CVE-2020-10711: Linux Kernel SELinux DOS Vulnerability

CVE-2020-10711 Overview

A NULL pointer dereference vulnerability was discovered in the Linux kernel's SELinux subsystem in versions before 5.7. This flaw occurs while importing the Commercial IP Security Option (CIPSO) protocol's category bitmap into the SELinux extensible bitmap via the ebitmap_netlbl_import routine. The vulnerability allows a remote network user to crash the system kernel, resulting in a denial of service condition.

Critical Impact

Remote attackers can exploit this NULL pointer dereference to crash the Linux kernel, causing system-wide denial of service without requiring authentication.

Affected Products

• Linux Kernel (versions before 5.7)
• Red Hat Enterprise Linux 6.0, 7.0, 8.0
• Red Hat OpenStack 13
• Red Hat Virtualization Host 4.0
• Red Hat 3scale 2.0
• Debian Linux 8.0, 9.0, 10.0
• openSUSE Leap 15.1, 15.2
• Canonical Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, 19.10, 20.04 LTS

Discovery Timeline

• May 12, 2020 - Vulnerability disclosed via OpenWall OSS Security Discussion
• May 22, 2020 - CVE-2020-10711 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-10711

Vulnerability Analysis

This vulnerability exists in the SELinux network labeling subsystem's handling of CIPSO (Commercial IP Security Option) protocol data. The flaw is classified as CWE-476 (NULL Pointer Dereference) and allows remote network-based attacks with high complexity requirements.

The core issue lies in how the kernel processes CIPSO restricted bitmap tags. When the cipso_v4_parsetag_rbm routine processes an incoming CIPSO restricted bitmap tag, it incorrectly sets a security attribute flag indicating that a category bitmap is present, even when no memory has been allocated for this bitmap. Subsequently, when SELinux attempts to import this category bitmap through the ebitmap_netlbl_import routine, it dereferences the NULL pointer, causing a kernel panic.

Root Cause

The root cause is a logic error in the CIPSO tag parsing code where the category bitmap presence flag is set unconditionally during tag processing. The cipso_v4_parsetag_rbm function fails to properly validate that the category bitmap has been successfully allocated before setting the MLS_CAT security attribute flag. This creates a state inconsistency where SELinux's ebitmap_netlbl_import function expects valid bitmap data but receives a NULL pointer instead.

Attack Vector

The vulnerability can be exploited remotely over the network by sending specially crafted CIPSO-labeled IP packets to a vulnerable system. The attack does not require authentication or user interaction, though the attack complexity is considered high due to the specific network conditions required.

An attacker would craft network packets containing malformed CIPSO restricted bitmap tags designed to trigger the vulnerable code path. When the target kernel processes these packets and attempts to apply SELinux labeling, the NULL pointer dereference causes an immediate kernel panic, resulting in system crash and denial of service.

The exploitation requires:

1. Network access to the target system
2. SELinux and CIPSO/NetLabel functionality enabled on the target
3. Crafted CIPSO packets with malformed restricted bitmap tags

Detection Methods for CVE-2020-10711

Indicators of Compromise

• Unexpected system crashes or kernel panics on Linux systems with SELinux enabled
• Kernel oops messages referencing ebitmap_netlbl_import or cipso_v4_parsetag_rbm functions
• Network traffic containing unusual CIPSO-labeled IP packets
• Repeated system reboots without apparent cause on network-facing servers

Detection Strategies

• Monitor kernel logs for NULL pointer dereference messages in SELinux or NetLabel subsystems
• Implement network intrusion detection rules for anomalous CIPSO protocol traffic
• Deploy crash dump analysis to identify kernel panics originating from the SELinux bitmap handling code
• Use SentinelOne's kernel-level monitoring to detect exploitation attempts targeting the SELinux subsystem

Monitoring Recommendations

• Enable kernel crash dump collection to capture evidence of exploitation attempts
• Configure network monitoring for unusual CIPSO-labeled traffic patterns
• Monitor system availability and unexpected reboot events on SELinux-enabled systems
• Implement alerting for kernel oops messages containing references to affected functions

How to Mitigate CVE-2020-10711

Immediate Actions Required

• Update the Linux kernel to version 5.7 or later where the vulnerability has been patched
• Apply vendor-specific security patches from your Linux distribution
• Consider temporarily disabling CIPSO/NetLabel functionality if not required for operations
• Implement network filtering to block potentially malicious CIPSO-labeled traffic at the perimeter

Patch Information

Security patches are available from multiple vendors:

• Red Hat: Red Hat CVE Report
• Debian: Debian Security Advisory #4698 and Debian Security Advisory #4699
• Ubuntu: Multiple Ubuntu Security Notices including USN-4411-1, USN-4412-1, USN-4413-1, USN-4414-1, and USN-4419-1
• openSUSE: Security announcements available via openSUSE Security Announcement
• NetApp: NetApp Security Advisory

Workarounds

• Disable CIPSO protocol support in the kernel if not required for your environment
• Implement network-level filtering to drop CIPSO-labeled packets from untrusted sources
• Use network segmentation to limit exposure of vulnerable systems to potential attack traffic
• Consider disabling SELinux network labeling features temporarily until patches can be applied

# Check current kernel version
uname -r

# For RHEL/CentOS systems, update kernel packages
yum update kernel

# For Debian/Ubuntu systems, update kernel packages
apt-get update && apt-get upgrade linux-image-$(uname -r)

# Verify SELinux status
sestatus

# To temporarily disable NetLabel CIPSO support (requires reboot)
# Add to kernel boot parameters: netlabel.cipso_v4=0