CVE-2021-1748: Apple iPadOS XSS Vulnerability

CVE-2021-1748 Overview

CVE-2021-1748 is an input validation vulnerability affecting Apple's WebKit-based URL processing across multiple operating systems. Processing a maliciously crafted URL can lead to arbitrary JavaScript code execution in the context of the targeted user. Apple addressed the issue with improved input sanitization in iOS 14.4, iPadOS 14.4, tvOS 14.4, and watchOS 7.3. The flaw is classified under [CWE-20] (Improper Input Validation) and requires user interaction, such as opening or following a crafted link.

Critical Impact
A network-based attacker can achieve arbitrary JavaScript execution on affected Apple devices by tricking a user into processing a crafted URL, with high impact to confidentiality, integrity, and availability.

Affected Products

Apple iOS prior to 14.4
Apple iPadOS prior to 14.4
Apple tvOS prior to 14.4
Apple watchOS 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

The vulnerability stems from insufficient validation of URL input within Apple's URL handling components shared across iOS, iPadOS, tvOS, and watchOS. When the system processes a maliciously crafted URL, the parser fails to properly sanitize attacker-controlled portions of the input. This allows JavaScript payloads embedded in the URL to execute in a privileged web context.

The issue maps to [CWE-20] Improper Input Validation. Exploitation requires user interaction, meaning the victim must open the crafted URL through a messaging app, email, document, or web page. Successful exploitation enables script execution that can read sensitive data, perform actions on behalf of the user, or chain into further browser-based attacks.

Root Cause

The root cause is missing or insufficient sanitization in the URL processing logic. Apple's advisory notes the fix was implemented through improved input sanitization, indicating the original parser accepted URL components that should have been rejected or properly encoded before being passed downstream to the JavaScript execution context.

Attack Vector

The attack vector is network-based with low complexity and no privileges required. An attacker hosts or distributes a crafted URL that triggers the parsing flaw when a user opens it on a vulnerable Apple device. Common delivery channels include phishing emails, SMS messages, malicious websites, and embedded links inside documents or third-party apps that hand URLs to the system handler.

No verified public exploit code or proof-of-concept is available for this CVE. Technical specifics are described in Apple Support Article HT212146, HT212148, and HT212149.

Detection Methods for CVE-2021-1748

Indicators of Compromise

Unexpected JavaScript execution or browser redirects following the opening of a URL on an unpatched iOS, iPadOS, tvOS, or watchOS device.
Outbound connections from mobile devices to unfamiliar domains immediately after a user taps a link in messaging or email applications.
Mobile device management (MDM) telemetry showing devices running OS versions earlier than iOS 14.4, iPadOS 14.4, tvOS 14.4, or watchOS 7.3.

Detection Strategies

Inventory all enrolled Apple devices through MDM and flag those running operating system builds older than the patched versions.
Inspect network proxy and DNS logs for users visiting suspicious URLs containing unusual encoding patterns or oversized URL components.
Correlate phishing email and SMS reports against device OS versions to prioritize triage for unpatched endpoints.

Monitoring Recommendations

Enable MDM compliance policies that alert on devices failing to meet minimum OS version requirements.
Monitor mobile threat defense telemetry for anomalous browser activity, redirect chains, and credential harvesting attempts.
Track user-reported phishing campaigns and pivot on URL patterns delivered to Apple device users.

How to Mitigate CVE-2021-1748

Immediate Actions Required

Update all Apple devices to iOS 14.4, iPadOS 14.4, tvOS 14.4, or watchOS 7.3 or later through Settings or MDM-pushed update policies.
Identify devices that cannot receive the update due to hardware constraints and apply compensating controls or replace them.
Reinforce user awareness training focused on not opening unsolicited links delivered through email, SMS, or messaging platforms.

Patch Information

Apple released patches addressing CVE-2021-1748 in the following updates: iOS 14.4 and iPadOS 14.4 (HT212146), watchOS 7.3 (HT212148), and tvOS 14.4 (HT212149). The vendor remediated the issue through improved input sanitization in the URL processing components.

Workarounds

No vendor-supplied workaround exists; patching to the fixed OS versions is the only complete mitigation.
Restrict use of unpatched devices for accessing sensitive corporate resources until updates are installed.
Configure secure email gateways and SMS filtering to strip or sandbox suspicious URLs before delivery to mobile users.

bash

# Verify Apple device OS version meets the patched baseline
# On macOS administration host querying via MDM API or on-device Settings
# iOS / iPadOS: Settings > General > About > Software Version (must be >= 14.4)
# watchOS:      Watch app > General > About > Version (must be >= 7.3)
# tvOS:         Settings > System > About > tvOS Version (must be >= 14.4)