CVE-2021-4193: Vim Out-of-bounds Read Vulnerability

CVE-2021-4193 Overview

CVE-2021-4193 is an out-of-bounds read vulnerability affecting Vim, the popular text editor. This vulnerability allows an attacker to potentially read memory beyond allocated buffer boundaries when processing specially crafted input. The vulnerability requires local access and user interaction to exploit, as a victim must open a malicious file in Vim.

Critical Impact

Successful exploitation could lead to disclosure of sensitive information from process memory, potentially exposing credentials, encryption keys, or other confidential data residing in adjacent memory locations.

Affected Products

• Vim (all versions prior to patch 8.2.3950)
• Fedora 34
• Debian Linux 9.0 and 10.0
• Apple macOS (multiple versions including macOS 10.15.7 with various security updates)
• Apple macOS 11.x (Big Sur) and 12.x (Monterey)

Discovery Timeline

• 2021-12-31 - CVE-2021-4193 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-4193

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-bounds Read). The flaw exists in how Vim handles position calculations when processing visual mode search patterns using /\%V. When the cursor position references a column beyond the actual end of a line, Vim fails to properly validate the boundary, resulting in memory being read past the allocated buffer.

The vulnerability requires local access to the system and user interaction—specifically, the victim must open a maliciously crafted file in Vim. Once triggered, the out-of-bounds read can expose sensitive information stored in adjacent memory regions, though it does not directly allow code execution or system modification.

Root Cause

The root cause lies in the charset.c source file where position handling logic assumed that the column position would always be within valid bounds of the line buffer. The original code had a special check for empty lines but failed to account for cases where the position column exceeded the actual line length in non-empty lines.

The vulnerable code path was triggered when using the /\%V search pattern (visual mode search), where the position could extend beyond the NUL terminator of the line string, causing Vim to read memory past the intended buffer boundary.

Attack Vector

The attack requires local system access and user interaction. An attacker would need to:

1. Craft a malicious file containing specific content that triggers the vulnerable code path
2. Convince a user to open this file in Vim
3. Have the user perform operations that invoke the visual mode search functionality

The following patch demonstrates how the vulnerability was addressed by adding proper boundary validation:

 	posptr = NULL;  // continue until the NUL
     else
     {
-	// Special check for an empty line, which can happen on exit, when
-	// ml_get_buf() always returns an empty string.
-	if (*ptr == NUL)
-	    pos->col = 0;
+	colnr_T i;
+
+	// In a few cases the position can be beyond the end of the line.
+	for (i = 0; i < pos->col; ++i)
+	    if (ptr[i] == NUL)
+	    {
+		pos->col = i;
+		break;
+	    }
 	posptr = ptr + pos->col;
 	if (has_mbyte)
 	    // always start on the first byte

Source: GitHub Vim Commit

The fix iterates through the buffer to find the actual NUL terminator and clamps the column position to prevent reading beyond the end of the line.

Detection Methods for CVE-2021-4193

Indicators of Compromise

• Unexpected Vim crashes or abnormal termination when opening specific files
• Memory access violations or segmentation faults during Vim visual mode operations
• Suspicious files with unusual content patterns designed to trigger search operations

Detection Strategies

• Monitor for Vim processes exhibiting unusual memory access patterns or crashes
• Implement file integrity monitoring to detect potentially malicious files targeting Vim
• Use endpoint detection tools to identify exploitation attempts through behavioral analysis
• Review system logs for repeated Vim crashes that may indicate exploitation attempts

Monitoring Recommendations

• Enable core dump analysis to capture crash details for forensic review
• Deploy memory safety tools like AddressSanitizer in development environments to detect out-of-bounds reads
• Monitor for unusual file access patterns where users open files from untrusted sources in Vim

How to Mitigate CVE-2021-4193

Immediate Actions Required

• Update Vim to version 8.2.3950 or later which includes the security patch
• Apply operating system vendor security updates for Fedora, Debian, and macOS
• Avoid opening untrusted files in Vim until the patch is applied
• Consider using alternative text editors temporarily for handling untrusted content

Patch Information

The vulnerability was fixed in Vim patch 8.2.3950. The fix is available in the official Vim GitHub repository. Multiple operating system vendors have released security updates:

• Debian: Security updates available via Debian LTS Announcement
• Fedora: Updates available through standard package management
• Apple macOS: Addressed in security updates HT213183, HT213256, and HT213343
• Gentoo: See GLSA 2022-08-32

Workarounds

• Restrict Vim usage to trusted files only until patching is complete
• Configure file associations to prevent automatic opening of potentially malicious files in Vim
• Use read-only mode (vim -R) when viewing untrusted files to limit potential impact
• Implement strict file type validation before opening files from external sources

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

# Update Vim on Fedora systems
sudo dnf update vim

# Verify Vim version includes the patch
vim --version | grep -i "8.2"