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From Christopher Schultz <ch...@christopherschultz.net>
Subject Re: [OT] Protecting against HTTP response splitting
Date Thu, 21 Apr 2011 13:52:55 GMT
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Konstantin,

On 4/20/2011 11:56 AM, Konstantin Kolinko wrote:
> 2011/4/20 Christopher Schultz <chris@christopherschultz.net>:
>>
>> I was considering scouring the URL/URI specs for exactly what characters
>> are allowed but then decided that I didn't really care: I was mostly
>> concerned with thwarting a response-splitting attack and avoiding \r and
>> \n does that.
> 
> See HTTP spec on what is allowed in headers.

It's in the RFC 822, ARPA Internet Text Messages, not the HTTP spec.

>> This isn't intended to be an outgoing HTTP header value validator.
>>
>> Technically, this is over-engineered because it looks for /either/ \r
>> /or/ \n, rather than \r\n which should be the only way to exploit such a
>> vulnerability. :)
> 
> You are wrong. This way is not the only one.

I suppose my implementation may be both simplistic and draconian. For
example, a header value such as "This isCRLF my value" is perfectly
valid and does not represent a response-splitting attack, though it
would trigger an error from my filter. Also, header values are allowed
to contain CRs and LFs, just not next to each other. I'm okay with
violating both of these provisions since my application doesn't make use
of them.

I'm interested in where you see another opportunity to split an HTTP
response using HTTP headers.

- -chris
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