Technical Analysis of "Run commands on company machines through CSV injection"

Overview: Mubassir Kamdar’s write-up, "Run commands on company machines through CSV injection," details the exploitation of CSV injection vulnerabilities. This technique involves injecting malicious formulas into CSV files which, when opened in spreadsheet programs like Microsoft Excel or LibreOffice Calc, can execute arbitrary commands on the user's machine.

Key Technical Details:

1. CSV Injection Basics:

   • CSV Definition: A CSV (Comma-Separated Values) file is a plain text file that contains data separated by commas and is widely used for data export and import.
   • Injection Vector: By inserting specially crafted strings into CSV fields, attackers can create malicious formulas. Spreadsheet programs execute these formulas upon rendering the CSV content.

2. Main Attack Vector:

   • Spreadsheet Formula Injection: Programs like Excel and LibreOffice Calc treat data beginning with =, +, -, or @ as formulas. An attacker can craft a payload that starts with these characters to inject a formula.
   • Example Payload: plaintext =CMD|' /C calc'!A0 This payload attempts to invoke the calculator program (calc) on Windows systems using a command execution formula in Excel.

3. Exploitation Steps:

   • Step 1: Injecting Payload: The attacker finds a way to input malicious data into a system that exports the data to a CSV file. This can involve user registration forms, feedback forms, or any other input vectors.
   • Step 2: Exporting to CSV: The system’s legitimate functionality exports the input data (containing the malicious payload) into a CSV file.
   • Step 3: Opening the file: When a privileged user (e.g., an administrator) opens the CSV file in Excel or any vulnerable spreadsheet software, the application interprets and executes the formula.

4. Technical Contexts and Vulnerability Points:

   • Export Functionality: Any web application functionality that exports user-controllable data into a CSV format is a potential vector for this attack.
   • Client-Side Execution: The exploitation depends on the spreadsheet software’s ability to execute injected formulas. Primarily targets Microsoft Excel and LibreOffice Calc.

5. Impact:

   • Command Execution: If an administrator opens the CSV file, it can lead to arbitrary command execution on their machine. For example: plaintext =cmd|'/C notepad'!A0 This would open Notepad on a Windows machine.
   • Arbitrary Code Execution: More complex payloads could lead to full system compromise depending on the attacker’s craftiness and the target’s environment.

6. Mitigations:

   • CSV Sanitization: Before exporting to CSV, ensure any data starting with formula characters (=, +, -, @) is either stripped or escaped. For example, a common escape technique is prepending the character ' (apostrophe), turning = into '=.
   • Safe Handling Practices: Train users to open untrusted CSV files with caution, preferably in environments where execution of formulas is disabled.
   • Content Security Policy (CSP): Implement strong CSP rules to minimize the risk of client-side code execution through web interfaces.
   • Update Software: Use the latest versions of spreadsheet programs that may have patched such vulnerabilities.

Key Takeaways:

• CSV Files Are Not Inert: They can be abused to inject executable content when interpreted by spreadsheet applications.
• User Input Sanitization: It is crucial to sanitize user inputs before exporting them to avoid including formula injection strings.
• Exploitation Context: The attack significantly depends on user behavior—specifically, opening the CSV files in susceptible spreadsheet software.
• Vulnerability Scope: Any system that exports user input to CSV can potentially be vulnerable if proper sanitization mechanisms are not in place.

Conclusion:

The write-up underscores the importance of comprehensive input validation and sanitization when dealing with data exports to CSV format. Given the typical use of CSV files for data interchange, understanding and mitigating the risk of CSV injection is essential for maintaining a secure application environment.

For full details, check the original blog post here.