CVE-2021-30663: Apple Safari RCE Vulnerability

CVE-2021-30663 Overview

CVE-2021-30663 is an integer overflow vulnerability affecting Apple's WebKit browser engine, which powers Safari and other web rendering components across multiple Apple platforms. The vulnerability arises from insufficient input validation when processing certain web content, allowing an attacker to trigger an integer overflow condition. When exploited, this flaw enables arbitrary code execution on affected devices simply by convincing a user to visit a maliciously crafted webpage.

Critical Impact

This vulnerability has been actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Processing maliciously crafted web content may lead to arbitrary code execution with the privileges of the current user.

Affected Products

• Apple Safari (versions prior to 14.1.1)
• Apple iOS (versions prior to 14.5.1)
• Apple iPadOS (versions prior to 14.5.1)
• Apple iOS 12.x (versions prior to 12.5.3)
• Apple macOS Big Sur (versions prior to 11.3.1)
• Apple tvOS (versions prior to 14.6)

Discovery Timeline

• 2021-09-08 - CVE-2021-30663 published to NVD
• 2025-10-23 - Last updated in NVD database

Technical Details for CVE-2021-30663

Vulnerability Analysis

CVE-2021-30663 is classified as CWE-190 (Integer Overflow or Wraparound), a memory corruption vulnerability within Apple's WebKit rendering engine. The flaw occurs when the browser processes specially crafted web content that triggers an integer overflow condition during memory allocation or size calculations.

Integer overflow vulnerabilities in web browsers are particularly dangerous because they can be exploited remotely with minimal user interaction—simply visiting a malicious webpage is sufficient. When an integer overflow occurs during memory operations, it can result in undersized buffer allocations, leading to subsequent buffer overflows when data is written to these incorrectly sized memory regions.

The network-based attack vector combined with the low complexity of exploitation makes this vulnerability highly attractive to threat actors. Successful exploitation grants attackers the ability to execute arbitrary code within the context of the browser process, potentially leading to full device compromise, data theft, or installation of persistent malware.

Root Cause

The root cause of CVE-2021-30663 lies in insufficient input validation within WebKit's content processing routines. When handling certain malformed or oversized input values, arithmetic operations can exceed the maximum representable value for the integer data type being used. This causes the value to wrap around to a small or negative number, which is then used for subsequent memory operations.

Apple addressed this vulnerability by implementing improved input validation to ensure that values used in size calculations cannot overflow and that proper bounds checking is performed before memory allocation and data processing operations.

Attack Vector

The attack is conducted remotely over the network by hosting malicious web content on an attacker-controlled server or by injecting malicious content into legitimate websites through compromise or advertising networks. The attack requires user interaction—specifically, the victim must navigate to or be redirected to the malicious webpage.

Once the victim's browser begins rendering the malicious content, the integer overflow is triggered during content processing. This can corrupt memory structures, overwrite function pointers, or manipulate control flow to achieve arbitrary code execution. Given that this vulnerability affects the core WebKit engine, it impacts not only Safari but also any application that uses WebKit for rendering web content.

Detection Methods for CVE-2021-30663

Indicators of Compromise

• Unexpected crashes or instability in Safari, Safari WebContent processes, or applications using WebKit
• Unusual network connections originating from browser processes to suspicious external hosts
• Evidence of WebKit exploit payloads in web traffic logs or proxy records
• Suspicious JavaScript or HTML content with abnormally large numeric values or complex nested structures
• Unexpected child processes spawned by Safari or WebKit-based applications

Detection Strategies

• Monitor system logs for WebKit process crashes with memory corruption signatures or unusual exit codes
• Implement web content filtering to detect and block known exploit patterns targeting WebKit integer overflows
• Deploy endpoint detection solutions capable of identifying post-exploitation behaviors such as unauthorized process spawning
• Utilize browser sandboxing and exploit mitigation technologies to detect breakout attempts

Monitoring Recommendations

• Enable detailed logging for Safari and WebKit processes to capture crash reports and memory access violations
• Monitor network traffic for connections to suspicious domains or IP addresses following browser activity
• Implement file integrity monitoring on critical system directories to detect unauthorized modifications
• Review security advisories and threat intelligence feeds for emerging exploit techniques targeting this vulnerability

How to Mitigate CVE-2021-30663

Immediate Actions Required

• Update all affected Apple devices to the patched versions immediately: iOS 14.5.1, iPadOS 14.5.1, iOS 12.5.3, Safari 14.1.1, macOS Big Sur 11.3.1, and tvOS 14.6
• Prioritize patching as this vulnerability is confirmed to be actively exploited in the wild
• Enable automatic updates on all Apple devices to ensure timely application of security patches
• Review enterprise mobile device management (MDM) policies to enforce minimum OS version requirements

Patch Information

Apple has released security updates across multiple products to address CVE-2021-30663. Detailed patch information is available in the following Apple Security Advisories:

• Apple Support Document HT212335 - iOS 14.5.1 and iPadOS 14.5.1
• Apple Support Document HT212336 - macOS Big Sur 11.3.1
• Apple Support Document HT212341 - iOS 12.5.3
• Apple Support Document HT212532 - Safari 14.1.1
• Apple Support Document HT212534 - tvOS 14.6

This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, which requires federal agencies to remediate by specified deadlines.

Workarounds

• If immediate patching is not possible, consider using an alternative browser that does not rely on WebKit until updates can be applied
• Implement network-level web filtering to block access to known malicious domains associated with WebKit exploits
• Restrict browsing to trusted websites only on vulnerable devices pending patch deployment
• Consider temporarily disabling JavaScript execution in Safari, though this may significantly impact browsing functionality

# Verify current macOS version to ensure patch is applied
sw_vers -productVersion
# Expected output for patched system: 11.3.1 or later

# Verify Safari version on macOS
/Applications/Safari.app/Contents/MacOS/Safari --version
# Expected output for patched system: 14.1.1 or later

# Check iOS/iPadOS version via command line (for managed devices)
# Settings > General > About > Software Version
# Expected: 14.5.1 or later (or 12.5.3 for older devices)