CVE-2020-36773 in Ghostscript
Summary
by MITRE • 02/04/2024
Artifex Ghostscript before 9.53.0 has an out-of-bounds write and use-after-free in devices/vector/gdevtxtw.c (for txtwrite) because a single character code in a PDF document can map to more than one Unicode code point (e.g., for a ligature).
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Analysis
by VulDB Data Team • 02/25/2024
The vulnerability identified as CVE-2020-36773 represents a critical memory corruption issue within the Artifex Ghostscript PDF rendering engine affecting versions prior to 9.53.0. This flaw exists within the txtwrite device implementation located in devices/vector/gdevtxtw.c, specifically manifesting as both out-of-bounds write and use-after-free conditions. The vulnerability stems from the improper handling of character encoding when processing PDF documents containing ligature sequences, where a single character code can legitimately map to multiple Unicode code points during text rendering operations.
The technical exploitation of this vulnerability occurs when Ghostscript processes PDF documents containing specific ligature characters that require mapping to multiple Unicode code points. During the text writing process, the software fails to properly validate or allocate sufficient memory for the expanded character representation, leading to buffer overflows that can result in arbitrary code execution. The use-after-free component emerges when memory allocated for character processing is freed but subsequently accessed during the ligature handling routine, creating potential exploitation vectors for remote code execution attacks.
This vulnerability impacts the core PDF rendering functionality of Ghostscript, which is widely used across enterprise environments for document processing, printing, and conversion services. The out-of-bounds write condition can corrupt adjacent memory regions, potentially allowing attackers to overwrite critical program structures or inject malicious code, while the use-after-free aspect provides additional attack surface for exploitation. The vulnerability is particularly concerning as it can be triggered through simple PDF document processing without requiring user interaction, making it suitable for automated exploitation in web-based attack scenarios.
The security implications extend beyond simple memory corruption to encompass potential privilege escalation and system compromise when Ghostscript is executed with elevated privileges. This vulnerability aligns with CWE-121 Stack-based Buffer Overflow and CWE-416 Use After Free categories, representing a classic memory safety issue that can be exploited through improper bounds checking and memory management. From an ATT&CK perspective, this vulnerability maps to T1059.007 Command and Scripting Interpreter: JavaScript and T1203 Exploitation for Client Execution, as it enables remote code execution through PDF document processing. Organizations using Ghostscript for document handling, web applications processing PDF uploads, or print servers relying on Ghostscript for document conversion are particularly vulnerable to this issue.
Mitigation strategies include immediate upgrading to Ghostscript version 9.53.0 or later, which contains the necessary patches to address the character encoding validation issues. Additionally, implementing strict PDF document validation and sanitization processes can reduce the risk of exploitation, particularly in environments where untrusted PDF documents are processed. Network segmentation and access controls should be implemented to limit exposure of systems running Ghostscript, while regular security monitoring and vulnerability scanning should be conducted to identify any potential exploitation attempts. Organizations should also consider implementing sandboxing mechanisms around PDF processing services to contain potential exploitation attempts and prevent lateral movement within compromised systems.