CVE-2021-3875: Vim Heap-Based Buffer Overflow Vulnerability

CVE-2021-3875 Overview

CVE-2021-3875 is a heap-based buffer overflow vulnerability affecting the Vim text editor. This memory corruption flaw occurs during search operations with range specifications, potentially leading to application crashes and denial of service conditions. The vulnerability is triggered when Vim processes specially crafted input that causes the editor to access memory beyond allocated buffer boundaries.

Critical Impact

A local attacker can exploit this heap-based buffer overflow to crash Vim, causing denial of service. While the vulnerability requires user interaction (opening a malicious file or executing a crafted command), successful exploitation can disrupt user workflows and potentially lead to data loss in unsaved editing sessions.

Affected Products

• Vim (all versions prior to patch 8.2.3489)
• Fedora 33
• Fedora 35

Discovery Timeline

• 2021-10-15 - CVE-2021-3875 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3875

Vulnerability Analysis

The vulnerability resides in Vim's command execution handling within the src/ex_docmd.c source file. When processing search commands that follow address specifications (using / or ? patterns), Vim fails to properly validate line number boundaries before setting the cursor position. This leads to an ml_get error where the application attempts to access a line number that exceeds the actual number of lines in the current buffer.

The flaw is classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), indicating that improper memory operations occur during the vulnerable code path execution. When exploited, this causes Vim to read or write data outside the intended memory region, resulting in application instability.

Root Cause

The root cause lies in insufficient boundary checking when handling line number addresses during search operations. Specifically, when a search pattern follows another address in a command, Vim would set the cursor line position directly from the provided lnum value without verifying that this value falls within the valid range of lines (1 to curbuf->b_ml.ml_line_count). This allows an attacker to specify a line number greater than the total lines in the buffer, triggering the heap-based buffer overflow.

Attack Vector

The attack requires local access and user interaction. An attacker must convince a user to open a malicious file or execute a specially crafted command sequence within Vim. The exploitation involves:

1. Crafting a Vim command that specifies a search pattern with an out-of-bounds line address
2. Having the victim execute this command, either directly or through a malicious modeline/script
3. Vim attempts to access the invalid line position, causing memory corruption

The following patch demonstrates the fix implemented in Vim patch 8.2.3489:

// When '/' or '?' follows another address, start from
// there.
-		    if (lnum != MAXLNUM)
-			curwin->w_cursor.lnum = lnum;
+		    if (lnum > 0 && lnum != MAXLNUM)
+			curwin->w_cursor.lnum =
+			    lnum > curbuf->b_ml.ml_line_count
+				       ? curbuf->b_ml.ml_line_count : lnum;

// Start a forward search at the end of the line (unless
// before the first line).

Source: GitHub Vim Commit

The fix adds two critical checks: ensuring lnum > 0 and clamping the line number to the maximum valid line count (curbuf->b_ml.ml_line_count) if it exceeds the buffer bounds.

Detection Methods for CVE-2021-3875

Indicators of Compromise

• Vim processes crashing unexpectedly with segmentation faults or memory access errors
• System logs showing ml_get errors from Vim execution
• Vim core dumps indicating heap corruption patterns
• Unusual Vim command sequences in shell history involving search patterns with large line numbers

Detection Strategies

• Monitor for Vim process crashes and analyze core dumps for heap overflow signatures
• Implement file integrity monitoring to detect potentially malicious Vim configuration files or modelines
• Deploy endpoint detection rules that alert on repeated Vim crashes within short time periods
• Review system audit logs for suspicious command-line invocations of Vim with complex address patterns

Monitoring Recommendations

• Configure process monitoring to track Vim process stability and abnormal terminations
• Enable core dump collection for forensic analysis of potential exploitation attempts
• Use SentinelOne's behavioral AI to detect anomalous Vim execution patterns indicative of exploitation
• Monitor for the creation of suspicious Vim script files or configuration modifications

How to Mitigate CVE-2021-3875

Immediate Actions Required

• Update Vim to version 8.2.3489 or later that includes the security patch
• Review and restrict Vim modeline processing by setting set nomodeline in vimrc
• Audit any custom Vim scripts or plugins for potentially malicious command sequences
• Consider using SentinelOne endpoint protection to detect and prevent exploitation attempts

Patch Information

The vulnerability has been addressed in Vim patch 8.2.3489. The fix modifies the src/ex_docmd.c file to add proper boundary validation when setting cursor positions during search operations. Organizations should update through their package manager or compile from source using the patched version.

Security advisories have been published for multiple distributions:

• Fedora Package Announcements for Fedora 33 and 35
• Gentoo GLSA 202208-32

Workarounds

• Disable modeline processing by adding set nomodeline to your ~/.vimrc configuration
• Avoid opening untrusted files directly in Vim; use cat or less first to inspect content
• Run Vim in restricted mode (vim -Z) when editing files from untrusted sources
• Consider using Neovim or alternative editors for handling potentially malicious files

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

# Alternatively, create a secure vimrc for untrusted file editing
echo "set nomodeline" > ~/.vimrc.secure
echo "set modelines=0" >> ~/.vimrc.secure
# Use with: vim -u ~/.vimrc.secure untrusted_file