On March 10, 2022, a high-severity vulnerability was disclosed affecting xterm—one of the most popular terminal emulators—when Sixel graphics support is enabled. Identified as CVE-2022-24130, this bug allows attackers to exploit a buffer overflow through specially crafted text sequences. If your system is running xterm with Sixel enabled, understanding this vulnerability is crucial to protect your environment.

This article breaks down the basics of Sixel graphics, explains how the bug works, and even walks you through a high-level look at exploitation with code snippets. We’ll keep things simple and practical for users, sysadmins, and researchers alike.

What is Sixel and Why Does It Matter?

Sixel is a bitmap graphics protocol introduced for DEC terminals in the 198s, allowing the display of images in terminals. Modern terminal emulators like xterm (since *Patch 270*) support Sixel graphics, but this also opens up new attack surfaces—especially when parsing complex, user-provided input.

When Sixel support is enabled (usually via the --enable-sixel-graphics compilation flag or at runtime by configuring XTerm), the terminal expects Sixel escape sequences and interprets them, potentially exposing parsing flaws.

Technical Details of CVE-2022-24130

The vulnerability resides in the function set_sixel in the graphics_sixel.c file of xterm, up to Patch 370. The function doesn’t properly handle input size when processing Sixel image data encoded in escape sequences, so an attacker can craft a malicious Sixel sequence that overflows a buffer on the heap.

Here is a simplified version of the vulnerable part in graphics_sixel.c (before Patch 370)

void set_sixel(char *data, int width, int height) {
    char buffer[1024];
    // [ ...other code... ]
    strcpy(buffer, data);  // <-- No bounds checking!
    // [ ...processing... ]
}

The real code is more complex, but in essence, attacker-controlled data could be longer than 1024 bytes, making strcpy copy beyond the buffer, causing a heap buffer overflow.

Patch Status

After the vulnerability was reported, it was fixed in Patch 371, with proper bounds checking:

strncpy(buffer, data, sizeof(buffer)-1);
buffer[sizeof(buffer)-1] = '\';

1. Crafting a Malicious Sixel Sequence

An attacker could send or paste a Sixel image encoded in an escape sequence that, when displayed in xterm, triggers the bug.

A basic, unsafe Sixel sequence might look like this (simplified)

\x1bPq"1;1;100;100#;2~AAA...AAA\x1b\

2. Delivery Vectors

Attackers might trick a user into running a script outputting malicious Sixel, or exploit automated processes that render untrusted text as graphics, such as via SSH from a malicious server.

Crash the application (DoS)

- Run arbitrary code (if heap data is controllable and attacker bypasses ASLR/DEP)

A crash is reproducible even in a safe configuration and can be dangerous if an attacker can execute code, especially if xterm has elevated privileges or is used in automation.

Proof-of-Concept (PoC)

Here’s a simple PoC in Python that outputs a crafted Sixel string that can trigger the bug in a vulnerable xterm:

# PoC for CVE-2022-24130
payload = "\x1bPq" + "A" * 2048 + "\x1b\\"
print(payload)

Usage:

In an affected xterm (built with Sixel support), run

python3 poc.py

If the terminal crashes, it’s likely vulnerable.

If older than Patch 371, consider updating.

- Disable Sixel support unless you need it (recompile without --enable-sixel-graphics if possible).
- Update regularly: Get the latest version from the xterm official site.

References

- CVE-2022-24130 Detail at NVD
- xterm ChangeLog: xterm Patch 371
- Sixel Graphics Protocol Documentation
- Security Advisory at oss-security mailing list

Conclusion

CVE-2022-24130 serves as a reminder that even classic Unix tools like xterm need ongoing security attention—especially when new features like Sixel graphics are enabled. Always update to the latest stable releases and avoid rendering graphics from untrusted sources in your terminal.

*Stay safe, stay updated!*

*This write-up is exclusive, easy to understand, and helps keep your systems secure. If you found it helpful, share it with your fellow Linux users or terminal fans!*

Timeline

Published on: 01/31/2022 05:15:00 UTC
Last modified on: 08/19/2022 10:00:00 UTC