CVE-2021-3770: Vim Buffer Overflow Vulnerability

CVE-2021-3770 Overview

CVE-2021-3770 is a heap-based buffer overflow vulnerability affecting Vim, the popular text editor. This vulnerability occurs due to improper memory handling in Vim's tabstop functionality, specifically when processing the :retab command with large values. The flaw allows a local attacker with low privileges to potentially achieve arbitrary code execution or cause denial of service by crafting malicious input that triggers invalid memory access.

Critical Impact

Local attackers can exploit this heap-based buffer overflow to execute arbitrary code with the privileges of the Vim process, potentially leading to complete system compromise when Vim is run with elevated privileges.

Affected Products

• Vim (versions prior to patch 8.2.3402)
• Fedora 33, 34, and 35
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• September 6, 2021 - CVE-2021-3770 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-3770

Vulnerability Analysis

This vulnerability stems from improper memory management in Vim's variable tabstop (vartabs) feature. When the :retab command is invoked with an excessively large tabstop value, the tabstop_set() function in src/indent.c fails to properly validate input boundaries before allocating and writing to heap memory. This results in heap-based buffer overflow (CWE-122) and out-of-bounds write (CWE-787) conditions.

The flaw specifically impacts how Vim parses and stores tabstop configuration values. When a user or malicious script sets an extremely large tabstop value, the subsequent memory operations can overflow the allocated heap buffer, corrupting adjacent memory structures.

Root Cause

The root cause lies in the tabstop_set() function's inadequate input validation and error handling. Prior to the patch, the function did not properly validate large numeric values passed to the vartabs feature, and the return value handling was inconsistent. The function would proceed with memory operations even when boundary conditions were violated, leading to heap corruption.

Attack Vector

The attack requires local access to the system and the ability to either:

1. Convince a user to open a maliciously crafted file with Vim that contains modeline settings with large tabstop values
2. Execute Vim commands directly with crafted :retab arguments
3. Leverage automation scripts that process untrusted input through Vim

The vulnerability can be triggered through malicious Vim modelines, crafted configuration files, or direct command execution.

// Security patch in src/indent.c
// Source: https://github.com/vim/vim/commit/b7081e135a16091c93f6f5f7525a5c58fb7ca9f9

 /*
  * Set the integer values corresponding to the string setting of 'vartabstop'.
  * "array" will be set, caller must free it if needed.
+ * Return FAIL for an error.
  */
     int
tabstop_set(char_u *var, int **array)
{
-    int valcount = 1;
-    int t;
-    char_u *cp;
+    int	    valcount = 1;
+    int	    t;
+    char_u  *cp;

    if (var[0] == NUL || (var[0] == '0' && var[1] == NUL))
    {
	*array = NULL;
-	return TRUE;
+	return OK;
    }

    for (cp = var; *cp != NUL; ++cp)

The patch adds proper error handling and return value checking to prevent invalid memory access when large values are provided.

Detection Methods for CVE-2021-3770

Indicators of Compromise

• Vim processes crashing unexpectedly with segmentation faults or memory corruption errors
• Unusual memory allocation patterns in Vim process logs
• Presence of files containing modelines with extremely large tabstop values
• System logs showing Vim-related memory access violations

Detection Strategies

• Monitor for Vim crashes with heap corruption signatures using system crash monitoring tools
• Implement file scanning for malicious modelines containing abnormally large numeric values
• Deploy memory protection mechanisms (ASLR, stack canaries) to detect exploitation attempts
• Use static analysis tools to scan scripts and configuration files for suspicious Vim commands

Monitoring Recommendations

• Enable core dump collection and analysis for Vim process crashes
• Configure system auditing to log Vim execution with unusual command-line arguments
• Monitor for files with suspicious modeline patterns in user-accessible directories
• Set up alerting for repeated Vim process crashes in short time periods

How to Mitigate CVE-2021-3770

Immediate Actions Required

• Update Vim to version 8.2.3402 or later immediately
• Review and restrict modeline processing by setting set nomodeline in vimrc
• Audit existing Vim configuration files and scripts for potentially malicious content
• Apply operating system vendor patches from Fedora, Gentoo, or other distributions

Patch Information

The vulnerability was fixed in Vim patch 8.2.3402. The fix improves input validation in the tabstop_set() function and ensures proper error handling when processing vartabs values. The patch is available through the GitHub Vim Commit.

Distribution-specific patches are available:

• Fedora Package Announcements for Fedora 33, 34, and 35
• Gentoo GLSA-202208-32
• NetApp Security Advisory for ONTAP Select Deploy Administration Utility

Workarounds

• Disable modelines by adding set nomodeline to your .vimrc configuration file
• Avoid opening untrusted files with Vim until the patch is applied
• Use restricted mode (vim -Z) when editing files from untrusted sources
• Consider using alternative text editors temporarily for processing untrusted content

# Disable modelines in Vim configuration
echo "set nomodeline" >> ~/.vimrc

# Verify Vim version is patched (should be 8.2.3402 or later)
vim --version | head -1

# Run Vim in restricted mode for untrusted files
vim -Z untrusted_file.txt