CVE-2021-3781: Artifex Ghostscript RCE Vulnerability

CVE-2021-3781 Overview

A trivial sandbox escape flaw was discovered in the Ghostscript interpreter that allows attackers to bypass the -dSAFER sandbox protection by injecting specially crafted pipe commands. This vulnerability enables arbitrary command execution on the target system within the context of the Ghostscript interpreter, posing severe risks to confidentiality, integrity, and system availability.

Critical Impact
This sandbox escape vulnerability allows attackers to execute arbitrary commands on the system by bypassing Ghostscript's -dSAFER protection mechanism through malicious document processing.

Affected Products

Artifex Ghostscript 9.50
Artifex Ghostscript 9.52
Artifex Ghostscript 9.53.3
Artifex Ghostscript 9.54.0
Fedora Project Fedora 34

Discovery Timeline

2022-02-16 - CVE CVE-2021-3781 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3781

Vulnerability Analysis

This vulnerability represents a Command Injection flaw (CWE-78) combined with Improper Input Validation (CWE-20) in the Ghostscript interpreter. The -dSAFER option is designed to restrict potentially dangerous operations when processing PostScript and PDF documents, creating a sandbox environment. However, the flaw allows attackers to circumvent this sandbox entirely by injecting specially crafted pipe commands into documents processed by Ghostscript.

When a malicious document is processed, the injected pipe commands are executed with the same privileges as the Ghostscript process, which could be elevated depending on the system configuration. This is particularly dangerous because Ghostscript is widely used in document processing pipelines, print servers, and web applications for PDF rendering and conversion.

Root Cause

The root cause lies in insufficient input validation within the Ghostscript interpreter's pipe handling mechanism. The -dSAFER sandbox mode fails to properly sanitize or restrict pipe command constructs, allowing specially crafted PostScript or PDF content to escape the sandbox boundaries and execute arbitrary system commands.

Attack Vector

The attack is network-accessible, requiring low privileges and no user interaction. An attacker can exploit this vulnerability by:

Crafting a malicious PostScript or PDF document containing embedded pipe commands
Delivering the document to a target system that processes it with Ghostscript
When the document is processed (even with -dSAFER enabled), the injected pipe commands execute
The attacker gains arbitrary command execution in the context of the Ghostscript interpreter process

This vulnerability has a scope change impact, meaning exploitation can affect resources beyond the vulnerable component's security scope, potentially compromising the entire host system.

The exploitation mechanism involves injecting pipe commands within PostScript constructs that bypass the -dSAFER restrictions. For detailed technical information about the vulnerability mechanism, refer to the Ghostscript CVE-2021-3781 Details advisory.

Detection Methods for CVE-2021-3781

Indicators of Compromise

Unexpected child processes spawned by Ghostscript (gs) processes
Anomalous network connections originating from Ghostscript processes
Suspicious PostScript or PDF files containing pipe command syntax (e.g., %pipe%)
Unusual system command execution patterns following document processing events

Detection Strategies

Monitor process creation events where the parent process is gs or ghostscript for suspicious command line arguments
Implement file scanning rules to detect PostScript/PDF documents containing pipe command injection patterns
Deploy endpoint detection rules that alert on Ghostscript processes attempting to spawn shells or execute system utilities
Review application logs for Ghostscript errors or warnings that may indicate exploitation attempts

Monitoring Recommendations

Enable comprehensive logging for document processing services and print spoolers
Configure SentinelOne Singularity to monitor Ghostscript process behavior and detect anomalous child process spawning
Implement network segmentation to limit the impact of potential compromise from document processing systems
Set up alerts for Ghostscript processes accessing sensitive files or making unexpected network connections

How to Mitigate CVE-2021-3781

Immediate Actions Required

Update Artifex Ghostscript to a patched version immediately
Audit systems for any versions of Ghostscript in the affected range (9.50, 9.52, 9.53.3, 9.54.0)
Review document processing workflows to identify exposure points
Consider temporarily disabling Ghostscript-based document processing if immediate patching is not possible

Patch Information

Artifex has released security updates to address this vulnerability. Organizations should apply the latest Ghostscript security patches as documented in the official advisory. For detailed patch information, refer to the Ghostscript CVE-2021-3781 Details page and the Red Hat Bug Report #2002271. Fedora users should update to patched versions available through their distribution's package manager.

Workarounds

Implement strict input validation for all documents before Ghostscript processing
Use application sandboxing (containers, SELinux, AppArmor) to limit Ghostscript process capabilities
Disable or restrict Ghostscript usage in automated document processing pipelines until patched
Deploy network-level controls to prevent compromised document processors from initiating outbound connections

bash

# Configuration example
# Verify installed Ghostscript version
gs --version

# Check if Ghostscript is in use on the system
which gs
rpm -qa | grep ghostscript  # For RPM-based systems
dpkg -l | grep ghostscript  # For Debian-based systems

# Update Ghostscript on Fedora systems
sudo dnf update ghostscript

# Apply SELinux confinement for additional protection
sudo semanage fcontext -a -t ghostscript_exec_t /usr/bin/gs
sudo restorecon -v /usr/bin/gs