Struts vulnerability (s2-045)

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- Summary of vulnerabiliry

- When a request is conducted to Strunts application, built-in ServletFilter is executed.

   If content_type contains the strings "multipart/form-data",
org.apache.struts2.dispatcher.multipart.JakartaMultiPartRequest#parse() is called.
   (which is built in default parser of Struts core)
   So, if we don't modify the defaut settings of parser, attackers can reveledging the 
vulnerability by sending crafted requests contains multipart/form-data in content_type.

- The cause of the vulnerabiliry is the follwing:

     In processs of JakartaMultiPartRequest#parse(), an Exception is thrown and 
buildErrorMessage() is called if content_type is invalid format.
     During handling Exception object in buildErrorMessage(), malicious OGNL specified 
in content_type is executed, which leads to arbitory code execetion.

- Mitigation

- Reject malicious request by using ServletFilter or WAF.

    The following is an example of ServletFilter which retuns server error against malicious 
Content-Type containing OGNL expression.


    public void doFilter(ServletRequest request, ServletResponse response,
           FilterChain chain) 
           throws, javax.servlet.ServletException

        System.out.println("Servlet Filter: "+this.getClass().getName()+"Called.");
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        String contentType = httpRequest.getHeader(CONTENT_TYPE);
        String uri = httpRequest.getRequestURI();
        BufferedReader reader = null;
        String body = "";
        	reader = httpRequest.getReader();
        	Stream lines = reader.lines();
        	body = lines.collect(Collectors.joining("\r\n"));
        } catch (IOException e) {
        	// skip filter
        	chain.doFilter(request, response);
        } finally{
        Pattern p = Pattern.compile(SIGNATURE_OGNL);
        if (contentType!=null && !contentType.toLowerCase(Locale.ENGLISH).startsWith(MULTIPART) && 
        	System.out.println("Malicious Content-Type:"+contentType);
        	throw new ServletException(ERROR_INVALID_REQUEST);
        } else if (p.matcher(uri).find()){
        	System.out.println("Malicious URI:"+uri);
        	throw new ServletException(ERROR_INVALID_REQUEST);
        } else if (p.matcher(body).find()){
        	System.out.println("Malicious Request body:"+body);
        	throw new ServletException(ERROR_INVALID_REQUEST);
        chain.doFilter(request, response);

Example of Servlet filter


- Switch parser for multipart/form-data from JakartaMultiPartRequest to another.

     In 2.3.18 later, JakartaStreamMultiPartRequest is available. 
     We can switch parser by setting the following property in struts configuration 
(e.g. struts.xml)
     <constant name="struts.multipart.parser" value="jakarta-stream" />
     Then the follwing class is executed instead of JakartaMultiPartRequest:



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- Code execution mechanism and notice on security filters

Stack trace in vulnerable codes:
--> JakartaMultiPartRequest#buildErrorMessage()
-->parser.evaluate(openChars, expression, ognlEval, maxLoopCount);
--> OgnlTextParser.evaluate()
--> com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecute()
-->Ognl.parseExpression(expression); **

Let's focus on ** : tree = Ognl.parseExpression(expression);
Here is expression example:

parseExpression() parses expression and return object containing evaluated ognl codes.
If spaces are contained before and after "."(dot) in expression, they are trimmed during parsing,
meaning that attacker could bypass security filters such as WAF.

If full package name "ognl.OgnlContext" is specified as filter, attackers can bypass it
by sending requests containing spaces for instance "ognl. OgnlContext".

Based on above, we had better to specify only class name as filters
for instance: OgnlContext , OgnlUtil etc.

To record malicious request in Content-Type:
set LogFormat in httpd.conf

LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" \"%{Content-Type}i\" \"%{Content-Disposition}i\"" combined

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This page contains a single entry by gamzatti published on March 9, 2017 9:36 PM.

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