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:




Comments:

Hi Carol,

What a great article series. Love it!

A couple other suggestions to mitigate this vulnerability:
- Run the app server as a non privileged account (not root)
- Consider running the app server in a Solaris container. This limits the damage if the system is compromised.

Posted by Warren Strange on October 08, 2009 at 06:53 AM EDT #

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