CVE-2020-25211: Linux Kernel Buffer Overflow Vulnerability

CVE-2020-25211 Overview

CVE-2020-25211 is a buffer overflow vulnerability in the Linux kernel through version 5.8.7 affecting the netfilter connection tracking subsystem. Local attackers who are able to inject conntrack netlink configuration can overflow a local buffer in ctnetlink_parse_tuple_filter within net/netfilter/nf_conntrack_netlink.c. Successful exploitation can cause system crashes or trigger the use of incorrect protocol numbers, potentially leading to denial of service conditions or integrity violations.

Critical Impact

Local attackers with elevated privileges can exploit this vulnerability to crash the system or corrupt netfilter connection tracking data, potentially disrupting network security filtering and monitoring functions.

Affected Products

• Linux Kernel through version 5.8.7
• Debian Linux 9.0 and 10.0
• Fedora 31 and 32

Discovery Timeline

• September 9, 2020 - CVE-2020-25211 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25211

Vulnerability Analysis

This vulnerability is classified as a buffer overflow (CWE-120) in the Linux kernel's netfilter connection tracking subsystem. The flaw resides in the ctnetlink_parse_tuple_filter function, which processes netlink messages for connection tracking configuration. When parsing tuple filter data from netlink attributes, the function fails to properly validate input boundaries, allowing a local attacker with sufficient privileges to overflow a stack-allocated buffer.

The vulnerability requires local access and elevated privileges to exploit, as the attacker must be able to send specially crafted netlink messages to the conntrack subsystem. The impact is primarily on system integrity and availability—an attacker can corrupt memory leading to incorrect protocol number usage in connection tracking, or cause a kernel panic resulting in denial of service.

Root Cause

The root cause is insufficient input validation in the ctnetlink_parse_tuple_filter function. When parsing netlink attributes containing connection tracking tuple data, the code does not adequately verify that the provided data fits within the expected buffer boundaries. This allows an attacker to provide malformed or oversized input that exceeds the allocated buffer space, resulting in a classic buffer overflow condition.

The fix, identified by commit 1cc5ef91d2ff, addresses this by implementing proper bounds checking on the netlink attribute data before copying it into the local buffer structure.

Attack Vector

The attack requires local access to the system with privileges sufficient to send netlink messages to the netfilter conntrack subsystem (typically requiring CAP_NET_ADMIN capability). The attacker crafts a malicious netlink message containing oversized or malformed tuple filter data and sends it to the kernel's netfilter subsystem.

The exploitation process involves:

1. Establishing a netlink socket connection to the netfilter subsystem
2. Crafting a netlink message with conntrack tuple filter attributes containing data designed to overflow the target buffer
3. Sending the malicious message, triggering the overflow in ctnetlink_parse_tuple_filter
4. Achieving either a system crash (denial of service) or corruption of protocol number handling in connection tracking

Due to the local attack vector and privilege requirements, this vulnerability is most concerning in multi-tenant environments where users may have container or namespace-level network administration capabilities that could be leveraged to reach the vulnerable code path.

Detection Methods for CVE-2020-25211

Indicators of Compromise

• Unexpected kernel panics or system crashes with stack traces referencing ctnetlink_parse_tuple_filter or related netfilter functions
• Anomalous netlink socket activity from processes that should not be interacting with connection tracking
• Connection tracking entries showing unexpected or invalid protocol numbers
• System log entries indicating memory corruption in the netfilter subsystem

Detection Strategies

• Monitor for kernel oops or panic messages containing references to nf_conntrack_netlink.c or ctnetlink_parse_tuple_filter
• Implement audit rules on netlink socket creation and usage by non-system processes
• Deploy kernel-level monitoring to detect unusual patterns in conntrack netlink message sizes or frequencies
• Use SentinelOne's Linux agent to detect kernel-level exploitation attempts and abnormal system behavior

Monitoring Recommendations

• Enable kernel crash dump collection (kdump) to capture forensic evidence of exploitation attempts
• Configure system logging to capture netfilter-related kernel messages at debug level during investigation periods
• Monitor for processes acquiring CAP_NET_ADMIN capability unexpectedly
• Implement network namespace and container escape detection for environments where this capability may be delegated

How to Mitigate CVE-2020-25211

Immediate Actions Required

• Update the Linux kernel to a patched version (5.8.8 or later, or apply distribution-specific patches)
• Apply security patches from your distribution vendor (Debian, Fedora, etc.)
• Restrict CAP_NET_ADMIN capability to only essential processes and users
• Review container and namespace configurations to ensure network capabilities are appropriately limited

Patch Information

The vulnerability was fixed in the upstream Linux kernel via commit 1cc5ef91d2ff. Distribution-specific patches are available through:

• Debian Security Advisory DSA-4774
• Debian LTS Announcements for Debian 9 and 10
• Fedora Package Announcements for Fedora 31 and 32
• NetApp Security Advisory ntap-20201009-0001 for affected NetApp products

Workarounds

• Limit processes with CAP_NET_ADMIN capability to reduce the attack surface
• Use mandatory access control (SELinux, AppArmor) to restrict which processes can access netfilter netlink sockets
• In containerized environments, avoid granting NET_ADMIN capability to containers unless absolutely necessary
• Monitor and audit all netlink socket usage on production systems

# Check current kernel version
uname -r

# Verify if patches are applied (example for Debian-based systems)
apt list --installed | grep linux-image

# Restrict CAP_NET_ADMIN with setcap (example)
# Ensure only necessary binaries have this capability
getcap -r /usr/bin 2>/dev/null | grep net_admin

# Update kernel on Debian-based systems
apt update && apt upgrade linux-image-$(uname -r | cut -d'-' -f3)

# Update kernel on Fedora
dnf update kernel