CWE-401 (Missing Release of Memory after Effective Lifetime). Each failed connection attempt leaves orphaned memory allocations, and because these allocations are never freed, an attacker can deliberately trigger repeated connection failures to progressively consume available system memory.

The vulnerability is accessible via the network attack vector, requires no authentication, and involves low attack complexity. While it does not impact confidentiality or integrity, it poses a significant availability threat as sustained exploitation can render the system unresponsive or trigger out-of-memory conditions.

Root Cause

The root cause is improper memory management in the LLCP socket connection handling code. When the llcp_sock_connect function encounters an error during the connection establishment process, it fails to release previously allocated memory structures. This missing deallocation in the error handling path creates a memory leak that accumulates with each failed connection attempt.

The issue stems from insufficient cleanup logic in error-handling branches, where allocated resources should be freed before returning an error code to the caller. The fix requires ensuring all allocated memory is properly released regardless of whether the connection succeeds or fails.

Attack Vector

An attacker can exploit this vulnerability by initiating numerous NFC LLCP connection attempts to a vulnerable system. The attack requires network access to the target system's NFC stack. By repeatedly triggering connection failures, the attacker can cause memory to be consumed without release.

Since the vulnerability does not require user interaction or privileges, and the attack complexity is low, a remote attacker can systematically exhaust system memory. Over time, this leads to degraded system performance, application crashes due to memory allocation failures, and potentially complete system denial of service.

Detection Methods for CVE-2020-25672

Indicators of Compromise

• Gradual increase in kernel memory usage, particularly in slab allocations related to NFC/LLCP structures
• Unusual volume of NFC LLCP connection attempts in system logs
• Out-of-memory (OOM) killer events without corresponding legitimate memory-intensive workloads
• System instability or service degradation correlated with NFC subsystem activity

Detection Strategies

• Monitor kernel memory consumption using tools like /proc/meminfo and slabtop for anomalous growth patterns
• Implement logging and alerting for repeated NFC LLCP connection failures from the same source
• Deploy kernel-level instrumentation to track memory allocation and deallocation in the NFC subsystem
• Use endpoint detection solutions to identify exploitation attempts targeting kernel vulnerabilities

Monitoring Recommendations

• Establish baseline memory consumption metrics and alert on sustained deviations
• Configure system monitoring to track slab cache growth, particularly for NFC-related kernel objects
• Enable comprehensive logging for NFC subsystem events and connection attempts
• Implement network monitoring to detect unusual NFC protocol traffic patterns

How to Mitigate CVE-2020-25672

Immediate Actions Required

• Apply kernel security updates from your Linux distribution that address CVE-2020-25672
• Disable the NFC subsystem if not required by removing or blacklisting NFC kernel modules (nfc, llcp)
• Limit network access to systems with NFC capabilities to reduce attack surface
• Monitor affected systems for signs of memory exhaustion or denial of service

Patch Information

Security patches addressing this vulnerability have been released by multiple Linux distributions and vendors:

• Debian: Security updates announced via Debian LTS Announcement
• Fedora: Updates available through Fedora Package Announcements
• NetApp: Advisory published at NetApp Security Advisory

Organizations should update to the latest patched kernel versions provided by their distribution.

Workarounds

• Blacklist NFC kernel modules to prevent loading: add blacklist nfc and blacklist llcp to /etc/modprobe.d/blacklist.conf
• Implement memory resource limits using cgroups to contain potential impact
• Deploy network segmentation to restrict access to NFC-capable systems
• Consider using mandatory access control (SELinux/AppArmor) to limit NFC subsystem interactions

# Disable NFC modules on systems where NFC is not required
echo "blacklist nfc" | sudo tee /etc/modprobe.d/nfc-blacklist.conf
echo "blacklist llcp" | sudo tee -a /etc/modprobe.d/nfc-blacklist.conf
echo "blacklist nfc_llcp" | sudo tee -a /etc/modprobe.d/nfc-blacklist.conf

# Unload modules if currently loaded
sudo modprobe -r nfc_llcp llcp nfc 2>/dev/null || true

# Verify modules are not loaded
lsmod | grep -i nfc