Re: [hybi] Handshake was: The WebSocket protocol issues.

[Scott Ferguson <[e-mail]>]

Maciej Stachowiak wrote:
> On Sep 26, 2010, at 6:22 PM, Greg Wilkins wrote:
>
>   
>> 2010/9/27 Maciej Stachowiak <[e-mail]>:
>>     
>>> Here is an email from February where I summarized some of the risks from
>>> cross-protocol attacks for WebSocket:
>>>       
>> Maciej,
>>
>> thanks for the reference, but I didn't and still don't fully
>> understand the actual risk posed by those descriptions and that we are
>> still shadow boxing against perceived rather than real threats.
>>     
>
> The reason I am skeptical of your proposed ping-poing handshake as a defense against cross-protocol attacks is that the *only* defense protecting WebSocket servers from attacks over HTTP is the assumed inability of browser-hosted HTTP clients to send whatever carries the random nonce, or something that looks sufficiently similar. Your summary doesn't specify exactly how the nonce is sent, but I'll assume it is using a header that normally cannot be sent via cross-site XHR or similar APIs.
>   

Yes, the "ping" packet is missing a hash, which is needed to validate 
the browser as a websocket browser. Modifying Greg's proposal:

C1: c-nonce=...

S2: H(c-nonce, "WebSocket")
      s-nonce=...

C3: H(s-nonce, "WebSocket")

With that modification, the client does not send websocket data until 
the server has been validated as a websocket server, and the server does 
not process websocket data until the client has been validated as a 
websocket client. (There's no need for a S4 "pong" packet since the 
server is already validated as websocket when the client validates S2.) 
That protocol is pretty much the minimum required, and doesn't add 
overhead beyond the current handshake.

I'd propose two additional changes that aren't required, strictly 
speaking, but would be helpful for other security-related reasons:

1. Use "WEBSOCKET" instead of "GET" as the HTTP method. This is helpful 
because it lets validating servers quickly reject websocket requests 
(e.g. well-behaved HTTP servers), and lets server administrators use 
existing security frameworks (which are often HTTP method based), to 
specify websocket security easily. With the current "GET", it's somewhat 
of a problem for a server administrator to specify a different policy 
for GET vs websockets.

2. Use a new AUTH or HELLO opcode/packet for the C3 hash instead of 
overloading PING. This would make the packet's purpose clear and give a 
space for any future challenge-based authentication to put its 
credentials in a future spec extension. (I'd recommend any 
authentication extension/C3-payload use HTTP style authentication 
headers so the extension could just use existing security handshakes.)

> A potential improvement would be to require the server to produce a random number, which is included in the ping and must be in the pong in addition to the hash. In that case, an attacker could not queue up the correct pong without looking at the server's response at all. But going down this road results in increasingly elaborate schemes, along the lines of the -76 handshake. It's also likely there are more complicated flaws that we simply haven't thought of yet. I don't think we can stake the security of this protocol just on our own ability to think up attacks within a period of only a few weeks.
>   

No, it's different from -76, because the properties of the nonce/hash 
pattern are well known. The flaw with -76 is just what you describe: 
it's inventing new security patterns and is likely to have complicated 
flaws. Sticking to a well-known pattern is the right way to go.

-- Scott
>
> As I have thought about these issues, I am increasingly convinced that an NPN-style solution is much more robust. Attempting to make a TLS connection to a vanilla HTTP server, or an HTTPS server that does not understand NPN, will reject the connection at a very low level, greatly limiting the potential for shenanigans. Browser clients in general do not offer APIs that would allow a Web attacker any control of the outgoing TLS handshake, and the TLS layer would fail a bad connection before server software even had the opportunity to make a mistake. This approach seems much more robust to me. Rather than just barely being secure, it fails hard before the attacker can even do anything tricky. I think it is much likely there would be future attacks. I hope the WG strongly considers the NPN approach, despite the costs and challenges imposed by using TLS.
>
> Regards,
> Maciej
>
>
>
>
>   
>>> 1) Hostile JavaScript could use WebSocket for a cross-protocol attack against
>>> vanilla HTTP resources or non-HTTP servers.
>>> If the attacker can trick a non-WebSocket server into echoing back chosen text (for
>>> example through something in the URL part of the request), then they could make
>>> it give what appears to be a valid WebSocket handshake response. This could result
>>> in unauthorized access.
>>>       
>> I don't understand "unauthorized access" in this context.   If a
>> server, HTTP or otherwise want to protect it's content, then it needs
>> to have both authentication and authorization, which typically means
>> that the client will possess some credentials that the server must
>> check.     If the server does not protect its resources or the
>> attacker already has access to credentials (or can trick the
>> browser/user into providing credentials), then the easiest way to
>> access a non-websocket server would be with existing HTTP mechanisms
>> in the browser.
>>
>> Is there anything about WS that would make a vulnerable HTTP server
>> easier to exploit than just using the existing HTTP mechanisms
>> available in the browser? ie are there any exploit HTTP requests that
>> can be generated using the WS client that could not be generated
>> anyway?
>>
>>
>>
>>     
>>> 2) Cross-site XMLHttpRequest (using CORS or XDomainRequest) could be used for a
>>> cross-protocol attack against WebSocket resources, potentially violating integrity (though not confidentiality).
>>>
>>> The WebSocket protocol currently does not require any checking of the client handshake. However, any
>>> WebSocket server that performs any side effects in response to messages from the client has a security
>>> vulnerability if it does not check correctness of the handshake request from the client.
>>>       
>> I think that any server that "performs any side effects in response to
>> messages from the client" will have a security vulnerability if they
>> do not correctly check the authentication and authorization of the
>> client. Eventually most browsers will have WS available to the
>> javascript, so a server that is so vulnerable will be able to be
>> exploited just with WS instead of trying to trick HTTP.
>>
>> The read question should be - are there any WS
>> authentication/authorization mechanisms that can be subverted by using
>> a HTTP mechanism instead of a the WS API?
>>
>>
>> I still think that we should take reasonable measures to prevent cross
>> protocol handshakes - even if there are no identified real security
>> issues.  But I think that the usage of Sec- headers and a simple nonce
>> will give very good protection and that we only need do more if there
>> is a real clearly identified risk.
>>
>> regards
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>>     
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