CVE-2020-36773: Artifex Ghostscript Use-After-Free Flaw

CVE-2020-36773 Overview

CVE-2020-36773 is a memory corruption vulnerability affecting Artifex Ghostscript before version 9.53.0. The vulnerability exists in devices/vector/gdevtxtw.c (txtwrite device) and involves both an out-of-bounds write and use-after-free condition. The flaw is triggered when processing PDF documents where a single character code maps to more than one Unicode code point, such as ligatures (e.g., "fi" or "fl" ligatures).

This vulnerability allows remote attackers to potentially achieve arbitrary code execution by crafting malicious PDF documents that exploit improper memory handling during text extraction operations.

Critical Impact

Remote attackers can exploit this memory corruption vulnerability through malicious PDF documents to potentially execute arbitrary code or cause application crashes, affecting confidentiality, integrity, and availability of affected systems.

Affected Products

• Artifex Ghostscript 9.51
• Artifex Ghostscript 9.52
• Artifex Ghostscript 9.52.1
• Artifex Ghostscript 9.53.0-rc1
• Artifex Ghostscript 9.53.0-rc2

Discovery Timeline

• 2024-02-04 - CVE CVE-2020-36773 published to NVD
• 2025-05-22 - Last updated in NVD database

Technical Details for CVE-2020-36773

Vulnerability Analysis

This vulnerability is classified under CWE-416 (Use After Free). The memory corruption occurs in the txtwrite device component of Ghostscript, specifically within the devices/vector/gdevtxtw.c file. The txtwrite device is responsible for extracting text content from PDF documents and converting it to plain text output.

The vulnerability is triggered when Ghostscript processes PDF documents containing characters that map to multiple Unicode code points. Ligatures are common examples of this scenario—a single glyph identifier in the PDF may represent combined characters like "fi" or "ffi". When the txtwrite device encounters such mappings, it fails to properly handle the memory allocation required for the expanded character output.

The dual nature of this vulnerability—combining both out-of-bounds write and use-after-free conditions—makes it particularly dangerous. The out-of-bounds write allows corruption of adjacent memory regions, while the use-after-free condition enables attackers to potentially control freed memory contents, creating pathways for code execution.

Root Cause

The root cause lies in improper memory management within the text extraction code path. When a PDF character code maps to multiple Unicode code points (as with ligatures), the code does not correctly account for the expanded buffer size needed. This leads to writing beyond allocated buffer boundaries. Additionally, memory that has already been freed may be accessed during subsequent operations, resulting in the use-after-free condition.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious PDF document containing specially constructed character mappings (e.g., ligature definitions)
2. Delivering the malicious PDF to a victim system where Ghostscript processes it
3. When Ghostscript's txtwrite device processes the document for text extraction, the memory corruption is triggered

Common attack scenarios include:

• Processing untrusted PDF documents through document conversion workflows
• Server-side PDF processing applications that use Ghostscript
• Print spoolers and document preview systems
• Automated document processing pipelines

The vulnerability can be exploited remotely through any application or service that uses Ghostscript to process PDF documents, making it particularly dangerous in enterprise document processing environments.

Detection Methods for CVE-2020-36773

Indicators of Compromise

• Unexpected crashes or segmentation faults in Ghostscript processes when processing PDF documents
• Abnormal memory consumption patterns in Ghostscript or applications using the Ghostscript library
• Process termination with memory corruption error messages referencing gdevtxtw.c or txtwrite device
• Suspicious PDF files with unusual character mapping or ligature definitions in document logs

Detection Strategies

• Monitor for Ghostscript process crashes with memory-related error codes, particularly during text extraction operations
• Implement file integrity monitoring for Ghostscript binary and library files to detect potential exploitation artifacts
• Deploy endpoint detection solutions capable of identifying exploitation attempts targeting document processing applications
• Analyze incoming PDF documents for anomalous character mapping structures before processing

Monitoring Recommendations

• Enable verbose logging for Ghostscript operations to capture detailed processing information during PDF handling
• Monitor system logs for repeated Ghostscript failures or unusual process behavior patterns
• Implement application-level monitoring for memory allocation anomalies in document processing workflows
• Configure alerting for any unexpected child process spawning from Ghostscript processes

How to Mitigate CVE-2020-36773

Immediate Actions Required

• Update Artifex Ghostscript to version 9.53.0 or later immediately on all affected systems
• Audit systems for Ghostscript installations and identify all applications that depend on Ghostscript for PDF processing
• Implement input validation and sandboxing for untrusted PDF document processing until patches are applied
• Consider disabling the txtwrite device if text extraction functionality is not required

Patch Information

Artifex has addressed this vulnerability in Ghostscript version 9.53.0. The fix is available through the Ghostscript Git Commit 8c7bd78 which corrects the memory handling for character codes that map to multiple Unicode code points.

Organizations should update to Ghostscript 9.53.0 or later by downloading from the official Ghostscript releases page. Additional details about the vulnerability can be found in the Ghostscript Bug Report #702229.

For Linux distributions, check with your distribution vendor for updated packages. The openSUSE Bug Report #1177922 tracks the fix for openSUSE/SUSE systems.

Workarounds

• Restrict processing of untrusted PDF documents until patches can be applied to production systems
• Implement network segmentation to isolate document processing servers from critical infrastructure
• Run Ghostscript processes in sandboxed environments with limited privileges and restricted system access
• Disable the txtwrite device by removing or restricting access to text extraction functionality if not operationally required

# Configuration example
# Check current Ghostscript version
gs --version

# Verify the installation is patched (should be 9.53.0 or later)
# If version is below 9.53.0, update immediately

# On Debian/Ubuntu systems
sudo apt-get update && sudo apt-get install ghostscript

# On RHEL/CentOS systems
sudo yum update ghostscript

# On openSUSE/SUSE systems
sudo zypper update ghostscript

# Restrict txtwrite device access if patching is not immediately possible
# Add to Ghostscript configuration or use command-line restrictions
# -dSAFER -dNOPLATFONTS options provide additional hardening