CVE-2025-26595 - Critical Buffer Overflow in X.Org/XWayland XkbVModMaskText Function

In early 2025, a serious security issue was found in X.Org and XWayland—two important components at the heart of many Linux and Unix graphical environments. This vulnerability, logged as CVE-2025-26595, stems from a classic programming mistake: a buffer overflow in the function XkbVModMaskText(). Anyone exploiting this bug could potentially run malicious code with the permissions of the X server, which almost always means getting complete control of your desktop session.

This write-up explains the bug, showcases relevant code, links to additional resources, and demonstrates the real-world risk with an exploit description. All details are written in a clear and simple style.

What: An unchecked buffer overflow vulnerability

- How: The code copies data into a stack-allocated buffer without checking if there’s enough space

At Risk: Most Linux distributions running X.Org and Wayland as their graphical server backend

The Problem

The risky function, XkbVModMaskText(), is responsible for collecting and formatting X keyboard (XKB) virtual modifier info (think keyboard layout stuff) into a readable string. To do this, it declares a fixed-length buffer on the stack (meaning it is NOT dynamically resizable). Then, it proceeds to copy modifier names into this buffer—without checking if everything fits.

The Vulnerable Code

Here’s a simplified code sample to show how the vulnerability occurs. This is adapted from the original code seen in X server repositories:

#define BUF_SIZE 100

int XkbVModMaskText(unsigned int mask, char *buf, XkbDescPtr xkb) {
    char temp[BUF_SIZE]; // Fixed-size stack buffer
    int n = ;

    // Loops through all virtual modifier names
    for (int i = ; i < XkbNumVirtualMods; i++) {
        if (mask & (1 << i)) {
            // Directly copies name to buffer, regardless of total length!
            strcpy(temp + n, xkb->names->vmods[i]);
            n += strlen(xkb->names->vmods[i]); // advances n without checks
        }
    }

    // ... more code ...
    strcpy(buf, temp); // copies temp out
    return 1;
}

What's wrong?
If enough “virtual modifier” names exist, their total length might surpass 100 bytes (the buffer size). This excess will overwrite nearby memory—classic stack buffer overflow.

Original References

- X.Org Security Advisory (XSA-2025-01)
- Official X.Org Patch Pull Request
- NVD Entry for CVE-2025-26595
- Ubuntu Security Notice

How an Attacker Could Use This

If an attacker can control or influence the names of virtual modifiers (for example, via a crafted keyboard layout file or through interaction with an X application), they could cause the buffer overflow to:

Overwrite the stack with chosen data, potentially controlling the instruction pointer

- Trigger execution of attacker-controlled code, giving them access to the X server or even root privileges (if the X server runs as root or with special capabilities)

Simple Exploit Scenario

Suppose an attacker supplies modifier names, each being very long (say, 50+ bytes), and enough of them so their total exceeds the 100-byte buffer. For controlled conditions, they insert shellcode (special binary instructions) into the modifier data, causing the server to crash or execute their code.

Pseudo-exploit code to trigger overflow

1. Craft a keyboard map with 10 virtual modifiers, each named "AAAAAAAAAA... (repeats, 50 times)".
2. Load keyboard map into X.Org or XWayland via xkbcomp or xmodmap.
3. Call a function (locally or via a malicious X client) that triggers XkbVModMaskText().
4. Observe buffer overflow, leading to a server crash or code execution.

Upstream patch example

// Before:
strcpy(temp + n, xkb->names->vmods[i]);

// After:
strncat(temp + n, xkb->names->vmods[i], BUF_SIZE - n - 1);

// Or, better:
if (n + strlen(xkb->names->vmods[i]) < BUF_SIZE) {
    strcpy(temp + n, xkb->names->vmods[i]);
    n += strlen(xkb->names->vmods[i]);
}

Restrict who can connect to your X session.

- Check vendor/security advisories for more details.

Conclusion

CVE-2025-26595 is a high-severity stack buffer overflow flaw in X.Org and XWayland, potentially giving attackers a way to crash or take over graphical environments in Linux and Unix desktops. It is crucial for both users and administrators to apply updates quickly, especially on multi-user systems or publicly accessible infrastructure.

Stay safe: keep your system up to date and monitor official advisories.

Timeline

Published on: 02/25/2025 16:15:38 UTC
Last modified on: 03/21/2025 17:50:22 UTC