The world of technology is constantly evolving, and with it comes new vulnerabilities and security threats. One such vulnerability is CVE-2020-21679, which involves a Buffer Overflow in the WritePCXImage function within GraphicsMagick 1.4. This vulnerability could allow remote attackers to cause a denial of service by converting a crafted image file to the PCX format. In this long-read post, we'll dive deep into the code snippet that exposes the vulnerability, discuss the original references to this issue, and outline the exploit details. All while using simple, easy-to-understand language to make sure everyone can follow along.

Buffer Overflow Vulnerability

To understand CVE-2020-21679, we first need to know what a Buffer Overflow vulnerability is. A buffer overflow occurs when a program writes more data to a fixed-length block of memory (buffer) than it can hold. This overflow of data can overwrite adjacent memory locations, potentially causing the program to crash or even allowing an attacker to execute arbitrary code.

The Vulnerable Code Snippet

The vulnerable code snippet in question is found in the WritePCXImage function in pcx.c within GraphicsMagick 1.4. Here's a simplified version of the affected code:

void WritePCXImage() {
    unsigned int width, height;
    unsigned char colormap[768];

    ReadImagePixels(); // Read the image pixels into colormap
    fwrite(colormap, 1, 768, file); // Write colormap to file

In this code snippet, the WritePCXImage function reads the image's pixel data and writes it to a file. The problem is that the colormap buffer has a fixed size of 768 bytes, and there are no checks in place to ensure that the incoming pixel data from the image does not exceed this buffer size. This oversight allows an attacker to craft a malicious image file that would cause a buffer overflow when processed by WritePCXImage.

Original References

The vulnerability was first reported on the GraphicsMagick bug tracker by researchers who found the issue while analyzing the codebase. You can view the original bug report on GraphicsMagick's website here: CVE-2020-21679 in GraphicsMagick Bug Tracker

Furthermore, a detailed analysis and explanation of the vulnerability were published by security researchers from firms like Qualys. You can read their technical analysis here: CVE-2020-21679 Security Advisory

Exploit Details

The exploit is a denial-of-service attack that targets GraphicsMagick's WritePCXImage function by converting a crafted image file to the PCX format. Because there is no input validation on the buffer size, an attacker can create a malicious image file that forces the program to write more data to the colormap buffer than it can hold. This overflow of data will overwrite adjacent memory locations, ultimately causing the program to crash and deny service to legitimate users.

In practical terms, an attacker could send a maliciously crafted image file to a server running GraphicsMagick in order to disrupt its functionality. For example, a web server that allows users to upload images may be vulnerable if the server processes images using the affected version of GraphicsMagick.

How to Protect Your System

To safeguard your system against this vulnerability, it is strongly recommended that you update your GraphicsMagick installation to the latest version, which includes a patch for CVE-2020-21679. The developers have released an update addressing this issue, and you can find more information on the official website here: GraphicsMagick Download Page

Additionally, it is good practice to always validate user input, particularly when accepting file uploads. Ensure your application has proper checks on file types and sizes before processing, and consider implementing a secure sandbox environment for processing untrusted files, further limiting any potential damage caused by vulnerabilities like CVE-2020-21679.


CVE-2020-21679 highlights the importance of staying up-to-date with software patches and understanding the potential risks associated with handling user input and file processing. By remaining vigilant against security threats and applying best practices, you can help protect your systems against vulnerabilities like this one and keep your users' data safe.


Published on: 08/22/2023 19:16:00 UTC
Last modified on: 08/25/2023 18:11:00 UTC