OWASP Top 10 number 3: Malicious File Execution

Number 3 in the Top 10 most critical web application security vulnerabilities identified by the Open Web Application Security Project (OWASP) is Malicious File Execution, which occurs when attacker's files are executed or processed by the web server. This can happen when an input filename is compromised or an uploaded file is improperly trusted.

Examples

• file is accepted from the user without validating content
• filename is accepted from the user

In the example below a file name is accepted from the user and appended to the server's filesystem path.

// get the absolute file path on the server's filesystem 
String dir = servlet.getServletContext().getRealPath("/ebanking")
// get input file name
String file = request.getParameter(“file”); 
//  Create a new File instance from pathname string   
File f = new File((dir + "\\" + file).replaceAll("\\\\", "/")); 

If the filename was compromised to  ../../web.xml , it might allow access to web server properties

Malicious File Execution can result in:

• files loaded from another server and executed within the context of the web server
• modifying paths to gain access to directories on the web server
• malicious scripts put into a directory with inadequate access controls

Protecting against Malicious File Execution

• the Java EE Security Manager should be properly configured to not allow access to files outside the web root.
• do not allow user input to influence the path name for server resources
  • Inspect code containing a file open, include, create, delete...
• firewall rules should prevent new outbound connections to external web sites or internally back to any other server. Or isolate the web server in a private subnet
• Upload files to a destination outside of the web application directory.
  • Enable virus scan on the destination directory.

Java specific Protecting against Malicious File Exection

Use the OWASP ESAPI  HTTPUtilities interface:

• The ESAPI HTTPUtilities interface is a collection of methods that provide additional security related to HTTP requests, responses, sessions, cookies, headers, and logging.

  The HTTPUtilities getSafeFileUploads method uses the Apache Commons FileUploader to parse the multipart HTTP request and extract any files therein

  public class HTTPUtilities     public void getSafeFileUploads(java.io.File tempDir,                                java.io.File finalDir)                         throws ValidationException

  

References and More Information:

• Top 10 most critical web application security vulnerabilities
• Open Web Application Security Project (OWASP)
• OWASP TOP 10 FOR JAVA EE
• OWASP Enterprise Security API
• OWASP ESAPI  Overview Presentation