In recent times, a new vulnerability was discovered in the widely used libX11 library. This vulnerability, indexed as CVE-2023-43786, has been found to cause an infinite loop within the PutSubImage() function. As a result, a local user can exploit this to consume all available system resources and cause a service denial condition. In this post, we will take a detailed look at this vulnerability, discuss its potential impact on affected systems, and provide guidance on how to mitigate it effectively.

Vulnerability Details

To understand this vulnerability, we first need to have a grasp of the libX11 library and the role it plays. libX11 is a widely used library in Unix-like operating systems that implements the client-side of the X Window System protocol, which provides basic functionality for graphical user interfaces. A key component of this functionality is the PutSubImage() function.

This function is designed to copy a rectangular portion of an image from a source drawable to a destination drawable. In certain circumstances, however, the function can enter an infinite loop. This occurs when specific conditions are met – such as when the width of the source image is larger than that of the destination image.

When an attacker manages to exploit this flaw, they can consume an excessive amount of system resources, causing other processes to fail or become unresponsive. This is a classic denial of service (DoS) situation, which can impact the affected system's availability and functionality.

The problematic infinite loop stems from the section of code responsible for handling the source and destination width mismatch. Below is a snippet of the vulnerable code section within the PutSubImage() function:

while (remaining_width > )
    int portion_width = min(remaining_width, dwidth);
    int nbytes = GetLongs(nbox_list[i] * (nbox_list[i] >  ? portion_width : ), NN);
    if (dwidth < swidth)
        // Infinite loop when dwidth > swidth
        remaining_width -= portion_width;

As the code snippet indicates, if the destination width (dwidth) is greater than the source width (swidth), it will never update the remaining_width variable, leading to an unending loop.

Original References

The bug report detailing this vulnerability was first submitted to the X.Org Foundation's GitLab repository by the user OssFuzz, who identified the issue using the Google OSS-Fuzz project. You can find the original issue report at this link:

Github issue reference

Additionally, the X.Org Foundation has released an advisory that provides comprehensive information on CVE-2023-43786. You can read it here:

X.Org Foundation Advisory

Exploit Details

For an adversary to exploit this vulnerability, they would be required to create a malicious X client program that makes a specific Xlib call to the affected PutSubImage() function. The attacker must then trick a user with an active X session to execute the malicious client on their system. Once the client runs, it can potentially cause a DoS condition by consuming all available system resources.


The X.Org Foundation, in response to the issue, has released a patched version of the libX11 library (1.7.4), which resolves the vulnerability. To protect your system from potential exploitation of CVE-2023-43786, it is crucial to update your libX11 library to version 1.7.4 or later.


The CVE-2023-43786 vulnerability, caused by an infinite loop issue within the PutSubImage() function of libX11, can have serious implications for affected systems. With the potential to cause a DoS condition, it endangers the availability and performance of the compromised machine. By understanding the vulnerability, how it can be exploited, and the steps required to mitigate it, we can effectively protect our systems from CVE-2023-43786 and maintain their security.


Published on: 10/10/2023 13:15:22 UTC
Last modified on: 11/07/2023 04:21:29 UTC