CVE-2024-21885: A Detailed Analysis of the X.Org Server Heap Buffer Overflow Vulnerability

A recently discovered security vulnerability, CVE-2024-21885, impacts the X.Org Server, a widely used implementation of the X Window System that provides the foundation for graphical user interfaces on Unix and Linux-based operating systems. This flaw exists within the XISendDeviceHierarchyEvent function and can lead to a heap buffer overflow condition, with significant consequences including a crash of the affected application or even remote code execution in SSH X11 forwarding environments. This post will cover the technical details of the vulnerability, a code snippet, and links to the original references and exploit details.

Description of Vulnerability

The vulnerability exists in the handling of device IDs in the xXIHierarchyInfo struct, which is part of the X Input Extension. When a new device is added to the hierarchy, it is possible for the allocated array length to be exceeded, thereby causing a heap buffer overflow condition. This can result in the corruption of adjacent memory and potentially lead to crashes or remote code execution. The flaw is particularly dangerous when used in conjunction with SSH X11 forwarding, as this allows an attacker to access the X server remotely and exploit the vulnerability.

Here's a simplified example of the vulnerable function in the X.Org server

void XISendDeviceHierarchyEvent(DeviceIntPtr dev, int flags) {
    xXIHierarchyInfo *info;
    int num = ;
    ...
    for (DeviceIntPtr tmp = inputInfo.devices; tmp; tmp = tmp->next) {
        info = &xcalloc(1, sizeof(xXIHierarchyInfo));
        ...
        info[num].deviceid = tmp->id;
        num++;
    }
    ...
    if (flags & XISlaveAdded) {
        // The potential for heap buffer overflow lies here
        info[num].deviceid = new_device_id;
        info[num].num_slaves++;
    }
    ...
}

The problem arises when a new device ID is added to the xXIHierarchyInfo struct without checking if there is enough space allocated. It is possible to exhaust the allocated array length and cause a heap buffer overflow.

Original References

1. X.Org Server: https://www.x.org/wiki/
2. Security Advisory: https://lists.x.org/archives/xorg-announce/2024-January/003078.html
3. X Input Extension: https://www.x.org/releases/X11R7.7/doc/inputproto/XI2proto.txt

Exploit Details

At this point, no public proof-of-concept exploits are available for CVE-2024-21885. However, a skilled attacker could potentially craft a malicious X client that triggers the heap buffer overflow by sending crafted messages through the X protocol. In addition, leveraging the vulnerability in an SSH X11 forwarding scenario might enable remote access and the subsequent execution of arbitrary code.

Possible Mitigations

To mitigate the CVE-2024-21885 vulnerability, the developers of X.Org Server have provided patched versions that properly check for sufficient space in the xXIHierarchyInfo struct before adding new device IDs. Users are advised to update their X.Org Server software to the latest patched version. Meanwhile, in scenarios where the X server is accessed through SSH X11 forwarding, applying additional access controls and network segmentation can help reduce the risk of a successful exploit.

Conclusion

In conclusion, CVE-2024-21885 is a significant vulnerability affecting the X.Org server – a critical component of graphical user interfaces in Unix and Linux systems. The flaw can cause heap buffer overflow conditions, resulting in application crashes or remote code execution. Users should take immediate steps to apply the necessary patches and security controls to protect their systems against potential exploitation.

Timeline

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