Vim is one of the world’s most popular text editors, used by developers, sysadmins, and even casual power users across Linux, macOS, and Windows. Despite its reputation for reliability and security, *no software is immune to vulnerabilities.*

In May 2022, a serious flaw was reported: CVE-2022-1619, a heap-based buffer overflow discovered in the cmdline_erase_chars function within the Vim codebase. This flaw affects all Vim versions prior to 8.2.4899 and could potentially be exploited for crashing the app, code execution, or memory corruption. Below, we’ll explain this vulnerability in plain terms, show sample code, and discuss how an attacker might exploit this bug.

What is CVE-2022-1619?

This Common Vulnerabilities and Exposures (CVE) entry covers a coding mistake in Vim’s handling of certain command line actions. Specifically, the vulnerability lies in the cmdline_erase_chars function, which is used internally to erase characters from the command line input area in Vim.

This bug leads to a heap-based buffer overflow — that means the program could write data past the end of dynamically-allocated memory, corrupting adjacent areas of the heap. Such flaws can crash Vim, allow data corruption, or, in a worst-case scenario, let an attacker execute arbitrary code.

How Does the Vulnerability Work?

Inside Vim, when you press backspace or erase characters in the command-line mode, a function called cmdline_erase_chars is triggered. Due to a missing check, if an attacker can make Vim erase more characters than what's present in the buffer, Vim will try to access memory it doesn’t own.

Let’s look at a simplified snippet (based on the Vim source)

static void
cmdline_erase_chars(int num)
{
    if (cmdline_current_len < num)
        num = cmdline_current_len;
    memmove(cmdline_buffer + cmdline_pos - num,
            cmdline_buffer + cmdline_pos,
            cmdline_current_len - cmdline_pos);
    cmdline_current_len -= num;
}

The Issue:
Let’s say, due to a bug elsewhere or crafted input, num becomes larger than cmdline_pos or miscalculated. The function can subtract too much and cause memmove() to write data out of bounds.

How Can This Be Exploited?

- Local exploitation: An attacker could use a malicious Vimscript, modeline, or a crafted file to trigger the code path and crash Vim or even try code execution.
- Remote exploitation: Less likely, but possible if you use Vim to edit a file fetched from the network or through email.

Minimal Proof-of-Concept

Although an actual working exploit is tricky (needs exact heap layout), a basic crash can be triggered by running certain malformed inputs in command line mode. For example, a script that manipulates cmdline_erase_chars directly or indirectly:

" WARNING: Only run this on a safe test machine!
:call feedkeys('qqqqqqqqq' . "\<C-u>" . 'q', 'x')


*(This is conceptual; a real PoC would depend on specific internal Vim state and is typically not public for safety reasons.)*

Real-World Risk

- Crashing Vim: On triggering, Vim can instantly crash, leading to denial-of-service for that session.
- Memory Corruption: Vim’s heap could be corrupted, which is particularly dangerous on systems where Vim runs with higher privileges.
- Remote Code Execution: In rare cases, specially-crafted inputs could allow attackers to execute code in your session — for example, opening a specially crafted file.

How Was It Fixed?

The maintainers of Vim released a patch in 8.2.4899 which adds proper boundary checking before performing memory operations.

Fixed Function (simplified)

if (cmdline_pos < num)
    num = cmdline_pos; // Prevent writing before buffer start
memmove(cmdline_buffer + cmdline_pos - num, ...);

Update Vim: Upgrade to version 8.2.4899 or later.

- On Ubuntu/Debian: sudo apt update && sudo apt install vim

Check Your Plugins: Some plugins or scripts could indirectly trigger this path.

4. Prefer Sandboxed Editing: Use restricted modes like vim -Z or run inside containers when opening suspicious files.

References & Further Reading

- CVE-2022-1619 at NVD
- Original Patch in vim/vim GitHub
- Vim Release Notes (8.2.4899)
- Heap Buffer Overflows Explained (OWASP)

Summary

CVE-2022-1619 is a real danger for anyone using Vim before 8.2.4899, potentially allowing attackers to crash the editor or worse. The fix is quick — just update! If you can’t, be extra cautious with unknown files, and keep your system’s Vim patched regularly to avoid nasty surprises.

Timeline

Published on: 05/08/2022 10:15:00 UTC
Last modified on: 08/26/2022 20:20:00 UTC