CVE-2020-13753: WebKitGTK Sandbox Escape RCE Vulnerability

CVE-2020-13753 Overview

CVE-2020-13753 is a critical sandbox escape vulnerability affecting the bubblewrap sandbox implementation in WebKitGTK and WPE WebKit prior to version 2.28.3. The vulnerability stems from the sandbox's failure to properly block access to CLONE_NEWUSER and the TIOCSTI ioctl, enabling attackers to escape the sandbox and execute commands on the underlying system.

The CLONE_NEWUSER capability could potentially be exploited to confuse xdg-desktop-portal, which manages access to resources outside the sandbox. Additionally, the TIOCSTI ioctl can be abused to directly execute commands outside the sandbox by injecting keystrokes into the controlling terminal's input buffer, a technique similar to CVE-2017-5226.

Critical Impact

This vulnerability allows complete sandbox escape with network-accessible attack vector, potentially enabling remote attackers to execute arbitrary commands on affected systems running vulnerable WebKitGTK or WPE WebKit versions.

Affected Products

• WebKitGTK versions prior to 2.28.3
• WPE WebKit versions prior to 2.28.3
• Fedora 31
• Debian Linux 10.0
• Ubuntu Linux 18.04 LTS, 19.10, and 20.04 LTS
• openSUSE Leap 15.1

Discovery Timeline

• July 14, 2020 - CVE-2020-13753 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-13753

Vulnerability Analysis

This sandbox escape vulnerability represents a fundamental weakness in the bubblewrap sandbox's security boundary enforcement. The bubblewrap sandbox is designed to isolate web content from the underlying system, but this implementation failed to restrict two dangerous Linux kernel features.

The first issue involves CLONE_NEWUSER, a Linux kernel feature that allows unprivileged users to create new user namespaces. When combined with xdg-desktop-portal (a D-Bus service that provides applications with controlled access to system resources), an attacker could potentially manipulate the portal's permission model to gain unauthorized access to resources outside the intended sandbox boundaries.

The second issue involves the TIOCSTI ioctl (Terminal I/O Control Set Terminal Input), which allows a process to inject characters into the terminal's input queue as if they were typed by the user. This is particularly dangerous because it enables direct command injection into the parent shell session, completely bypassing sandbox restrictions.

Root Cause

The root cause is improper input validation (CWE-20) in the bubblewrap sandbox configuration within WebKitGTK and WPE WebKit. The sandbox failed to apply proper seccomp-bpf (Secure Computing with Berkeley Packet Filter) rules to block the CLONE_NEWUSER flag for the clone() system call and did not filter the TIOCSTI ioctl request. This oversight allowed sandboxed processes to access these dangerous capabilities that should have been explicitly denied.

Attack Vector

The vulnerability is exploitable via network-based attack vectors, as web content processed by WebKitGTK or WPE WebKit could contain malicious code designed to exploit these sandbox weaknesses. An attacker could craft a malicious webpage that, when rendered by an application using the vulnerable WebKit engine, executes code to:

1. Create a new user namespace using CLONE_NEWUSER to manipulate xdg-desktop-portal permissions
2. Use TIOCSTI ioctl calls to inject commands into the parent terminal's input buffer, achieving command execution outside the sandbox

The TIOCSTI attack technique is particularly effective when the browser or application is launched from a terminal, as the injected commands execute with the privileges of the terminal session rather than the sandboxed process.

Detection Methods for CVE-2020-13753

Indicators of Compromise

• Unexpected clone() system calls with CLONE_NEWUSER flag from WebKitGTK or WPE WebKit processes
• Anomalous ioctl() calls with TIOCSTI requests from sandboxed browser processes
• Unusual process creation patterns originating from web rendering processes
• Unexpected interactions with xdg-desktop-portal from web browser components

Detection Strategies

• Monitor for TIOCSTI ioctl calls using Linux audit subsystem rules targeting terminal device interactions
• Implement seccomp-bpf monitoring to detect attempts to create user namespaces from web browser processes
• Deploy behavioral analysis to identify web rendering processes attempting to escape their sandbox boundaries
• Review system logs for unexpected D-Bus communications with xdg-desktop-portal from web content processes

Monitoring Recommendations

• Enable Linux audit rules to log clone() system calls with namespace-related flags
• Configure SentinelOne agents to monitor for sandbox escape attempts and terminal injection attacks
• Implement process lineage monitoring to detect command execution chains originating from web browser sandboxes
• Set up alerts for unusual ioctl patterns on /dev/tty devices from sandboxed processes

How to Mitigate CVE-2020-13753

Immediate Actions Required

• Update WebKitGTK and WPE WebKit to version 2.28.3 or later immediately
• Apply distribution-specific security updates from Fedora, Debian, Ubuntu, or openSUSE
• Consider temporarily disabling or restricting applications using vulnerable WebKit versions until patched
• Avoid launching WebKit-based applications from terminal sessions until patched

Patch Information

The WebKit project addressed this vulnerability in WebKit Changeset 262368. Distribution-specific patches are available through the following security advisories:

• Debian Security Advisory DSA-4724
• Ubuntu Security Notice USN-4422-1
• Fedora Package Announcement
• Gentoo GLSA 202007-11
• openSUSE Security Announcement

Workarounds

• Run WebKit-based applications without a controlling terminal to prevent TIOCSTI exploitation
• Use additional sandboxing layers such as Flatpak or Firejail with restrictive profiles
• Disable user namespace creation system-wide if not required by setting kernel.unprivileged_userns_clone=0
• Isolate WebKit-based applications in separate user accounts or containers

# Disable unprivileged user namespaces system-wide (temporary workaround)
echo 'kernel.unprivileged_userns_clone=0' | sudo tee /etc/sysctl.d/99-disable-userns.conf
sudo sysctl -p /etc/sysctl.d/99-disable-userns.conf

# Verify WebKitGTK version on Debian/Ubuntu
dpkg -l | grep webkit2gtk
# Update to patched version
sudo apt update && sudo apt upgrade libwebkit2gtk-4.0-37