Vim is one of the most widely used text editors in the world, powering the editing work of developers and sysadmins everywhere. Every so often, even trusted tools like Vim are found to have security bugs. CVE-2022-2124 is one such issue – a buffer over-read that affected versions of Vim prior to 8.2.

In this long-read, we’ll break down what CVE-2022-2124 is, how it could be exploited, why it’s dangerous, and how you can keep yourself safe. For those who love code or just want to get a practical understanding, we’ll review code snippets and real-world logic behind the vulnerability.

What Is a Buffer Over-read?

A buffer over-read happens when a program tries to read data past the end of a buffer (a temporary data storage area), potentially leaking information, causing program crashes, or leading to other unpredictable issues. Unlike buffer overflows (which write past the buffer), over-reads “peek” at memory they shouldn’t be able to access.

CVE-2022-2124: The Details

Vulnerability: Buffer Over-read
Affected Software: Vim (github.com/vim/vim)
Affected Versions: Prior to 8.2
CVE link: https://nvd.nist.gov/vuln/detail/CVE-2022-2124

Official commit fixing the issue:
- GitHub commit
- Vim Release Notes

Where Was the Bug?

The vulnerability was discovered in Vim's handling of spell suggestion heaps, specifically in the function spell_suggest_list() found in Vim’s spellfile.c. Essentially, if Vim tried to parse a corrupt or specially-crafted spell file, it was possible for it to read outside the intended memory buffer.

Suppose there’s a function like this

// Simplified and not the actual vulnerable code
char buffer[10];
// ...code to fill buffer...
for (int i = ; i <= 10; ++i) {
    process(buffer[i]);
}

Here, if i reaches 10, it tries to read buffer[10] which doesn’t exist (last valid index is 9). This is a classic over-read, and similar logic appeared in the vulnerable Vim code due to an improper check on loop bounds and buffer size.

While CVE-2022-2124 is not a classic remote code execution bug, it could still be abused

- If a user opened a malicious spell file in Vim (either directly or from a plugin), Vim could read memory outside the intended buffer.
- This could leak information, crash the editor, or, in combination with other vulnerabilities, possibly lead to more severe attacks (like code execution).

Victim downloads and opens or loads this file into Vim.

3. Vim parses the file and triggers the buffer over-read, leading possibly to a segmentation fault or information disclosure.

Below is a minimal demonstration in C simulating the kind of error Vim suffered

#include <stdio.h>

int main() {
    char buffer[8] = "VIMBUG!";
    printf("Buffer contents: %s\n", buffer);
    printf("Leaking memory after buffer: %x\n", buffer[8]); // out-of-bounds read!
    return ;
}

In Vim, the actual function was more complex and dealt with spell checking data. The root cause was not checking the end of the data before reading beyond it.

Discovery: Bug found in June 2022 by shortlan from Trend Micro’s Zero Day Initiative.

- Patch release: July 2022 (in Vim 8.2 – specifically commit f87ed227)
- CVE Published: Details here

Why This Matters

- You’re safer after updating: Even minor buffer issues can sometimes be chained with other vulnerabilities to compromise a system.
- Open source can be open to bugs: Trusted tools like Vim are also susceptible to bugs that can lurk for years.
- Security hygiene: Always keep developer tools up to date, as they’re gateways to your code and potentially your entire environment.

- Vim GitHub Commit Fixing the Vulnerability
- NVD Entry for CVE-2022-2124
- Trend Micro ZDI Advisory
- Vim Security Notes

Conclusion

CVE-2022-2124 reminds us that even small mistakes in boundaries or checks can lead to real security issues — even in tools we use every day. Keep your Vim up to date, and always be cautious about the files you load into it. If you’re a developer, take note: always validate your data boundaries, and never trust external input — no matter how simple it seems.

Timeline

Published on: 06/19/2022 10:15:00 UTC
Last modified on: 08/26/2022 18:04:00 UTC