CVE-2023-5535: Vim Use After Free Vulnerability

CVE-2023-5535 Overview

CVE-2023-5535 is a Use After Free vulnerability affecting Vim, the popular text editor, in versions prior to v9.0.2010. This memory corruption flaw occurs in the buf_contents_changed() function, where autocommands can trigger buffer modifications leading to use-after-free conditions. An attacker who can entice a user to open a specially crafted file may be able to exploit this vulnerability to execute arbitrary code in the context of the affected process.

Critical Impact

This vulnerability allows local attackers to potentially achieve arbitrary code execution by exploiting a use-after-free condition triggered through malicious file handling. Successful exploitation requires user interaction but can result in complete compromise of confidentiality, integrity, and availability.

Affected Products

• Vim versions prior to 9.0.2010
• Fedora 37
• Fedora 38
• Fedora 39

Discovery Timeline

• 2023-10-11 - CVE-2023-5535 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-5535

Vulnerability Analysis

This Use After Free vulnerability exists within Vim's buffer handling code, specifically in the buf_contents_changed() function located in src/buffer.c. The core issue stems from improper handling of autocommands during buffer operations, which can trigger unexpected side effects such as wiping buffers while they are still being referenced.

When Vim processes certain buffer operations, autocommands may execute and modify or free buffer memory that is still in use by the calling function. After the autocommand completes, the original code continues to access memory that has been freed, creating a classic use-after-free scenario that can lead to memory corruption and potential code execution.

The attack requires local access and user interaction, meaning a victim must be convinced to open a maliciously crafted file that triggers the vulnerable code path.

Root Cause

The root cause is the failure to properly block autocommands before performing buffer operations in buf_contents_changed(). Autocommands in Vim are powerful hooks that execute automatically in response to certain events, but they can have "nasty side-effects like wiping buffers" as noted in the patch comments. Without blocking these autocommands, a race condition exists where buffer memory can be freed by an autocommand while still being accessed by the parent function, resulting in use-after-free.

Attack Vector

The attack vector is local, requiring an attacker to craft a malicious file that, when opened in Vim, triggers autocommands during buffer content checking operations. The attacker would need to:

1. Create a file with specially crafted content or associated autocommands
2. Convince a user to open this file in a vulnerable version of Vim
3. Trigger the buf_contents_changed() function with conditions that allow autocommands to free the buffer
4. Leverage the resulting use-after-free condition for code execution or denial of service

// Security patch from src/buffer.c
// Source: https://github.com/vim/vim/commit/41e6f7d6ba67b61d911f9b1d76325cd79224753d

 	return TRUE;
    }

+    // We don't want to trigger autocommands now, they may have nasty
+    // side-effects like wiping buffers
+    block_autocmds();
    if (ml_open(curbuf) == OK
	    && readfile(buf->b_ffname, buf->b_fname,
				  (linenr_T)0, (linenr_T)0, (linenr_T)MAXLNUM,

The patch adds a call to block_autocmds() before performing buffer operations, preventing autocommands from executing during this critical section and eliminating the use-after-free condition.

Detection Methods for CVE-2023-5535

Indicators of Compromise

• Unexpected Vim crashes or segmentation faults when opening files
• Core dumps from Vim processes indicating memory corruption
• Suspicious files with unusual autocommand configurations in user directories
• Evidence of exploitation attempts in system logs showing Vim process anomalies

Detection Strategies

• Monitor for Vim process crashes with memory corruption signatures using system monitoring tools
• Implement file integrity monitoring for Vim configuration files (.vimrc, autoload scripts)
• Deploy endpoint detection rules to identify suspicious file patterns designed to trigger autocommand abuse
• Use memory safety tools (ASan, Valgrind) during development to detect use-after-free conditions

Monitoring Recommendations

• Enable core dump collection and analysis for Vim processes on critical systems
• Monitor for unusual Vim process behavior including unexpected child processes or network connections
• Review Vim configuration files for potentially malicious autocommand definitions
• Track Vim version deployment across infrastructure to ensure patched versions are in use

How to Mitigate CVE-2023-5535

Immediate Actions Required

• Update Vim to version 9.0.2010 or later immediately
• Apply security updates from your Linux distribution's package manager
• Advise users to avoid opening untrusted files in Vim until patched
• Review and audit any custom Vim autocommand configurations for suspicious entries

Patch Information

The vulnerability has been addressed in Vim version 9.0.2010. The fix adds proper autocommand blocking before buffer operations to prevent the use-after-free condition. The security patch is available in the GitHub Vim Commit.

Distribution-specific updates are available:

• Fedora users should apply updates via Fedora Package Announcements

For additional details, see the Huntr Vulnerability Bounty report.

Workarounds

• Disable autocommands when opening untrusted files by launching Vim with vim -u NONE flag
• Use the --noplugin flag to prevent plugin-based autocommands from loading
• Configure Vim to prompt before executing autocommands from untrusted sources
• Consider using alternative text editors for reviewing untrusted files until Vim is updated

# Configuration example
# Update Vim on Fedora-based systems
sudo dnf update vim-enhanced vim-common

# Update Vim on Debian/Ubuntu-based systems
sudo apt update && sudo apt upgrade vim

# Verify installed Vim version
vim --version | head -1
# Should show version 9.0.2010 or higher

# Launch Vim without autocommands for untrusted files (workaround)
vim -u NONE --noplugin untrusted_file.txt