Vim is one of the most popular text editors among developers, system admins, and power users. But like any piece of software, it can have vulnerabilities. One of these was CVE-2022-1927: a _buffer over-read_ problem found in the GitHub repository vim/vim before version 8.2. In this post, let’s understand this vulnerability, how it works, see a code snippet, and discuss the exploit.

What is a Buffer Over-read?

A _buffer over-read_ happens when software reads more data from a buffer (a chunk of memory) than what actually exists. Imagine you are reading a line from a book, but you keep going past the end of the page into blank paper. This can cause all sorts of problems—crashes, strange outputs, or even security leaks. In the case of Vim, if a crafted file is opened, this issue can be triggered.

CVE-2022-1927 Overview

CVE-2022-1927 affects Vim versions before 8.2. It allows local attackers to crash Vim, or possibly read sensitive memory data, by convincing the user to open a specially crafted file. The issue was discovered and responsibly reported on GitHub, and fixed in later releases.

Severity: Medium (DoS & Information Disclosure)

Scope: Local (user must open a file)

Reference Link:
GitHub Security Advisory GHSA-86x4-xq55-v7v2
Official CVE Details

Understanding the Vulnerable Code

Vim parses window resizing commands (:resize), where the bug lurks. Let’s look at a (simplified and rephrased) pseudo-code to explain how the problem happens:

// Vulnerable Function
void do_window_resize(char *cmd) {
    char buf[21];
    int len = strlen(cmd);

    // Vulnerable! If cmd longer than 20 chars, we read outside buf
    for (int i = ; i < len; i++) {
        buf[i] = cmd[i];  // No bounds check!
    }

    // ...process buf here...
}

In the real source, this function failed to properly check the input length before copying it into buf, which is 20 bytes, but if cmd is longer, it writes/reads out of bounds. While the bug is subtle, a crafted command or file can trigger it and cause Vim to misbehave.

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With affected Vim versions (pre-8.2), this can result in a crash, strange resizing, or reading unrelated memory. In some cases, memory contents may show up in the status bar—info disclosure!

The Patch

The fix is simple: properly check the length of the variable before processing it.

// Patch: Add bounds checking!
if (len > 20) len = 20;
for (int i = ; i < len; i++) {
    buf[i] = cmd[i];
}
buf[len] = '\';  // Null-terminate for safety

Reference:
Upstream Fix Commit

Who is affected?

- Anyone using Vim older than 8.2 and running unknown or untrusted scripts/files

Upgrade to Vim 8.2 or later – patches included

- Use trusted files/scripts only

Conclusion

_CVE-2022-1927_ is a classic example of why input validation and buffer size awareness are crucial in C programming, especially in a widely-used tool like Vim.

Advice: Always keep your editors up to date, and avoid opening untrusted files. For developers, always check buffer sizes before copying or reading data.

Further Reading

- GHSA-86x4-xq55-v7v2
- Vim Release Notes
- CWE-126: Buffer Over-read

Timeline

Published on: 05/29/2022 14:15:00 UTC
Last modified on: 08/26/2022 18:52:00 UTC