CVE-2021-1748: Apple iPadOS XSS Vulnerability

CVE-2021-1748 Overview

CVE-2021-1748 is an input validation vulnerability affecting multiple Apple operating systems including iOS, iPadOS, tvOS, and watchOS. The vulnerability stems from improper input sanitization when processing URLs, which can be exploited by attackers to execute arbitrary JavaScript code on affected devices. This cross-site scripting (XSS) style vulnerability could allow attackers to steal sensitive information, hijack user sessions, or perform malicious actions in the context of the affected application.

Critical Impact

Processing a maliciously crafted URL may lead to arbitrary JavaScript code execution on iOS, iPadOS, tvOS, and watchOS devices, potentially compromising user data and device security.

Affected Products

• Apple iOS (versions prior to 14.4)
• Apple iPadOS (versions prior to 14.4)
• Apple tvOS (versions prior to 14.4)
• Apple watchOS (versions prior to 7.3)

Discovery Timeline

• 2021-04-02 - CVE-2021-1748 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1748

Vulnerability Analysis

This vulnerability is classified as CWE-20 (Improper Input Validation), indicating that the affected Apple operating systems failed to properly validate and sanitize URL input before processing. When a user interacts with a maliciously crafted URL, the lack of proper input sanitization allows embedded JavaScript code to execute in an unintended context.

The attack requires user interaction—specifically, a user must click on or otherwise process the malicious URL. This could occur through phishing emails, malicious websites, or compromised applications that present crafted URLs to the user. Once triggered, the JavaScript executes with the privileges of the application processing the URL, potentially allowing access to sensitive data, cookies, session tokens, or the ability to perform actions on behalf of the user.

Root Cause

The root cause of CVE-2021-1748 is insufficient input validation and sanitization in the URL processing components of iOS, iPadOS, tvOS, and watchOS. The affected code paths did not adequately filter or escape special characters and JavaScript payloads embedded within URLs before rendering or executing them. This oversight allowed attackers to craft URLs containing malicious JavaScript that would bypass validation checks and execute when the URL was processed by the system.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no special privileges but necessitating user interaction. An attacker could exploit this vulnerability through several methods:

1. Phishing campaigns - Sending crafted URLs via email or messaging platforms
2. Malicious websites - Embedding crafted URLs in web pages that redirect or open on Apple devices
3. Social engineering - Convincing users to click on specially crafted links shared through various channels
4. Compromised applications - Apps that process external URLs could inadvertently trigger the vulnerability

The vulnerability does not require authentication and can be exploited remotely over the network, making it accessible to a wide range of attackers. The malicious URL, when processed, causes arbitrary JavaScript to execute, potentially leading to data theft, session hijacking, or further exploitation of the device.

Detection Methods for CVE-2021-1748

Indicators of Compromise

• Unusual JavaScript execution or unexpected web content rendering in native applications
• Network traffic containing suspiciously crafted URLs with embedded script payloads
• Unexpected application behavior or crashes when processing URLs from external sources
• User reports of suspicious redirects or pop-ups after clicking links on Apple devices

Detection Strategies

• Monitor network traffic for URLs containing potential JavaScript injection patterns such as javascript:, encoded script tags, or event handlers
• Implement URL filtering and inspection at network perimeter devices to identify malicious URL patterns
• Deploy endpoint detection and response (EDR) solutions like SentinelOne to detect anomalous JavaScript execution behavior on managed Apple devices
• Review application logs for URL processing errors or exceptions that may indicate exploitation attempts

Monitoring Recommendations

• Enable comprehensive logging on mobile device management (MDM) solutions to track URL interactions
• Configure web proxy solutions to log and analyze all URL traffic from Apple devices
• Implement real-time alerting for detection of known malicious URL patterns
• Monitor for unusual outbound connections following URL interactions that may indicate successful exploitation

How to Mitigate CVE-2021-1748

Immediate Actions Required

• Update all affected Apple devices to the patched versions: iOS 14.4, iPadOS 14.4, tvOS 14.4, or watchOS 7.3 or later
• Educate users about the risks of clicking on suspicious or unknown URLs
• Implement network-level URL filtering to block potentially malicious URLs before they reach devices
• Review and restrict application permissions for URL handling where possible

Patch Information

Apple has released security updates addressing this vulnerability. Organizations and users should apply the following updates immediately:

• iOS and iPadOS: Update to version 14.4 or later - Apple Support Article HT212146
• tvOS: Update to version 14.4 or later - Apple Support Article HT212148
• watchOS: Update to version 7.3 or later - Apple Support Article HT212149

For enterprise environments, use MDM solutions to push updates to all managed devices and verify compliance.

Workarounds

• Restrict URL scheme handlers and limit which applications can process external URLs until patches are applied
• Implement strict content security policies on managed devices to limit JavaScript execution contexts
• Use network proxies to inspect and sanitize URLs before they reach Apple devices
• Configure email clients and messaging applications to disable automatic link previews and require explicit user action for URL processing