CVE-2023-4735: Vim Buffer Overflow Vulnerability

CVE-2023-4735 Overview

CVE-2023-4735 is an out-of-bounds write vulnerability affecting the Vim text editor in versions prior to 9.0.1847. The vulnerability exists in the do_addsub() function within src/ops.c, where insufficient bounds checking during binary number conversion can lead to memory corruption. Successful exploitation requires user interaction, such as opening a maliciously crafted file, but could result in arbitrary code execution with the privileges of the Vim process.

Critical Impact

This out-of-bounds write vulnerability could allow attackers to achieve code execution or cause application crashes through malicious input, affecting both Vim and Apple macOS systems that include the vulnerable Vim component.

Affected Products

• Vim versions prior to 9.0.1847
• Apple macOS 14.1

Discovery Timeline

• 2023-09-02 - CVE-2023-4735 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-4735

Vulnerability Analysis

The vulnerability resides in Vim's do_addsub() function, which handles arithmetic operations on numbers within text buffers. Specifically, when converting numbers to binary representation, the code iterates through bits without properly validating the output buffer boundaries. The loop that writes binary digits ('0' or '1') to buf2 lacks an upper bound check against the buffer size (NUMBUFLEN), allowing writes beyond the allocated buffer space when processing specially crafted input values.

This out-of-bounds write (CWE-787) can corrupt adjacent memory regions, potentially overwriting critical data structures or function pointers. An attacker who can control the input processed by this function could leverage this memory corruption to achieve arbitrary code execution within the context of the Vim process.

Root Cause

The root cause is a missing boundary condition check in the binary number conversion loop within do_addsub(). The original code did not validate that the write index i remained within the NUMBUFLEN buffer bounds before writing binary digit characters. This oversight allows the loop to continue writing past the end of buf2 when processing numbers with large binary representations.

Attack Vector

The attack vector requires local access and user interaction. An attacker must convince a user to open a specially crafted file in Vim that triggers the vulnerable code path. The malicious file would contain number values designed to cause the binary conversion loop to exceed the output buffer boundaries. While user interaction is required, the attack does not require authentication or elevated privileges.

// Vulnerable code before patch (src/ops.c)
for (i = 0; bit > 0; bit--)
    buf2[i++] = ((n >> (bit - 1)) & 0x1) ? '1' : '0';

// Patched code with bounds checking
for (i = 0; bit > 0 && i < (NUMBUFLEN - 1); bit--)
    buf2[i++] = ((n >> (bit - 1)) & 0x1) ? '1' : '0';

Source: GitHub Vim Commit

Detection Methods for CVE-2023-4735

Indicators of Compromise

• Unexpected Vim crashes or segmentation faults when opening specific files
• Anomalous memory access patterns in Vim processes detected by memory protection tools
• Suspicious files containing unusual numeric sequences designed to trigger arithmetic operations

Detection Strategies

• Monitor for abnormal Vim process behavior including unexpected crashes or memory access violations
• Deploy endpoint detection rules to identify exploitation attempts targeting memory corruption in text editors
• Implement file integrity monitoring to detect potentially malicious files before user interaction

Monitoring Recommendations

• Enable core dump collection for Vim processes to capture crash artifacts for forensic analysis
• Configure application-level logging to track file operations and identify suspicious input files
• Use SentinelOne's behavioral AI to detect post-exploitation activities following memory corruption

How to Mitigate CVE-2023-4735

Immediate Actions Required

• Update Vim to version 9.0.1847 or later immediately
• For macOS users, apply the security updates referenced in Apple Support Article HT213984
• Review systems for potentially malicious files that may have been used in exploitation attempts

Patch Information

The vulnerability has been addressed in Vim version 9.0.1847. The fix adds a boundary check (i < (NUMBUFLEN - 1)) to the binary conversion loop in do_addsub(), preventing writes beyond the allocated buffer. The patch is available via the official Vim GitHub repository. Apple has also addressed this vulnerability in their macOS updates as documented in the Huntr Bounty Report.

Workarounds

• Avoid opening untrusted files in Vim until the patch is applied
• Use alternative text editors for handling files from untrusted sources
• Consider running Vim in sandboxed or restricted environments to limit potential impact

# Verify Vim version to confirm patched status
vim --version | head -1
# Expected: VIM - Vi IMproved 9.0 (patch 1847 or higher)

# Update Vim via package manager (example for apt-based systems)
sudo apt update && sudo apt install vim