CVE-2020-14331: Linux Kernel Privilege Escalation Flaw

CVE-2020-14331 Overview

A flaw was found in the Linux kernel's implementation of the invert video code on VGA consoles when a local attacker attempts to resize the console, calling an ioctl VT_RESIZE, which causes an out-of-bounds write to occur. This vulnerability allows a local user with access to the VGA console to crash the system, potentially escalating their privileges on the system. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.

Critical Impact

Local privilege escalation vulnerability in the Linux kernel VGA console subsystem allowing attackers with physical console access to corrupt memory, crash the system, or potentially gain elevated privileges through out-of-bounds write operations.

Affected Products

• Linux Kernel (versions prior to fix, including 5.8.0-rc1 through 5.8.0-rc6)
• Red Hat Enterprise Linux 7.0
• Red Hat Enterprise Linux 8.0

Discovery Timeline

• 2020-09-15 - CVE CVE-2020-14331 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-14331

Vulnerability Analysis

This vulnerability exists within the Linux kernel's VGA console subsystem, specifically in the invert video code path. The flaw manifests when a local attacker with console access invokes the VT_RESIZE ioctl to resize the virtual terminal console. During this operation, the kernel fails to properly validate the dimensions being set, resulting in an out-of-bounds write condition (CWE-787).

The vulnerability requires physical access to the VGA console, which limits remote exploitation scenarios but remains a significant risk in environments where physical access controls may be inadequate, such as shared servers, kiosks, or systems with inadequate physical security measures.

Root Cause

The root cause of this vulnerability lies in improper boundary checking within the VGA console's video inversion code. When the VT_RESIZE ioctl is called, the kernel performs screen buffer operations without adequately verifying that the new console dimensions remain within allocated memory boundaries. This oversight allows an attacker to specify dimensions that cause write operations to exceed the bounds of allocated screen buffers, corrupting adjacent kernel memory.

Attack Vector

The attack requires physical access to the system's VGA console. An attacker with local low-privileged access can trigger the vulnerability by:

1. Opening a VGA console session on the target system
2. Invoking the VT_RESIZE ioctl with crafted parameters
3. Triggering the invert video code path with malicious console dimensions
4. Exploiting the resulting out-of-bounds write to corrupt kernel memory

The out-of-bounds write can be leveraged to overwrite critical kernel data structures, potentially leading to privilege escalation from a low-privileged user to root access. The attack does not require user interaction once the attacker has console access.

The vulnerability is triggered through the ioctl interface by calling VT_RESIZE with manipulated parameters. When the kernel processes this resize request and subsequently executes the invert video routine, it writes data beyond the allocated screen buffer boundaries. Technical details and discussion can be found in the OpenWall OSS Security Discussion.

Detection Methods for CVE-2020-14331

Indicators of Compromise

• Unexpected system crashes or kernel panics originating from the VGA console subsystem
• Anomalous VT_RESIZE ioctl calls detected in system audit logs
• Memory corruption signatures in kernel crash dumps referencing console or VGA driver code paths
• Unusual privilege changes following VGA console activity

Detection Strategies

• Enable kernel auditing for ioctl system calls, specifically monitoring for VT_RESIZE operations
• Deploy kernel integrity monitoring to detect unauthorized memory modifications
• Implement SentinelOne's runtime protection to detect privilege escalation attempts following console operations
• Monitor for unusual patterns of console resize operations that may indicate exploitation attempts

Monitoring Recommendations

• Configure audit rules to log all VT_RESIZE ioctl calls: auditctl -a always,exit -F arch=b64 -S ioctl -F a1=0x5609 -k vt_resize
• Review kernel logs regularly for VGA console-related warnings or errors
• Deploy endpoint detection solutions capable of monitoring kernel-level exploitation attempts
• Implement physical access controls and monitor for unauthorized console access

How to Mitigate CVE-2020-14331

Immediate Actions Required

• Update Linux kernel to the latest patched version available for your distribution
• Apply vendor-specific security patches from Red Hat, Debian, or your distribution vendor
• Restrict physical console access to authorized personnel only
• Review and audit systems with VGA console access for signs of compromise

Patch Information

Patches have been released by major Linux distributions to address this vulnerability. Red Hat has documented this issue in Bug Report #1858679. Debian has released advisories in September 2020 and October 2020. The upstream kernel discussion is available via the OpenWall Linux Kernel Discussion. Administrators should update to patched kernel versions as soon as possible.

Workarounds

• Disable VGA console access if not required for system operation by blacklisting the VGA console module
• Implement strict physical access controls to prevent unauthorized console access
• Use serial console or remote management interfaces instead of VGA console where possible
• Consider using SELinux or AppArmor policies to restrict ioctl access to the VGA console subsystem

# Restrict physical console access and audit VT_RESIZE calls
# Add audit rule to monitor VT_RESIZE ioctl operations
auditctl -a always,exit -F arch=b64 -S ioctl -F a1=0x5609 -k vt_resize_monitor

# Verify kernel version after patching
uname -r

# Check for available kernel updates (RHEL/CentOS)
yum check-update kernel

# Check for available kernel updates (Debian/Ubuntu)
apt list --upgradable | grep linux-image