CVE-2021-1789: Apple iPadOS RCE Vulnerability

CVE-2021-1789 Overview

CVE-2021-1789 is a type confusion vulnerability in WebKit that affects multiple Apple products including iOS, iPadOS, macOS, tvOS, watchOS, and Safari. The vulnerability arises from improper state handling within the WebKit rendering engine. When a user visits a maliciously crafted webpage, an attacker can exploit this type confusion issue to achieve arbitrary code execution on the victim's device. This vulnerability has been added to CISA's Known Exploited Vulnerabilities catalog, indicating active exploitation in the wild.

Critical Impact

Successful exploitation allows remote attackers to execute arbitrary code on affected Apple devices through maliciously crafted web content. CISA has confirmed this vulnerability is being actively exploited.

Affected Products

• Apple iOS (prior to 14.4) and iPadOS (prior to 14.4)
• Apple macOS Big Sur (prior to 11.2), macOS Catalina, and macOS Mojave (prior to Security Update 2021-001)
• Apple Safari (prior to 14.0.3)
• Apple tvOS (prior to 14.4)
• Apple watchOS (prior to 7.3)
• WebKitGTK
• Fedora 32 and 33

Discovery Timeline

• 2021-04-02 - CVE-2021-1789 published to NVD
• 2025-10-23 - Last updated in NVD database

Technical Details for CVE-2021-1789

Vulnerability Analysis

This vulnerability is classified as CWE-843 (Access of Resource Using Incompatible Type, also known as Type Confusion). Type confusion vulnerabilities occur when a program allocates or initializes a resource using one type but later accesses it using a type that is incompatible with the original type. In the context of WebKit's JavaScript engine, this can lead to memory corruption when the engine incorrectly interprets object types during execution.

The vulnerability resides in WebKit, Apple's open-source web browser engine that powers Safari and serves as the foundation for web rendering across all Apple platforms. When processing maliciously crafted web content, the engine fails to properly validate object types during state transitions, allowing an attacker to manipulate memory in unexpected ways.

Root Cause

The root cause stems from inadequate state handling within WebKit's JavaScript execution engine. When certain JavaScript operations are performed, the engine may incorrectly assume the type of an object, leading to type confusion. This occurs because the state management logic does not properly track or validate type information across all execution paths, creating an opportunity for attackers to craft specific JavaScript code that triggers the confusion condition.

Attack Vector

The attack is network-based and requires user interaction. An attacker must convince a victim to visit a malicious webpage or open a malicious document that renders web content. The exploitation flow typically involves:

1. Victim visits an attacker-controlled webpage or is redirected to malicious content
2. The webpage contains specially crafted JavaScript that triggers the type confusion
3. The type confusion corrupts memory structures within WebKit
4. The attacker leverages the memory corruption to achieve arbitrary code execution
5. Code executes with the privileges of the browser process

Since Safari and WebKit-based browsers are the default rendering engines on Apple devices, this vulnerability presents a significant attack surface. The vulnerability affects not only Safari but any application using WebKit for rendering web content.

Detection Methods for CVE-2021-1789

Indicators of Compromise

• Unusual WebKit or Safari process crashes followed by unexpected system behavior
• Network connections to suspicious domains immediately following web browsing activity
• Unexpected child processes spawned from Safari, WebKit-based apps, or web content processes
• Evidence of memory corruption in WebKit crash logs or system diagnostics

Detection Strategies

• Monitor for abnormal JavaScript execution patterns or excessive memory allocation in WebKit processes
• Implement endpoint detection rules for suspicious process chains originating from browser processes
• Deploy network monitoring to detect communication with known malicious infrastructure
• Review crash reports for WebKit and Safari for patterns consistent with type confusion exploitation

Monitoring Recommendations

• Enable comprehensive logging for browser processes and WebKit-related activities
• Configure SentinelOne agents to monitor for behavioral indicators of browser-based exploitation
• Implement web filtering to block access to known malicious domains serving exploit content
• Set up alerts for unusual process behavior following web browsing sessions

How to Mitigate CVE-2021-1789

Immediate Actions Required

• Update all Apple devices to the patched versions: iOS/iPadOS 14.4, macOS Big Sur 11.2, Safari 14.0.3, tvOS 14.4, and watchOS 7.3
• For macOS Catalina and Mojave systems, apply Security Update 2021-001 immediately
• Update WebKitGTK on Linux systems to the latest patched version
• Fedora users should apply the security updates via the package manager
• Prioritize patching given the CISA Known Exploited Vulnerabilities listing

Patch Information

Apple has released security updates addressing this vulnerability across all affected platforms. Organizations should reference the following Apple Security Advisories:

• Apple Security Update HT212146 - iOS 14.4 and iPadOS 14.4
• Apple Security Update HT212147 - macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave
• Apple Security Update HT212148 - Safari 14.0.3
• Apple Security Update HT212149 - watchOS 7.3
• Apple Security Update HT212152 - tvOS 14.4

Linux distributions using WebKitGTK should refer to Gentoo GLSA 2021-04-03 and the Fedora package announcements for their respective patches.

Workarounds

• Restrict browsing to trusted websites only until patches can be applied
• Consider using content blockers or browser extensions that limit JavaScript execution on untrusted sites
• Implement network-level filtering to block known malicious domains
• Where possible, use non-WebKit browsers on unpatched systems as a temporary measure

# Verify Apple software versions on macOS
sw_vers
# Check Safari version
/Applications/Safari.app/Contents/MacOS/Safari --version
# On iOS devices, navigate to Settings > General > About to verify iOS version
# Ensure all devices show versions at or above the patched releases