CVE-2025-66476

Executive Summary

CVE-2025-66476 is a high-severity uncontrolled search path vulnerability affecting Vim on Windows. When using cmd.exe as the shell, Vim searches the current working directory before system paths when resolving external commands. This allows an attacker to place a malicious executable (e.g., findstr.exe, grep.exe, or any command invoked via :!, :grep, :make, or compiler commands) in the same directory as the file being edited. When Vim executes the command, it runs the attacker's payload instead of the legitimate system binary, leading to arbitrary code execution with the user's privileges.

The vulnerability is unpatched in versions prior to 9.1.1947. No public exploit code or proof-of-concept is available at this time. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the risk is significant for targeted attacks.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby). It is a core Vim binary issue on Windows. However, if Vim is distributed via package managers like Chocolatey or Scoop, users should ensure they update to the patched version.

CVSS Breakdown & Threat Level

• CVSS Score: 7.8 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Attack Vector: Local (requires user to open a file in a directory containing a malicious executable)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required (user must open a file in the attacker-controlled directory)
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

Threat Level: High. While not remotely exploitable, the impact is severe if an attacker can trick a user into opening a file from a malicious directory.

Deep Dive & Mechanics

The vulnerability stems from Windows' default command resolution behavior when using cmd.exe. When Vim executes an external command (e.g., :!dir, :grep, :make), it calls cmd.exe with the command string. cmd.exe searches for executables in the following order:

1. Current working directory
2. System PATH directories

If an attacker places a malicious findstr.exe (or any other command Vim might invoke) in the same directory as the file being edited, Vim will execute that malicious binary instead of the legitimate system tool.

Key affected Vim commands:

• :! (shell commands)
• :grep (uses findstr on Windows)
• :make (compiler commands)
• :compiler (compiler settings)
• Any plugin or script that calls external commands

Why this is dangerous: An attacker can craft a malicious executable that mimics the original command's behavior but also executes a payload (e.g., reverse shell, data exfiltration). The user would see normal output, making detection difficult.

Vulnerable vs. Patched Code

Vulnerable (pre-9.1.1947): Vim on Windows uses cmd.exe without sanitizing the command path. The shell resolution follows Windows default behavior, searching the current directory first.

// Simplified vulnerable code path
void execute_external_command(char *cmd) {
    // On Windows, this calls cmd.exe /c <cmd>
    // cmd.exe searches CWD first
    system(cmd);
}

Patched (9.1.1947+): Vim now explicitly uses absolute paths for known external commands or forces the shell to skip the current directory. The patch modifies the command resolution to use where.exe or similar to find the system binary first.

// Patched code path
void execute_external_command(char *cmd) {
    // Use absolute path for known commands
    char *safe_cmd = resolve_system_path(cmd);
    // Or use cmd.exe /c with explicit path
    system(safe_cmd);
}

Note: The exact patch details are not publicly available, but the fix involves changing how Vim resolves external commands on Windows.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Windows machine with Vim < 9.1.1947
• Ability to place files in a directory the victim will open

Exploitation Steps:

1. Create a malicious executable that mimics a common Vim command (e.g., findstr.exe). The executable should:
   • Execute the original command's functionality (to avoid suspicion)
   • Run a payload (e.g., reverse shell, keylogger)

// malicious_findstr.c
#include <windows.h>
#include <stdlib.h>

int main() {
    // Execute original findstr with arguments
    system("C:\\Windows\\System32\\findstr.exe %*");
    
    // Execute payload
    system("powershell -enc <base64_payload>");
    
    return 0;
}

2. Compile the malicious executable:

gcc -o findstr.exe malicious_findstr.c

3. Place findstr.exe in a directory containing a text file.

4. Trick victim into opening the text file with Vim and running :grep or :!findstr.

Expected Result: The malicious findstr.exe runs, executing the payload.

Blue Team

Detection:

• Monitor for unexpected executables in directories where Vim is used
• Look for cmd.exe spawning unusual child processes
• Check for Vim processes executing commands from non-standard paths

Hunting Query (Sysmon):

<EventID>1</EventID>
<Image>cmd.exe</Image>
<ParentImage>vim.exe</ParentImage>
<CommandLine>*findstr*</CommandLine>

Prevention:

• Use set shell=C:\Windows\System32\cmd.exe in Vim config to force absolute path
• Restrict write permissions to directories where Vim is used
• Use AppLocker or WDAC to block untrusted executables

Indicators of Compromise (IOCs)

Detection Engineering

YARA Rule

rule CVE_2025_66476_Vim_Uncontrolled_Search_Path {
    meta:
        description = "Detects malicious executables exploiting CVE-2025-66476 in Vim on Windows"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66476"
        severity = "high"
    strings:
        $s1 = "findstr" nocase
        $s2 = "grep" nocase
        $s3 = "cmd.exe" nocase
        $s4 = "vim.exe" nocase
    condition:
        (uint16(0) == 0x5A4D) and  // MZ header
        (
            ($s1 or $s2) and
            (for any of ($s3, $s4): (# > 0))
        )
}

Sigma Rule

title: Vim Uncontrolled Search Path Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects Vim executing external commands from non-standard paths
author: Valters IT
date: 2025-04-10
references:
    - CVE-2025-66476
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        ParentImage|endswith: '\vim.exe'
        Image|endswith: '\cmd.exe'
        CommandLine|contains: 
            - 'findstr'
            - 'grep'
            - 'make'
            - 'compiler'
    filter:
        CommandLine|contains: 'C:\Windows\System32'
    condition: selection and not filter
falsepositives:
    - Legitimate use of Vim with custom shell scripts
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-66476 Vim Uncontrolled Search Path - Potential Payload"; flow:to_server,established; content:"|4d 5a|"; offset:0; depth:2; content:"findstr"; nocase; within:100; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

1. Upgrade Vim to version 9.1.1947 or later.
2. If upgrade is not possible, set the shell to use absolute paths:

   set shell=C:\Windows\System32\cmd.exe
   

3. Avoid opening files from untrusted directories with Vim.

Long-term:

• Implement application whitelisting (AppLocker, WDAC) to block untrusted executables.
• Use Vim's shellxquote and shellxescape options to sanitize commands.
• Monitor for suspicious process creation from Vim.

Workaround:

• Use :!C:\Windows\System32\findstr.exe explicitly instead of relying on path resolution.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert