CVE-2021-30661: Apple Safari Use-After-Free Vulnerability

CVE-2021-30661 Overview

CVE-2021-30661 is a use after free vulnerability affecting Apple's WebKit browser engine across multiple Apple platforms. The vulnerability exists due to improper memory management in WebKit, where memory is accessed after it has been freed. When a user visits a maliciously crafted webpage, an attacker can exploit this flaw to achieve arbitrary code execution on the affected device. Apple has confirmed that this vulnerability may have been actively exploited in the wild prior to patching.

Critical Impact

This vulnerability allows remote attackers to execute arbitrary code on affected Apple devices by tricking users into visiting malicious web content. Apple has acknowledged reports of active exploitation, and this vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog.

Affected Products

• Apple Safari (versions prior to 14.1)
• Apple iOS (versions prior to 14.5 and 12.5.3)
• Apple iPadOS (versions prior to 14.5)
• Apple macOS Big Sur (versions prior to 11.3)
• Apple tvOS (versions prior to 14.5)
• Apple watchOS (versions prior to 7.4)

Discovery Timeline

• 2021-09-08 - CVE-2021-30661 published to NVD
• 2025-10-23 - Last updated in NVD database

Technical Details for CVE-2021-30661

Vulnerability Analysis

CVE-2021-30661 is classified as CWE-416 (Use After Free), a memory corruption vulnerability that occurs when a program continues to use a pointer after the memory it references has been deallocated. In the context of WebKit, this vulnerability manifests during the processing of web content, where certain operations can trigger a condition in which freed memory is subsequently accessed.

Use after free vulnerabilities in browser engines are particularly dangerous because they can be triggered remotely by simply visiting a webpage. The attacker can craft specific HTML, CSS, or JavaScript content that causes the browser to free a memory object while retaining a reference to it. When this dangling pointer is later dereferenced, it can lead to memory corruption, information disclosure, or arbitrary code execution.

The network-based attack vector with low complexity makes this vulnerability highly accessible to attackers. The requirement for user interaction (visiting a malicious webpage) is the primary mitigating factor, though social engineering techniques can readily overcome this barrier.

Root Cause

The root cause of CVE-2021-30661 is improper memory management within the WebKit engine. Specifically, the vulnerability arises when memory associated with a particular object is freed, but references to that memory location are not properly invalidated. This creates a dangling pointer that, when subsequently accessed, leads to undefined behavior.

In typical use after free scenarios in JavaScript engines and DOM implementations, the issue often occurs during object lifecycle management, garbage collection interactions, or event handler execution. The freed memory may be reallocated for other purposes, and when the stale pointer is used, it can corrupt unrelated data structures or be manipulated to achieve code execution.

Attack Vector

The attack vector for CVE-2021-30661 is network-based, requiring an attacker to entice a victim to visit a maliciously crafted webpage. The attack flow typically follows this pattern:

1. The attacker creates a webpage containing specially crafted content designed to trigger the use after free condition in WebKit
2. The victim navigates to the malicious page using Safari or an application that utilizes WebKit for rendering web content
3. The malicious content triggers the memory management flaw, causing WebKit to access freed memory
4. The attacker leverages this memory corruption to gain arbitrary code execution within the context of the browser process
5. Depending on the exploitation technique, the attacker may achieve full device compromise or access to sensitive user data

The vulnerability affects WebKit-based browsing on all Apple platforms, meaning any application using WebKit for web content rendering is potentially affected. This includes Safari, Mail, and numerous third-party applications that embed web views.

Detection Methods for CVE-2021-30661

Indicators of Compromise

• Unusual Safari or WebKit-based application crashes, particularly when visiting unfamiliar websites
• Unexpected network connections from browser processes to suspicious external hosts
• Presence of unknown or unauthorized processes spawned by WebKit components
• Anomalous memory usage patterns in Safari or WebKit-related processes

Detection Strategies

• Monitor for WebKit crash reports with memory corruption signatures, especially those involving use after free patterns
• Implement network monitoring to identify connections to known malicious domains associated with exploitation campaigns
• Deploy endpoint detection solutions capable of identifying suspicious process behavior following web browser activity
• Enable application crash reporting and analysis to identify potential exploitation attempts

Monitoring Recommendations

• Configure centralized logging for all Apple device crash reports and security events
• Implement web filtering to block access to known malicious domains and newly registered suspicious domains
• Monitor for CISA KEV alerts and threat intelligence feeds related to WebKit exploitation activity
• Review browser history and network logs for devices exhibiting suspicious behavior

How to Mitigate CVE-2021-30661

Immediate Actions Required

• Update all affected Apple devices to the patched versions immediately: Safari 14.1, iOS 14.5 (or 12.5.3 for older devices), iPadOS 14.5, macOS Big Sur 11.3, tvOS 14.5, and watchOS 7.4
• Enable automatic updates on all Apple devices to ensure timely delivery of future security patches
• Educate users about the risks of visiting untrusted websites and clicking on suspicious links
• Consider implementing web content filtering to reduce exposure to potentially malicious sites

Patch Information

Apple has released security updates addressing CVE-2021-30661 across all affected platforms. The patches improve memory management within WebKit to prevent the use after free condition. Detailed information about the security updates is available through Apple's support documentation:

• Apple Support Document HT212317 - Safari 14.1
• Apple Support Document HT212318 - iOS 14.5 and iPadOS 14.5
• Apple Support Document HT212323 - macOS Big Sur 11.3
• Apple Support Document HT212324 - watchOS 7.4
• Apple Support Document HT212325 - tvOS 14.5
• Apple Support Document HT212341 - iOS 12.5.3

This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, requiring federal agencies to apply patches within specified timeframes.

Workarounds

• Restrict web browsing to trusted sites only until patches can be applied
• Use alternative browsers that do not rely on WebKit rendering where possible (note: on iOS, all browsers use WebKit)
• Implement network-level URL filtering to block access to known malicious domains
• Consider disabling JavaScript in Safari settings for high-risk environments, though this may significantly impact web functionality

# Verify current Safari version on macOS
/usr/bin/defaults read /Applications/Safari.app/Contents/Info.plist CFBundleShortVersionString

# Check for available software updates on macOS
softwareupdate --list

# Install all available security updates
softwareupdate --install --all