CVE-2021-40531: Sketch RCE Vulnerability via Library Feeds

CVE-2021-40531 Overview

CVE-2021-40531 is a critical remote code execution vulnerability affecting Sketch versions prior to 75 on macOS. The vulnerability exists in how Sketch handles library feeds, allowing malicious actors to bypass Apple's file quarantine security mechanism. When exploited, files are automatically downloaded and opened without the com.apple.quarantine extended attribute being applied, which is designed to protect users from running untrusted code downloaded from the internet.

This vulnerability is particularly dangerous because it enables remote code execution through seemingly legitimate design library functionality. An attacker could craft a malicious terminal profile that, when processed by Terminal.app, executes arbitrary commands via the CommandString parameter.

Critical Impact

Remote code execution through quarantine bypass allows attackers to execute arbitrary commands on victim machines without user interaction or security warnings.

Affected Products

• Sketch versions prior to 75
• Apple macOS (as the underlying platform)

Discovery Timeline

• 2021-09-06 - CVE-2021-40531 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40531

Vulnerability Analysis

This vulnerability falls under CWE-434 (Unrestricted Upload of File with Dangerous Type) and represents a security boundary bypass affecting macOS file quarantine protections. The core issue lies in Sketch's library feed handling mechanism, which fails to apply the com.apple.quarantine extended attribute to automatically downloaded files.

On macOS, the quarantine attribute serves as a critical security control that triggers Gatekeeper verification when users attempt to open files downloaded from the internet. When this attribute is missing, the operating system treats the file as trusted local content, bypassing security dialogs and code signing verification.

The attack chain involves crafting a malicious library feed that delivers a specially crafted terminal profile file. When Sketch processes this feed, it downloads the malicious file without quarantine protections. Subsequently, when the victim interacts with Terminal.app, the CommandString directive within the terminal profile executes attacker-controlled commands.

Root Cause

The root cause of this vulnerability is Sketch's failure to properly set the com.apple.quarantine extended attribute on files downloaded through its library feed functionality. This represents an improper implementation of macOS security best practices for applications that download and process external content. Applications that download files from the network should always apply quarantine attributes to ensure Gatekeeper can properly assess the trustworthiness of the content.

Attack Vector

The attack vector is network-based and can be exploited without user authentication or interaction. An attacker would need to:

1. Host a malicious library feed accessible to the target
2. Craft a terminal profile containing malicious CommandString directives
3. Convince the victim to add the malicious library feed to Sketch, or compromise an existing legitimate feed
4. When Sketch processes the feed, the malicious file is downloaded without quarantine protection
5. The terminal profile executes arbitrary commands when Terminal.app processes it

The attack demonstrates how trusted application functionality can be abused to bypass platform security controls. For detailed technical analysis, see the Jon Palmisc CVE-2021-40531 Analysis.

Detection Methods for CVE-2021-40531

Indicators of Compromise

• Files in Sketch library directories missing the com.apple.quarantine extended attribute
• Unexpected terminal profile files (.terminal extension) appearing on the system
• Unusual network connections from Sketch to untrusted library feed URLs
• CommandString execution in Terminal.app logs originating from downloaded profiles

Detection Strategies

• Monitor for files created by Sketch that lack the com.apple.quarantine extended attribute using xattr commands
• Implement network monitoring for Sketch library feed connections to untrusted domains
• Audit terminal profile files for suspicious CommandString directives
• Use endpoint detection to identify unexpected command execution chains originating from design application workflows

Monitoring Recommendations

• Enable macOS Unified Logging to capture file system extended attribute operations
• Monitor process creation events for Terminal.app spawning child processes from downloaded profiles
• Implement application allowlisting to restrict which terminal profiles can be executed
• Review Sketch library feed configurations periodically for unauthorized additions

How to Mitigate CVE-2021-40531

Immediate Actions Required

• Upgrade Sketch to version 75 or later immediately
• Review and remove any untrusted library feeds from Sketch configurations
• Audit systems for terminal profiles that may have been downloaded without quarantine attributes
• Scan for indicators of compromise on systems running vulnerable Sketch versions

Patch Information

Sketch has addressed this vulnerability in version 75. Users should update to the latest version through the official update mechanism or by downloading from the Sketch website. The fix ensures that all files downloaded through library feeds properly receive the com.apple.quarantine extended attribute, restoring macOS Gatekeeper protections.

For official patch details, see the Sketch Version 75 Update.

Workarounds

• Disable library feed functionality in Sketch until the update can be applied
• Remove all external library feed subscriptions temporarily
• Implement network-level blocking of untrusted library feed URLs
• Use macOS Security & Privacy settings to only allow apps from identified developers

# Check for files missing quarantine attribute in Sketch directories
find ~/Library/Application\ Support/com.bohemiancoding.sketch3 -type f -exec xattr -l {} \; | grep -L "com.apple.quarantine"

# Manually add quarantine attribute to suspicious files
xattr -w com.apple.quarantine "0081;00000000;Safari;" /path/to/suspicious/file