CVE-2022-1771: Vim Uncontrolled Recursion DoS Vulnerability

CVE-2022-1771 Overview

CVE-2022-1771 is an uncontrolled recursion vulnerability in the Vim text editor affecting versions prior to 8.2.4975. This vulnerability exists in the command line loop functionality and can be triggered when a malicious file or input causes recursive calls without proper depth limiting. When exploited, an attacker can cause a denial of service condition by crashing the Vim application through stack exhaustion.

Critical Impact

A recursive command line loop in Vim can lead to stack exhaustion and application crash, resulting in denial of service for users editing files with crafted malicious content.

Affected Products

• Vim versions prior to 8.2.4975
• All platforms where vulnerable Vim versions are installed
• Linux distributions shipping unpatched Vim packages

Discovery Timeline

• 2022-05-18 - CVE-2022-1771 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-1771

Vulnerability Analysis

This vulnerability is classified as CWE-674 (Uncontrolled Recursion), which occurs when a function calls itself repeatedly without implementing proper termination conditions or depth limits. In Vim's case, the recursive command line loop in src/ex_getln.c lacked a call depth counter, allowing unbounded recursion that could exhaust the call stack.

The impact is primarily availability-focused, as exploitation results in application crashes but does not compromise data confidentiality or integrity. The attack requires local access and user interaction—specifically, a user must open or interact with a specially crafted file that triggers the recursive condition.

Root Cause

The root cause of CVE-2022-1771 is the absence of recursion depth tracking in Vim's command line loop implementation. Without a static depth counter to limit how many times the function could recursively call itself, malicious input could trigger infinite recursion until the process ran out of stack space and crashed.

Attack Vector

The attack vector requires local access to the target system. An attacker must craft a malicious file or input sequence that, when processed by Vim, triggers the vulnerable recursive command line loop. User interaction is required as the victim must open or execute commands on the malicious content. While exploitation requires these conditions to be met, the attack complexity is low once the preconditions are satisfied.

The following patch demonstrates the fix implemented in Vim 8.2.4975:

    int		indent,		// indent for inside conditionals
    int		clear_ccline)	// clear ccline first
{
+    static int	depth = 0;	    // call depth
    int		c;
    int		i;
    int		j;

Source: GitHub Commit 51f0bfb

The fix adds a static depth variable to track recursion depth, enabling the function to detect and prevent excessive recursive calls that would otherwise lead to stack exhaustion.

Detection Methods for CVE-2022-1771

Indicators of Compromise

• Vim process crashes with segmentation fault or stack overflow errors
• Core dumps indicating stack exhaustion in ex_getln.c functions
• Unexpected Vim termination when opening or processing specific files

Detection Strategies

• Monitor for repeated Vim crashes with stack-related error signatures
• Implement file integrity monitoring on systems where Vim is used for sensitive operations
• Use version detection tools to identify Vim installations running versions prior to 8.2.4975

Monitoring Recommendations

• Enable crash reporting and logging for terminal applications including Vim
• Monitor system logs for segmentation fault events associated with Vim processes
• Deploy endpoint detection solutions capable of identifying denial of service attack patterns

How to Mitigate CVE-2022-1771

Immediate Actions Required

• Update Vim to version 8.2.4975 or later immediately
• Review systems for vulnerable Vim installations using package management tools
• Consider using alternative text editors temporarily if immediate patching is not possible
• Avoid opening untrusted files with vulnerable Vim versions

Patch Information

The vulnerability was fixed in Vim version 8.2.4975. The patch is available via the official Vim GitHub repository commit. Linux distributions have released updated packages addressing this vulnerability, including Gentoo GLSA 202208-32 and Gentoo GLSA 202305-16.

Workarounds

• Restrict Vim usage to trusted files only until the patch can be applied
• Use system-level resource limits (ulimit) to constrain stack size and mitigate crash impact
• Deploy application sandboxing to limit the effects of potential Vim crashes
• Consider read-only alternatives for viewing untrusted file content

# Check current Vim version
vim --version | head -1

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

# Update Vim on RHEL/CentOS/Fedora systems
sudo dnf update vim

# Set stack size limits as a temporary mitigation
ulimit -s 8192