CVE-2020-25211: Linux Kernel Buffer Overflow Vulnerability

CVE-2020-25211 Overview

CVE-2020-25211 is a buffer overflow vulnerability [CWE-120] in the Linux kernel netfilter subsystem affecting versions through 5.8.7. The flaw resides in the ctnetlink_parse_tuple_filter function within net/netfilter/nf_conntrack_netlink.c. Local attackers with sufficient privileges to inject conntrack netlink configuration can overflow a local buffer. Successful exploitation causes kernel crashes or triggers use of incorrect protocol numbers, leading to denial of service or integrity issues. The Linux kernel commit 1cc5ef91d2ff addresses the issue. Affected distributions include Debian 9 and 10, and Fedora 31 and 32.

Critical Impact
Local privileged attackers can crash the kernel or corrupt conntrack protocol handling by injecting malformed netlink configuration messages.

Affected Products

Linux Kernel versions through 5.8.7
Debian Linux 9.0 and 10.0
Fedora 31 and 32

Discovery Timeline

2020-09-09 - CVE-2020-25211 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25211

Vulnerability Analysis

The vulnerability exists in the netfilter conntrack netlink interface, which allows userspace processes to configure and query the kernel's connection tracking subsystem. The ctnetlink_parse_tuple_filter function processes netlink messages containing tuple filter attributes. The function fails to properly validate the size of incoming data before writing it to a local buffer.

An attacker with the CAP_NET_ADMIN capability can craft malformed netlink messages to overflow this buffer. The overflow either corrupts adjacent kernel memory or causes the function to operate on invalid protocol numbers. Both outcomes lead to integrity and availability impacts on the host system.

The issue is classified under [CWE-120] (Classic Buffer Overflow). The CVSS vector indicates local attack vector requiring high privileges, with no impact on confidentiality but high impact on integrity and availability.

Root Cause

The root cause is missing bounds checking in ctnetlink_parse_tuple_filter when handling user-supplied conntrack netlink attributes. The function copies attribute data into a fixed-size local buffer without verifying that the input length matches the expected protocol structure size. Mismatched lengths overflow the stack-allocated buffer and may corrupt return addresses or adjacent kernel data structures.

Attack Vector

Exploitation requires local access and elevated privileges on the target system, specifically CAP_NET_ADMIN to send conntrack netlink configuration. An attacker in a container with this capability or a compromised privileged process can trigger the overflow. The attacker constructs a netlink message with attribute payloads that exceed expected sizes for tuple filter parsing. Sending the message through the netfilter socket interface triggers the vulnerable code path. The vulnerability is not remotely exploitable and requires existing local privileges.

No verified public exploit code is available. Refer to the Linux Kernel Commit for the patched parsing logic.

Detection Methods for CVE-2020-25211

Indicators of Compromise

Unexpected kernel panics or oops messages referencing ctnetlink_parse_tuple_filter or nf_conntrack_netlink in dmesg and /var/log/kern.log
Unusual conntrack netlink activity from unprivileged or unexpected processes holding CAP_NET_ADMIN
Sudden connection tracking anomalies, including connections handled with incorrect protocol numbers

Detection Strategies

Monitor kernel logs for stack canary violations, KASAN reports, or oops events referencing netfilter conntrack code paths
Audit process capability sets to identify which workloads hold CAP_NET_ADMIN and could reach the vulnerable code
Inspect netlink socket usage via auditd rules tracking socket() calls with AF_NETLINK and NETLINK_NETFILTER protocol

Monitoring Recommendations

Centralize kernel log collection from all Linux hosts and alert on netfilter-related crash signatures
Track installed kernel versions across the fleet to identify hosts still running vulnerable releases prior to commit 1cc5ef91d2ff
Establish baselines for conntrack netlink traffic volume and alert on deviations from privileged services

How to Mitigate CVE-2020-25211

Immediate Actions Required

Apply vendor kernel updates from Debian (DSA-4774 and associated LTS advisories), Fedora, and other distributions referenced in the security advisories
Inventory Linux hosts running kernel versions at or below 5.8.7 and prioritize patching for systems exposing CAP_NET_ADMIN to untrusted workloads
Restrict container capabilities by removing CAP_NET_ADMIN from container security contexts unless explicitly required

Patch Information

The upstream fix is contained in Linux kernel commit 1cc5ef91d2ff94d2bf2de3b3585423e8a1051cb6, available from the Linux Kernel Commit Update. Distribution-specific patches are published in the Debian Security Advisory DSA-4774, the Debian LTS Advisory, the Fedora Package Announcement, and the NetApp Security Advisory NTAP-20201009-0001.

Workarounds

Drop CAP_NET_ADMIN from non-essential services and container runtimes using systemd CapabilityBoundingSet or container security policies
Use seccomp profiles to block socket() calls with AF_NETLINK for workloads that do not require netlink access
Apply mandatory access control policies (SELinux, AppArmor) to restrict which processes can issue conntrack netlink configuration messages

bash

# Configuration example: remove CAP_NET_ADMIN from a systemd service
# /etc/systemd/system/myservice.service.d/override.conf
[Service]
CapabilityBoundingSet=~CAP_NET_ADMIN
AmbientCapabilities=
NoNewPrivileges=yes