Vim is one of the most popular text editors in the world, trusted by millions of developers for decades. However, even the best software is not immune to critical bugs. In 2022, a vulnerability—tracked as CVE-2022-1898—was discovered in the Vim editor, specifically a Use-After-Free bug affecting versions before 8.2. This bug could allow attackers to crash Vim or even execute arbitrary code.

In this post, we’ll break down what the vulnerability is, how it happens, and how it could be exploited in a real-world scenario—with easy code explanations. If you use Vim, especially from the command line or on remote servers, keep reading!

What Is a Use-After-Free (UAF) Bug?

A *Use-After-Free* bug happens when an application continues to use memory after it's already been released (freed). This can lead to unpredictable behavior—sometimes letting attackers control what the program does, potentially running malicious code.

Vulnerability Name: CVE-2022-1898

- Affected Software: Vim (vim/vim GitHub repo)

Affected Versions: Up to, but not including 8.2

- Discovered By: Mitre Details Here
- Upstream Issue: Vim GitHub Issue #10549
- Patch: Vim Pull Request #10550

Where Did the Bug Happen?

The bug lived in the part of Vim that handled *modelines*. Modelines are a legacy Vim feature that lets people add configuration at the top or bottom of files, like this:

# vim: set ts=4 sw=4 :

Attackers discovered that with specially-crafted modelines, they could trick Vim into using memory after it was freed.

The Core Code Mistake

Let’s see a simplified version of what happened in the code (based on the original patch):

char_u *line = ml_get_buf(buf, lnum, FALSE);
...
if (some_condition) {
    vim_free(line);
    ...
    func_that_uses(line); // <- Unsafe! 'line' was just freed!
}

Potentially trick Vim into executing malicious code

All the attacker needs to do is trick you into opening a file with a "bad" modeline.

To trigger this bug, an attacker could use a file like

vi: set modeline modelines=1 tabstop=999999999999 :

How it works

- The above line, when parsed by Vim, triggers the vulnerable path in the code, potentially causing Vim to access freed memory.

Real-world Impact

- While just crashing Vim is annoying, a motivated attacker might arrange things in memory to take control of the process.

Original References

- NVD Entry for CVE-2022-1898
- GitHub Security Advisory
- Vim Patch Details

set nomodeline

`
- Never open files from untrusted sources, or double-check modeline usage.

---

## TL;DR (Too Long; Didn’t Read)

- *CVE-2022-1898* is a use-after-free bug in Vim affecting versions before 8.2.
- Opening a booby-trapped file with a crafted modeline could let an attacker exploit your system.
- The fix is simple: update Vim and disable modelines if you don’t use them.

---

## Summary Table

| Key           | Info                             |
|---------------|----------------------------------|
| Vulnerability | CVE-2022-1898                    |
| Impact        | Use-After-Free, Possible RCE     |
| Affected      | vim/vim < 8.2                    |
| Fixed in      | Vim 8.2+                         |
| How to fix    | Update Vim, disable modeline     |
| References    | NVD, Patch |

---

## Final Thoughts

This bug shows even mature, trusted open-source projects like Vim can have serious security issues. Make sure you're always updated—and be wary of obscure legacy features like modelines! For security, consider disabling them altogether.

If you manage systems where Vim is pre-installed (like servers), check their version and update as soon as possible. Happy (and safe) editing!

---

*Did you find this guide exclusive and helpful? Consider sharing to help others stay safe! For further reading, dig into the GitHub advisory.*

Timeline

Published on: 05/27/2022 09:15:00 UTC
Last modified on: 08/21/2022 06:15:00 UTC