In early 2025, a dangerous vulnerability shook the graphics stack that powers millions of desktops: CVE-2025-26598. At its core, it’s a simple programming oversight in X.Org and Xwayland that opens the door to memory corruption and potential privilege escalation. Here’s an exclusive deep-dive, written for all readers, technical or not.

What Is CVE-2025-26598?

Researchers discovered that in certain versions of X.Org Server and Xwayland, the function GetBarrierDevice() is supposed to look up a pointer device by its ID. If the device doesn’t exist, you’d expect it to return NULL. But due to faulty code logic, it instead returns the last device in the list, even if no match was found.

This mistake enables an out-of-bounds write—a classic memory safety bug that can be leveraged to hijack the system.

Let’s see how this bug comes up

DeviceIntPtr
GetBarrierDevice(ClientPtr client, XID deviceid)
{
    DeviceIntPtr dev = NULL;
    DeviceIntPtr *devs;
    int num_devs;
    int i;

    devs = get_pointer_devices(&num_devs);
    for (i = ; i < num_devs; i++) {
        if (devs[i]->id == deviceid) {
            dev = devs[i];
            break;
        }
    }
    // Here’s the bug:
    if (dev == NULL && num_devs > )
        dev = devs[num_devs - 1]; // Return the last element, even if not found!

    free(devs);
    return dev;
}

In plain English:
If deviceid is not found, instead of returning NULL (as the rest of the code expects), it returns the last pointer device (which could be invalid or completely unrelated).

How Does This Lead to Out-Of-Bounds Write?

Imagine the code expects a valid device or a NULL. When it gets the wrong device (or uninitialized memory), it can:

Be tricked into executing malicious code (privilege escalation)

An attacker could craft input (for example, using the X11 protocol) that attempts to interact with a non-existent device, triggering GetBarrierDevice() to misbehave.

Suppose a local attacker can send requests to the X server. They could try

# Pseudocode
device_id = 99999  # Non-existent device
# Send malicious xinput or protocol request to the X server using this ID
send_barrier_request(device_id)
# X server will return pointer to devs[num_devs - 1],
# Continuing with operations as if it’s a legit device
# Malicious code might overwrite memory at that device's address

Combine this with knowledge of memory layout, and the attacker might overwrite privileged structures and gain control over the X server process.

Impact

- Affected software: X.Org Server, Xwayland, likely many Linux distributions
- Attack surface: Local users (including unprivileged ones) or sandboxed applications can target the X server process running as root

Official References

- Security advisory from X.Org
- Debian Security Tracker for CVE-2025-26598
- CVE Details entry

The fix is to change

if (dev == NULL && num_devs > )
    dev = devs[num_devs - 1];

to simply

// Just return NULL if not found

Check with your distro to make sure you have the latest X.Org/Xwayland updates!

Final Thoughts

CVE-2025-26598 is a perfect reminder of why never to make assumptions in low-level C code—especially when dealing with memory management and device handling. If you’re running an environment that uses X.Org or Xwayland, update immediately.

Stay safe, keep your packages updated, and watch for future advisories!

Timeline

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