CVE-2023-48231: Vim Use-After-Free Vulnerability

CVE-2023-48231 Overview

CVE-2023-48231 is a Use After Free vulnerability affecting Vim, the popular open source command line text editor. When closing a window, Vim may attempt to access an already freed window structure, leading to potential application crashes. This memory safety issue occurs during window management operations and represents a classic use-after-free condition where dereferenced memory has already been deallocated.

Critical Impact

Exploitation of this vulnerability can cause Vim to crash when closing windows under certain conditions. While exploitation beyond denial of service (application crash) has not been demonstrated as viable, the underlying memory corruption could potentially be leveraged for more severe attacks in specific scenarios.

Affected Products

• Vim versions prior to 9.0.2106
• Fedora 37
• Fedora 38
• Fedora 39

Discovery Timeline

• 2023-11-16 - CVE-2023-48231 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-48231

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption flaw where the application continues to use a pointer after the memory it references has been freed. In Vim's case, the issue manifests in the win_close() function within src/window.c. When a window is being closed, certain autocommand events (specifically EVENT_BUFLEAVE) can be triggered, potentially invalidating the window structure before it's fully processed. The code continued to access the win pointer without verifying that the window remained valid after the autocommand execution.

The vulnerability requires user interaction—specifically, the user must be involved in closing a window that triggers the problematic code path. While the network attack vector indicates the vulnerability could be triggered through crafted files opened remotely, practical exploitation is limited to causing application denial of service through crashes.

Root Cause

The root cause lies in insufficient validation of the window pointer after autocommand execution in the win_close() function. When Vim processes a window close operation, it triggers buffer-related autocommands that can modify or invalidate window structures. Prior to the fix, the code assumed the window pointer remained valid after calling apply_autocmds(), creating a dangerous use-after-free condition when the autocommand invalidated the window.

Attack Vector

An attacker could exploit this vulnerability by crafting a malicious Vim script or configuration that triggers the use-after-free condition during window close operations. The attack requires user interaction to open the malicious content and perform window operations. The most likely attack scenario involves:

1. A specially crafted Vim file or plugin containing autocommands
2. User opens the file or loads the plugin
3. User closes a window, triggering EVENT_BUFLEAVE
4. Autocommand execution invalidates the window structure
5. Vim attempts to access the freed memory, causing a crash

The following patch was applied to src/window.c to address the vulnerability:

	    reset_VIsual_and_resel();	// stop Visual mode

	    other_buffer = TRUE;
+	    if (!win_valid(win))
+		return FAIL;
	    win->w_closing = TRUE;
	    apply_autocmds(EVENT_BUFLEAVE, NULL, NULL, FALSE, curbuf);
	    if (!win_valid(win))

Source: GitHub Vim Commit Details

The fix adds a validity check using win_valid(win) before accessing the window structure, returning FAIL if the window has been invalidated.

Detection Methods for CVE-2023-48231

Indicators of Compromise

• Unexpected Vim crashes during window close operations
• Segmentation fault errors in Vim process logs related to window management
• Core dumps containing references to win_close() or window structure access after free

Detection Strategies

• Monitor system logs for Vim segmentation faults or abnormal termination signals
• Implement application crash monitoring for Vim processes across endpoints
• Review Vim autocommand configurations for suspicious or untrusted plugins

Monitoring Recommendations

• Deploy endpoint detection to identify repeated Vim crashes that may indicate exploitation attempts
• Enable core dump collection to analyze potential exploitation patterns
• Monitor for unusual Vim plugin or script installations from untrusted sources

How to Mitigate CVE-2023-48231

Immediate Actions Required

• Upgrade Vim to version 9.0.2106 or later which contains the security fix
• Review and audit any custom Vim autocommands and plugins for potential abuse vectors
• Consider restricting Vim from opening untrusted files in environments with high security requirements

Patch Information

This vulnerability has been addressed in Vim commit 25aabc2b8ee1e19ced6f4da9d866cf9378fc4c5a, which is included in release version 9.0.2106 and later. The patch adds validation checks to ensure window structures remain valid after autocommand execution. Users should upgrade to the patched version through their package manager or by compiling from source.

• GitHub Vim Commit Details
• GitHub Security Advisory GHSA-8g46-v9ff-c765
• Fedora Package Announcement

Workarounds

• No official workarounds are available for this vulnerability
• Users should prioritize upgrading to the patched version as the primary remediation
• Consider using alternative text editors temporarily if upgrading is not immediately possible

# Upgrade Vim on Fedora systems
sudo dnf update vim

# Verify installed Vim version
vim --version | head -1

# Check for patch 2106 inclusion
vim --version | grep "2106"