CVE-2022-3352: Vim Use After Free Vulnerability

CVE-2022-3352 Overview

CVE-2022-3352 is a Use After Free vulnerability affecting Vim text editor versions prior to 9.0.0614. This memory corruption flaw occurs when an autocommand (specifically SpellFileMissing) attempts to delete a buffer that is still in use, leading to potential access of freed memory. Successful exploitation requires local access and user interaction, but can result in complete compromise of confidentiality, integrity, and availability on the affected system.

Critical Impact

Exploitation of this Use After Free vulnerability can allow attackers to execute arbitrary code with the privileges of the user running Vim, potentially leading to full system compromise through memory corruption attacks.

Affected Products

• Vim versions prior to 9.0.0614
• Fedora 35, 36, and 37
• Debian Linux 10.0

Discovery Timeline

• 2022-09-29 - CVE-2022-3352 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-3352

Vulnerability Analysis

This Use After Free vulnerability exists in Vim's buffer handling mechanism, specifically related to the SpellFileMissing autocommand functionality. When certain autocommands are triggered during spell checking operations, they can inadvertently delete a buffer that is still being actively referenced by the spell loading logic. Once the buffer is freed, subsequent code continues to access the now-invalid memory location, creating a classic Use After Free condition.

The vulnerability is particularly concerning because autocommands in Vim are powerful automation features that execute arbitrary Vim commands in response to specific events. In this case, the SpellFileMissing autocommand, which fires when a spell file cannot be found, could be crafted or manipulated to delete the current buffer while the spell loading routine is still iterating over it.

Root Cause

The root cause is the lack of proper buffer locking during spell file loading operations. When the spell loading code in src/spell.c iterates through buffers and triggers autocommands, it fails to increment the buffer's lock counter (b_locked), allowing the buffer to be deleted while still in use. This creates a dangling pointer situation where the spell loading code continues to reference the freed buffer memory.

Attack Vector

An attacker with local access could craft a malicious Vim configuration file (.vimrc or modeline) or specially crafted file that triggers the SpellFileMissing autocommand in a way that deletes the current buffer. When a user opens such a file or enables spell checking, the Use After Free condition is triggered, potentially allowing arbitrary code execution through heap manipulation techniques.

The following patch was applied to fix the vulnerability:

// Security patch in src/spell.c - patch 9.0.0614: SpellFileMissing autocmd may delete buffer
    sl.sl_slang = NULL;
    sl.sl_nobreak = FALSE;

+    // Disallow deleting the current buffer.  Autocommands can do weird things
+    // and cause "lang" to be freed.
+    ++curbuf->b_locked;

    // We may retry when no spell file is found for the language, an
    // autocommand may load it then.
    for (round = 1; round <= 2; ++round)

Source: GitHub Commit Update

Additionally, the error handling in src/buffer.c was improved:

     }
    }
    if (!can_unload)
-	semsg(_(e_attempt_to_delete_buffer_that_is_in_use_str), buf->b_fname);
+    {
+	char_u *fname = buf->b_fname != NULL ? buf->b_fname : buf->b_ffname;
+
+	semsg(_(e_attempt_to_delete_buffer_that_is_in_use_str),
+				fname != NULL ? fname : (char_u *)"[No Name]");
+    }
    return can_unload;
}

Source: GitHub Commit Update

Detection Methods for CVE-2022-3352

Indicators of Compromise

• Unexpected Vim crashes during spell checking operations
• Core dumps indicating memory corruption in buffer handling functions
• Suspicious .vimrc files or modelines containing complex autocommand definitions targeting SpellFileMissing events
• Unusual autocommand registrations that attempt to delete buffers during spell operations

Detection Strategies

• Monitor for Vim process crashes with memory corruption signatures, particularly segmentation faults during spell checking
• Audit .vimrc files and Vim configuration directories for suspicious autocommand definitions
• Implement file integrity monitoring on system-wide Vim configuration files
• Deploy endpoint detection solutions capable of identifying memory exploitation attempts in Vim processes

Monitoring Recommendations

• Enable core dump collection and analysis for Vim processes to identify exploitation attempts
• Configure audit logging on user and system Vim configuration files
• Monitor for unusual Vim plugin installations or modifications to spell-related configuration
• Track Vim process behavior for abnormal memory access patterns using endpoint detection tools

How to Mitigate CVE-2022-3352

Immediate Actions Required

• Upgrade Vim to version 9.0.0614 or later immediately
• Review and audit all Vim configuration files, including .vimrc, plugin files, and modelines
• Temporarily disable spell checking functionality if immediate patching is not possible
• Restrict untrusted file access and be cautious when opening files from untrusted sources

Patch Information

The vulnerability has been addressed in Vim version 9.0.0614 through commit ef976323e770315b5fca544efb6b2faa25674d15. The fix implements proper buffer locking by incrementing curbuf->b_locked before entering the spell file loading loop, preventing autocommands from deleting the buffer while it's in use. Distribution-specific patches are available:

• Fedora: Updated packages available for Fedora 35, 36, and 37 via Fedora Package Announcements
• Debian: Security advisory available via Debian LTS Security Advisory
• Gentoo: Update available per Gentoo GLSA 202305-16

Workarounds

• Disable autocommands temporarily by starting Vim with -u NONE or adding set noautocmd to configuration
• Disable spell checking with set nospell if patching is delayed
• Restrict Vim's ability to execute modelines by setting set nomodeline to prevent malicious file-embedded commands
• Run Vim in a sandboxed environment or with reduced privileges when handling untrusted files

# Configuration example - Add to .vimrc to disable potentially dangerous features
# Disable spell checking temporarily
set nospell

# Disable modeline parsing for untrusted files
set nomodeline

# Start Vim without loading any plugins or configuration
# vim -u NONE filename