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Sending multipart/form-data requests from Flash (with arbitrary headers)



Sending multipart/form-data requests from Flash (with arbitrary headers)
Sending multipart/form-data requests from Flash (with arbitrary headers)



Hello lists,

In my original "Forging HTTP request headers with Flash" paper 
(http://www.securityfocus.com/archive/1/441014), I mentioned forcing multipart/form-data 
input format to ensure that Flash's LoadVars isn't used to forge the request.
However, there's a work-around for the attacker - using Flash's XML object. The attacker 
can forge almost any practical input for multipart/form-data (as long as it's part of a 
well-formed XML document):

var req:XML=new XML("\r\n--bar\r\nContent-Disposition: form-data; 
name=field1\r\n\r\ndata1\r\n--bar\r\nContent-Disposition: form-data; 
name=field2\r\n\r\ndata2\r\n--bar--\r\n");
req.addRequestHeader("Content-Type","multipart/form-data; boundary=\"bar\"");
req.addRequestHeader("Referer","http://some.site/"); 
req.addRequestHeader("Expect","something");
req.send("http://www.vuln.site/path/to/script.cgi","_blank"); 

It seems that the capabilities of the XML object (XML.send), in terms of forging HTTP 
request headers, are similar to those of LoadVars (LoadVars.send). The above code works 
well in IE 6.0 (Flash 7 and 8).

The reader may note that there's a bit of data in the body before the first MIME part 
(that's the  start-element) as well as a bit of data after the last MIME part 
(). As it happens, the multipart Content-Type allows such "preamble" (data before the 
first multipart boundary delimiter) and "epilogue" (data after the last multipart boundary 
delimiter) - see RFC 2046 section 5.1.1, and indeed application engines (e.g. PHP) silently 
ignore those parts.

One shortcoming of the technique is that it cannot express all possible data. This is due 
to the fact that the XML object stores its XML data in an XML tree, and then serializes 
this tree to an XML document upon invocation of the send() method. This serialization 
doesn't duplicate the exact XML input (byte-wise, although logically the same tree would 
result), and it also guarantees that the serialized data is a well formed XML document. For 
example, any double quote marks will be converted to their XML entity representation 
("), likewise for several other symbols. Theoretically, this means that it's still 
possible to distinguish a Flash submission from a browser submission, by forcing data with 
those symbols to be sent, e.g. using hidden fields with those values. But I wouldn't 
recommend this practice as it doesn't sound like "The Right Thing" - it's way too 
implementation specific.

Bottom line: multipart/form-data submissions can be forged almost completely, headers and 
all, using Flash 7/8 (in IE 6.0).

-Amit

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