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Re: "actual content length", was: Handling multiple headers when only one is allowed

From: Adrien de Croy <adrien@qbik.com>
Date: Sat, 12 Jun 2010 01:24:23 +1200
Message-ID: <4C123907.2000607@qbik.com>
To: Dan Winship <dan.winship@gmail.com>
CC: Julian Reschke <julian.reschke@gmx.de>, Bil Corry <bil@corry.biz>, HTTP Working Group <ietf-http-wg@w3.org>, Michal Zalewski <lcamtuf@google.com>, Jeff Hodges <Jeff.Hodges@KingsMountain.com>, Adam Barth <ietf@adambarth.com>, "Yngve N. Pettersen (Developer Opera Software ASA)" <yngve@opera.com>

I raised this problem a while back.

All the browsers except Opera (limited case) make no complaint when a 
download is truncated.  This is whether it's chunked and doesn't receive 
a final 0 chunk, or whether there's a content length and the connection 
is closed (whether or not the server indicated it would close) prior to 
that many bytes being transferred.

I personally view this as highly problematic, and it's tied in with the 
work I've been doing recently with scanning at a proxy.

The reason it's problematic, is because every single proxy I've tested 
(TMG/ISA, WinRoute, WinGate, Webmarshall - admittedly there are many 
more) does something called either "drip-feeding" or "trickling".  If 
you're downloading a file through one of these proxies, they will send 
you a portion of the resource as it's coming down to the proxy.  When 
the proxy has received the whole file, it scans it and sends the rest if 
it's ok, but if it's not ok, it has 1 option only - abort the connection.

Since the browsers ignore the connection having been aborted, and 
present the downloaded file as if nothing was wrong, then any malware 
purveyor need only pad their malware out, so that the executable part 
will fall within the drip-feeding window.  It basically renders AV at 
gateway potentially useless.

If OTOH the browsers were to act on the fact that the download was 
aborted, this wouldn't be nearly as big a security risk.



On 12/06/2010 12:51 a.m., Dan Winship wrote:
> On 06/09/2010 08:24 AM, Julian Reschke wrote:
>>>      http://code.google.com/p/browsersec/wiki/Part1#Hypertext_Transfer_Protocol
>>> ...
>> Interesting.
>> That text mentions the test
>>    "Content-Length header value overrides actual content length?"
>> I have trouble understanding what this means... Unless the connection is
>> closed, or chunked encoding is in place, or the message is by definition
>> not having a body (HEAD response), there *is* no other signal than
>> Content-Length to find out the actual content length.
> It's about the case when you know the connection will be closed at the
> end of the response, but the actual number of bytes sent by the server
> before closing the connection doesn't match the Content-Length header.
> Eg, if you GET http://sourceforge.net/apps/wordpress/sourceforge, you'll
> get back a 302 response with "Content-Length: 338" and "Connection:
> close", followed by 8 bytes of body data before the server closes the
> connection. A client should consider that to be a truncated response,
> but as noted by the page above, most web browsers instead just assume
> that the Content-Length header was wrong. (I assume this was originally
> another case of bug-compatibility with IE to support broken web sites.)
> -- Dan

Adrien de Croy - WinGate Proxy Server - http://www.wingate.com
Received on Friday, 11 June 2010 13:25:04 UTC

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