CVE-2023-4738: Vim Heap Buffer Overflow Vulnerability

CVE-2023-4738 Overview

CVE-2023-4738 is a heap-based buffer overflow vulnerability affecting the Vim text editor prior to version 9.0.1848. This memory corruption flaw exists in the vim_regsub_both() function, which handles regular expression substitution operations. When exploited, an attacker could potentially achieve arbitrary code execution by crafting malicious input that triggers the buffer overflow condition during regex substitution processing.

Critical Impact

Successful exploitation of this heap-based buffer overflow could allow an attacker to execute arbitrary code with the privileges of the user running Vim, potentially leading to full system compromise if the user has elevated privileges.

Affected Products

• Vim versions prior to 9.0.1848
• Apple macOS 14.0

Discovery Timeline

• 2023-09-02 - CVE-2023-4738 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2023-4738

Vulnerability Analysis

The vulnerability resides in Vim's regular expression substitution functionality, specifically within the vim_regsub_both() function. The flaw occurs due to improper bounds checking when copying data during regex replacement operations. When processing certain substitution patterns, the code fails to validate that the destination buffer has sufficient space to accommodate the replacement string, leading to a heap-based buffer overflow condition (CWE-122, CWE-787).

The vulnerability requires local access and user interaction—typically opening a maliciously crafted file or executing a specially crafted command. Despite this requirement, the potential for arbitrary code execution makes this a significant security concern, particularly for users who regularly edit untrusted files or use Vim in automated workflows.

Root Cause

The root cause is insufficient boundary validation in the vim_regsub_both() function within src/regexp.c and related code in src/ex_cmds.c. The vulnerable code path fails to properly verify that the substitution length (sublen) does not exceed the available buffer space before performing memory copy operations. This allows an attacker to overflow the heap buffer by providing input that results in a replacement string larger than the allocated destination buffer.

Attack Vector

The attack requires local access and involves crafting a malicious Vim script, file, or command that triggers a regex substitution operation with specially crafted patterns. When Vim processes this input, the overflow occurs in the heap memory region, potentially allowing the attacker to:

1. Corrupt heap metadata to gain control of program execution
2. Overwrite adjacent heap objects to manipulate program behavior
3. Execute arbitrary code in the context of the Vim process

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

 		mch_memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
 		new_end += copy_len;
 
+		if (new_start_len - copy_len < sublen)
+		    sublen = new_start_len - copy_len - 1;
+
 #ifdef FEAT_EVAL
 		++textlock;
 #endif

Source: GitHub Vim Commit

Additional bounds checking was also added to src/regexp.c:

 	// "flags & REGSUB_COPY" != 0.
 	if (copy)
 	{
-	    if (eval_result[nested] != NULL)
+	    if (eval_result[nested] != NULL &&
+		    STRLEN(eval_result[nested]) < destlen)
 	    {
 		STRCPY(dest, eval_result[nested]);
 		dst += STRLEN(eval_result[nested]);

Source: GitHub Vim Commit

Detection Methods for CVE-2023-4738

Indicators of Compromise

• Unexpected Vim process crashes or core dumps during regex substitution operations
• Vim processes exhibiting abnormal memory consumption patterns
• Suspicious files containing crafted regex patterns or substitution commands
• Evidence of heap corruption in Vim crash logs or memory dumps

Detection Strategies

• Monitor Vim processes for abnormal termination signals (SIGSEGV, SIGABRT)
• Deploy endpoint detection rules to identify malicious file patterns targeting Vim
• Implement file integrity monitoring on Vim configuration files and commonly edited directories
• Use memory sanitizer tools (ASan, MSan) during testing to detect heap overflow attempts

Monitoring Recommendations

• Enable crash reporting and logging for Vim processes across the environment
• Monitor system logs for repeated Vim crashes that may indicate exploitation attempts
• Track Vim version inventory to identify systems running vulnerable versions
• Configure SentinelOne agents to monitor for suspicious Vim process behavior and memory anomalies

How to Mitigate CVE-2023-4738

Immediate Actions Required

• Upgrade Vim to version 9.0.1848 or later immediately
• For macOS users, apply Apple security updates as referenced in Apple Support Article HT213984
• Audit systems to identify all instances of vulnerable Vim installations
• Consider restricting Vim usage on sensitive systems until patching is complete

Patch Information

The vulnerability has been addressed in Vim version 9.0.1848. The fix adds proper bounds checking to ensure that substitution lengths do not exceed available buffer space. The patch is available in the official Vim repository at commit ced2c7394aafdc90fb7845e09b3a3fee23d48cb1.

For detailed patch information, see:

• GitHub Vim Commit
• Huntr Bounty Details
• Debian LTS Announcement

Workarounds

• Avoid opening untrusted files with Vim until patched
• Disable modelines by adding set nomodeline to your .vimrc configuration
• Run Vim with reduced privileges where possible
• Consider using alternative text editors for processing untrusted content

# Configuration example - Add to ~/.vimrc to disable modelines
set nomodeline
set modelines=0