Vim is the text editor that powers workflows for countless developers, sysadmins, and Linux users. But like any powerful tool, subtle bugs can sometimes slip in—especially in the dark corners of rarely-used settings. One such issue is CVE-2023-48232, a floating-point exception vulnerability that can crash Vim under specific but plausible circumstances.

This article exclusively explains what’s behind this bug, how it can be exploited, and what every Vim user should know. We’ll walk through the details, show a simplified code snippet of the bug, and explain how to fix it.

What is CVE-2023-48232?

At its core, CVE-2023-48232 is a crash bug—a bug that can cause Vim to terminate unexpectedly if you use certain settings. If your Vim is set up with:

A window border is present

And if a wrapped line continues right at the window border, a certain calculation in Vim’s code does a division by zero. This leads to a floating point exception, which crashes Vim. That’s it: as far as is known, this bug does not allow for running arbitrary code, nor does it leak information—it just bails out with a crash.

If you use window borders

...you might crash Vim when encountering certain wrapped lines.

Typical example in your .vimrc (dangerous!)

set cpo+=n
set so=5      " Smooth scrolling: keep 5 lines visible upwards and downwards

Vim, by default, does not set 'n' in 'cpo', so only users or scripts that add it are vulnerable.

Anatomy of the Bug

The problem occurs when Vim tries to calculate the line offset for display with all of the above conditions in place.

Here’s a simplified C-style pseudocode snippet inspired by the patch. The real vulnerable code lived in Vim’s display logic:

int screen_pos = window_width - line_length;
if (cpo_n_flag && screen_pos == ) {
    int offset = line_length / screen_pos; // Uh-oh! Division by zero if screen_pos == 
}

When line_length is so long it wraps exactly to the border, and cpo contains 'n', screen_pos can become zero.

Exploit Details

The exploit value is limited, as this is a DoS (Denial of Service) bug against a user’s own editor, not a remote or code execution exploit. Here’s how you might crash Vim in a vulnerable configuration:

Open a file with a single line that exceeds your terminal width enough so it wraps at the border.

3. Add/enable a vertical window split (:vsplit) or a border.

Move the cursor to force Vim to wrap the line and draw the border.

Result: Vim segfaults with a floating point exception.

Example file:

ThisIsAVeryVeryVeryVeryLongLineThatWillWrapAtTheWindowBorderIfYouHaveCpoNAndSmoothScrollingEnabled...

Here’s how you can reproduce it (if your older Vim is vulnerable)

:set cpo+=n
:set so=5
:vsplit
# Open a file with an extremely long single line and move cursor over the wrap

Or, using a command-line one-liner

# WARNING: this can crash Vim!
vim -c 'set cpo+=n | set so=5 | vsplit' long_suspect_line.txt

Fix and Mitigation

There are no runtime workarounds for vulnerable setups. The _only_ safe recovery is to upgrade Vim to version 9..2107 or newer.

The fix was introduced in commit cbb99f

> "Prevent floating point exception with non-default 'cpo-n' flag, overlong lines, window borders, and smooth scrolling..."

You can read the full patch and technical reasoning here:
🔗 Vim commit cbb99f

Upgrade Vim!

- Download latest Vim

References

- CVE Record for CVE-2023-48232
- Official Vim Patch (cbb99f)
- Vim Homepage

Summary

CVE-2023-48232 is a crash bug, not a remote code vulnerability, but it shows how even advanced editors can stumble when advanced options interact in unexpected ways. If you use custom cpo flags—especially 'n'—and smooth scrolling with window borders, update your Vim today. No workarounds are available except avoiding the n flag or upgrading. Stay safe and keep your Vim sharp!


Got questions or want more technical details? Check out the GitHub issue tracker or dive into the commit diff.

Timeline

Published on: 11/16/2023 23:15:08 UTC
Last modified on: 01/25/2024 21:37:57 UTC