CVE-2023-4781 - Heap-Based Buffer Overflow in Vim (Before 9..1873)

In September 2023, a serious vulnerability was disclosed in Vim, the popular text editor. This vulnerability, CVE-2023-4781, is a heap-based buffer overflow that affects versions prior to 9..1873. Buffer overflows in tools like Vim are dangerous because they can let an attacker execute their own code. Let's break down what this means, how it works, how it can be exploited, and—most importantly—how to keep yourself safe.

What Is a Heap Buffer Overflow?

Before jumping in, let's quickly explain: a heap buffer overflow happens when a program writes more data to a chunk of memory (the "heap") than it was allocated, potentially overwriting critical data, function pointers, or causing the program to crash or misbehave. This kind of flaw could be used by an attacker to execute arbitrary code.

Vulnerability: Heap-based Buffer Overflow

- Affected Software: Vim (before version 9..1873)
- CVE ID: CVE-2023-4781

Impact: Possibly arbitrary code execution

- Patch: Released in vim/vim commit

The Original Reference

The main references for CVE-2023-4781 are:
- GitHub Security Advisory GHSA-6gjj-v6c3-68w2
- NVD Entry for CVE-2023-4781
- Upstream fix in vim/vim PR #12689

How the Vulnerability Works

This vulnerability lies in how vim handles certain "registers." Vim has a feature called "registers" which are used to store and recall multiple text snippets. When retrieving a register with a specially crafted large name, vim did not check if the memory it was copying the name into was large enough to hold it—resulting in heap overflow.

The vulnerable function was something like this (simplified C code)

// Simplified, buggy function in register.c
static char_u *get_register_name(char_u *regname)
{
    char_u buf[100]; // Only 100 bytes allocated

    // Bad practice: copying the whole source without size check
    strcpy(buf, regname);
    // proceed to use buf...
}

Problem: If regname is longer than 100 bytes, this will smash data past the end of buf.

Let's imagine an attacker sends a special crafted input in a Vim session

1. They create a text file containing a line that invokes a Vim command with a register name of extreme length (more than 100 characters).

The attacker tricks a victim into opening this file in vulnerable Vim (prior to 9..1873).

3. When Vim tries to access this register, it copies the register name into the fixed-size buffer, causing an overflow.

A proof-of-concept input could look like this inside a Vim script file

" Create a large register name to trigger the overflow
let @{" . repeat("A", 200) . "} = 'exploit'
" Force access to the register
echo @{repeat("A", 200)}

If run, this would crash Vim, or (with more work) allow executing injected code, depending on the exploit development (note this is a hypothetical POC for illustration).

The effectiveness of an actual code execution exploit would depend on memory layout, protections like ASLR, and system configuration.

Real-World Impact

This is a local vulnerability: an attacker would need to get you to open a malicious file in Vim (for example, over email, or in a repository). Anyone using Vim to view untrusted files is at risk if they haven't updated!

How Was It Fixed?

The fuzzing report and issue were filed on GitHub, and a patch was quickly applied. The devs added proper bounds checking:

// Fixed code in register.c
if (STRLEN(regname) >= sizeof(buf))
    return NULL;
STRCPY(buf, regname);

This means that the function will reject oversized register names, stopping the buffer overflow.

macOS: If using Homebrew: brew upgrade vim

- Windows: Download the latest release from Vim.org

Summary

- CVE-2023-4781 allows attackers to crash or possibly run code via a heap buffer overflow in Vim versions before 9..1873.

References

- Vim Security Advisory GHSA-6gjj-v6c3-68w2
- Upstream fix commit 353c8a4
- CVE-2023-4781 entry at NVD

Timeline

Published on: 09/05/2023 19:15:00 UTC
Last modified on: 09/29/2023 17:15:00 UTC