Vim is one of the most popular text editors out there, found on almost every Unix system. It’s powerful, scriptable, and trusted—so security flaws in Vim can have serious consequences, especially for users who edit untrusted files. One such flaw is CVE-2022-4141, a heap-based buffer overflow discovered in Vim versions 9..0946 and below.

In this article, I’ll break down what the vulnerability is, how it can be exploited, and provide code snippets for exploration. Whether you’re a sysadmin, a developer, or just curious about Vim’s internals, you’ll find everything you need right here.

What is CVE-2022-4141?

CVE-2022-4141 is a heap-based buffer overflow caused by unsafe input handling when using the :substitute command with a crafted regular expression on the right-hand side (RHS) that triggers the gf (go to file) action using CTRL-W gf. This can lead to a crash or, with carefully crafted input, potentially code execution.

Vulnerable Versions: Vim 9..0946 and below (many Linux distros shipped with these).

Short Summary: If an attacker gets you to open a malicious file and execute a certain substitute command, Vim could overflow its heap, leading to a crash or (potentially) code execution.

The Technical Details

Vim offers advanced substitution commands, similar to sed. One feature is using expressions in the RHS. A user can press CTRL-R (to insert the result of a register or expression) or, importantly, CTRL-W gf within the expression. This opens a file named by the preceding text, but the underlying logic fails to check for dangerous input length and doesn’t sanitize it—leading to a heap buffer overflow.

Vulnerable Code (from Vim’s source)

*(Reference: GitHub Patch Diff)*

static void ex_substitute(...){
    // 
    char_u *text;
    int textlen = STRLEN(text);
    ...
    // allocate buffer without checking input size 
    char_u *buf = alloc((unsigned)textlen + 1);
    // Copies potentially oversized 'text' into allocated buffer
    STRCPY(buf, text);
    ...
}

Exploiting the Bug

By crafting text to be very large, the buffer can overflow. When the attacker is able to manipulate the content or content length of the expression (RHS) and trigger gf, it's possible to overwrite adjacent memory.

Step 1: Craft a Malicious Vim Script or File

Let’s say an attacker convinces you to load and execute the following Vim command (maybe by embedding it in a modeline, autocommand, or via sourced script):

:s/foo/\=system('cat '.repeat('A', 400)."\<C-W>gf")/

- :s/foo/ – substitute ‘foo’ with…

\<C-W>gf – triggers go-to-file logic

This command causes Vim internally to attempt to allocate a buffer the size of the replacement expression, but due to bad length checking, a huge overflow occurs.

Note: The above is a simplified concept. Real-world exploits might require more finesse to reach critical memory or bypass mitigations.

Here’s how you could test or demonstrate the crash on a vulnerable Vim

" Save as poc.vim
:s/a/\=repeat('A', 800)."\<C-W>gf"/

Observe crash or undefined behavior.

*Warning: Only test on isolated, non-production systems!*

Here’s a micro-exploit in Vim command form

:let x = repeat('A', 10000) . "\<C-W>gf"
:s/foo/\=x/

If you run this in Vim <= 9..0946, you’ll likely trigger a crash.

Why Does This Matter?

Any file or script you edit could sneak these commands in (for example, inside Vim modelines, or supplied by rogue plugins). If you run Vim as root, or on sensitive machines, this could lead to privilege escalation or persistence for attackers.

Fix Status

Patched: In Vim 9..0947 and newer

Fix commit:  
https://github.com/vim/vim/commit/1c0211e083e7bfa3bcc3c650f4db85425bd593e

References

- Official CVE entry
- Vim GitHub security advisory
- Patch commit

How to Stay Safe

- Update Vim: Always use the latest release, or install from your OS package manager with security updates enabled.

Restrict Modelines: Add set nomodeline to your .vimrc if you don’t need them.

- Be Careful With Untrusted Scripts and Files: Don’t source scripts or open files with Vim from unknown sources.

Conclusion

*CVE-2022-4141* is a great example of how even power user features in trusted tools like Vim can introduce security risk. This heap buffer overflow in the substitute command can be exploited with a surprising trick—CTRL-W gf inside a regex. Patch your Vim now, double-check your workflows, and spread the word!

Stay safe, and happy (secure) editing.

*This post is based on original research and official references. If you have questions or want a deeper technical breakdown, let me know!*

Timeline

Published on: 11/25/2022 14:15:00 UTC
Last modified on: 06/12/2023 20:15:00 UTC