CVE-2024-21886 - A Deep Dive Into the Heap Buffer Overflow Vulnerability in X.Org Server's DisableDevice Function

In this long-read post, we will discuss and analyze a critical vulnerability (CVE-2024-21886) found in the X.Org server's DisableDevice function. A heap buffer overflow, the vulnerability, can lead to an application crash or remote code execution in certain circumstances, specifically in SSH X11 forwarding environments. By exploring how this flaw works, what could be exploited, and taking a look at the code involved, we will develop a deeper understanding of this threat to security and potential mitigations.

Vulnerability Details

The CVE-2024-21886 vulnerability resides in the X.Org Server, which is an essential component of most Unix-based systems used to manage graphical displays. The flaw was discovered in the DisableDevice function, which is responsible for handling device deactivation requests.

This heap buffer overflow vulnerability allows an attacker to send a specific sequence of requests to the X.Org server. These requests can then cause a buffer overflow error, corrupting the heap memory and allowing the attacker to execute arbitrary code remotely.

Links to the original references for the vulnerability can be found in the following

1. The official CVE description: CVE-2024-21886
2. The X.Org security announcement: X.Org Security Advisory

Code Snippet

Here is a simple code snippet illustrating the vulnerable part of the code in the DisableDevice function from the X.Org Server:

void DisableDevice(DeviceIntPtr dev) {
    ...
    for (int i = ; i < dev->valuator->numAxes; i++) {
        if (dev->valuator->axisVal[i] < ) {
            dev->valuator->axisVal[i] = ;
        } else if (dev->valuator->axisVal[i] > MaxVal[i]) {
            dev->valuator->axisVal[i] = MaxVal[i];
        }
    }
    ...
}

In this code snippet, the loop iterates through an array valuator->axisVal, comparing values and setting new ones when necessary. The critical part here is that the loop uses the numAxes value as its boundary limit. If an attacker manages to manipulate this number, they could trigger a buffer overflow situation leading to memory corruption and potentially, remote code execution.

Exploit Details

To trigger this vulnerability, an attacker would typically exploit an X11 session in an SSH X11 forwarding environment. The most common scenario is when an X11 session is running on a client-side device and forwarding its display through an SSH connection to the target server. The attacker can then send specifically crafted malicious requests to the X.Org server via the SSH X11 tunnel.

Once the crafted requests are sent, the buffer overflow takes place, allowing the attacker to overwrite heap memory and gain control of the process's execution flow. This, in turn, allows them to execute arbitrary code remotely on the targeted system.

Mitigation

The maintainers of the X.Org Server have released a patch for this vulnerability (see X.Org Security Advisory). If you run an X.Org server, it is vital to apply the appropriate patch and update your system as soon as possible to protect against potential exploits.

Conclusion

Heap buffer overflow vulnerabilities, such as CVE-2024-21886, are critical to address and patch immediately. They can irreparably damage systems and put sensitive data at risk. By raising awareness and ensuring that security patches are applied promptly, we can make systems more resilient against potential attacks.

Timeline

Published on: 02/28/2024 13:15:08 UTC
Last modified on: 02/28/2024 14:06:45 UTC