CVE-2020-13974 Overview
An integer overflow vulnerability was discovered in the Linux kernel affecting versions 4.4 through 5.7.1. The vulnerability exists in drivers/tty/vt/keyboard.c and can be triggered when the k_ascii function is called multiple times in succession. This issue is tracked as CID-b86dab054059.
Critical Impact
Local attackers with low privileges could potentially exploit this integer overflow to compromise system confidentiality, integrity, and availability, though community debate exists regarding the practical exploitability of this vulnerability.
Affected Products
- Linux Kernel versions 4.4 through 5.7.1
- Debian Linux 9.0
- Canonical Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, and 20.04 LTS
Discovery Timeline
- June 9, 2020 - CVE-2020-13974 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2020-13974
Vulnerability Analysis
This vulnerability is classified as an Integer Overflow (CWE-190) affecting the Linux kernel's virtual terminal keyboard driver. The flaw resides in the k_ascii function within drivers/tty/vt/keyboard.c, which handles ASCII key input processing for virtual terminals.
When k_ascii is invoked repeatedly in rapid succession, an integer overflow condition can occur. Integer overflows happen when arithmetic operations produce values that exceed the maximum representable value for the data type, causing the value to wrap around unexpectedly. In the context of kernel code, such behavior can lead to incorrect memory calculations, buffer mismanagement, or other undefined behavior.
It is important to note that members of the Linux kernel community have debated whether this integer overflow actually leads to a practical security vulnerability in real-world scenarios. Exploitation would require local access to the system with low privileges, and the attacker would need to interact with the virtual terminal subsystem directly.
Root Cause
The root cause is improper handling of integer arithmetic in the k_ascii function. When this function processes ASCII input from the keyboard, repeated calls can cause an internal counter or accumulator variable to exceed its maximum value, resulting in an integer wrap-around. The code did not include adequate bounds checking to prevent this overflow condition, allowing the arithmetic to produce unexpected results.
Attack Vector
The attack vector is local, requiring an attacker to have authenticated access to the target system with low privileges. The attacker would need to interact with the virtual terminal keyboard subsystem, potentially through crafted input sequences that cause the k_ascii function to be invoked multiple times in succession. No user interaction beyond initial authentication is required for exploitation.
The vulnerability could theoretically be triggered by sending specific keyboard input sequences to the virtual terminal, causing the integer overflow in the affected code path.
Detection Methods for CVE-2020-13974
Indicators of Compromise
- Unusual activity or crashes associated with the virtual terminal subsystem (/dev/tty* devices)
- Kernel panic or oops messages referencing keyboard.c or the k_ascii function
- Unexpected behavior when interacting with virtual consoles
Detection Strategies
- Monitor kernel logs (dmesg, /var/log/kern.log) for errors or warnings related to the VT keyboard driver
- Implement kernel integrity monitoring to detect unexpected modifications to the affected driver module
- Deploy endpoint detection and response (EDR) solutions capable of monitoring kernel-level activity
Monitoring Recommendations
- Enable audit logging for virtual terminal access and suspicious keyboard input patterns
- Monitor for privilege escalation attempts following virtual terminal interactions
- Implement SentinelOne Singularity Platform for real-time kernel-level threat detection and behavioral analysis
How to Mitigate CVE-2020-13974
Immediate Actions Required
- Update the Linux kernel to a patched version that addresses CID-b86dab054059
- Apply vendor-provided security updates for Debian, Ubuntu, openSUSE, and Oracle Linux
- Restrict local access to systems running vulnerable kernel versions where possible
Patch Information
The Linux kernel maintainers have addressed this vulnerability through commit b86dab054059b970111b5516ae548efaae5b3aae in the mainline kernel repository. Multiple distributions have released security updates:
- Debian: Debian LTS Security Announcement August 2020
- Ubuntu: USN-4427-1, USN-4439-1, USN-4440-1, USN-4483-1, USN-4485-1
- openSUSE: OpenSUSE Security Announcement July 2020, OpenSUSE Security Announcement August 2020
- Oracle: Oracle Security Alert July 2022
Refer to the Linux Kernel Commit for technical details on the fix.
Workarounds
- Limit physical and remote access to virtual terminals on affected systems
- Consider disabling unused virtual terminals if not required for operations
- Implement strict access controls using SELinux or AppArmor to restrict VT subsystem access
# Example: Check current kernel version and verify if patched
uname -r
# Compare against patched kernel versions from your distribution
# Example: Restrict virtual terminal access using permissions
chmod 600 /dev/tty[1-6]
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
