In today's post, we will delve into a new vulnerability named CVE-2025-26598. This out-of-bounds write flaw was discovered in the popular display servers, X.Org and Xwayland, which are widely used in Linux distributions. We will analyze the issue in the GetBarrierDevice() function, which causes out-of-bounds memory access and results in unpredictable behavior. Furthermore, we'll share code snippets and links to original references to understand the exploit details.

Introduction

Before we get into the nitty-gritty, let's discuss X.Org and Xwayland briefly. X.Org is an open-source implementation of the X Window System, a graphical windowing system that provides the framework for building graphical user interfaces (GUIs). Xwayland is an X11 compatibility layer that runs on top of the Wayland display server protocol, offering support for X11 applications on Wayland compositor.

The Vulnerability - CVE-2025-26598

The out-of-bounds write flaw was found in the GetBarrierDevice() function, a subroutine responsible for finding the pointer device based on its device ID. This function should ideally return a NULL value if no match is found. However, due to improper error handling, the code returns the last element of the list, which may lead to out-of-bounds memory access.

Code Snippet

Here's a sample code snippet that demonstrates the faulty logic inside the GetBarrierDevice() function:

DeviceIntPtr
GetBarrierDevice(ClientPtr client, XID id)
{
    DeviceIntPtr dev, ret = NULL;
    int i;
    XaceSecurityLevel seclevel;

    for (i = inputInfo.numDevices, dev = inputInfo.devices;
         i > ;
         i--, dev = dev->next)
    {
        if (dev == inputInfo.numDevices)
            break;

        if (dev->id != id)
            continue;

        seclevel = XaceHookDeviceAccess(client, &dev, DixGetAttrAccess);
        if (seclevel == XaceSecurityLevelError)
            continue;

        ret = dev;
        break;
    }

    return ret;
}

As we can see, the code iterates through the list of devices, but it never explicitly sets the ret variable to NULL if no matching device ID is found. This results in the code returning the last element of the list, potentially causing out-of-bounds memory access.

Exploit Details

An attacker could exploit this vulnerability by sending a specifically crafted request that triggers the execution of the GetBarrierDevice() function with an incorrect device ID. This would cause the code to return the last element of the list instead of a NULL value, leading to out-of-bounds memory access and potentially causing the entire system to crash or allow remote code execution.

For a complete understanding of the issue, refer to these official sources that detail the vulnerability:

1. X.Org Server Security Advisory

2. Xwayland Security Advisory

3. National Vulnerability Database (NVD) Entry

Conclusion

CVE-2025-26598 was a critical out-of-bounds write flaw found in the X.Org and Xwayland display servers. It highlights the importance of thorough error handling and boundary checks when working with memory-related operations. Developers and system administrators should apply the necessary patches and updates to mitigate the risk of exploitation by potential attackers.

Timeline

Published on: 02/25/2025 16:15:38 UTC
Last modified on: 03/21/2025 17:51:13 UTC