[Fwd: NTLM HTTP Authentication is insecure by design - a new writeup by Amit Klein]

IIRC we _do_ have provisions in place to avoid this kind of problems.
If we are sure (Henrik? Duane?) it would be nice to reply to this post.

        Kinkie (who doesn't have the time to check the Source(tm))

-------- Forwarded Message --------
> From: Amit Klein (AKsecurity) <aksecurity@hotpop.com>
> To: bugTraq <bugtraq@securityfocus.com>
> Subject: NTLM HTTP Authentication is insecure by design - a new
> writeup by Amit Klein
> Date: Mon, 18 Jul 2005 19:40:32 +0200
>
> NTLM HTTP Authentication
> (and possibly other connection-oriented
> HTTP authentication and authorization protocols)
> is insecure by design
>
> Or
>
> NTLM Authentication and HTTP proxies
> don't mix
>
>
> Amit Klein, July 2005
>
>
>
> Introduction
> ============
>
> In "Meanwhile on the other side of the webserver"
> (http://www.securityfocus.com/archive/1/401866) I surveyed some
> possible attacks against a scenario wherein a proxy server is
> positioned in front of a web server, and that proxy server shares a
> single TCP connection to the server among several clients. In that
> write-up, I mentioned several problems related to HTTP Request
> Smuggling
> (http://www.watchfire.com/resources/HTTP-Request-Smuggling.pdf) and
> HTTP Response Splitting
> (http://www.sanctuminc.com/pdf/WhitePaper_HTTPResponse.pdf). These
> are attacks that make use of non-RFC HTTP requests (HTTP Request
> Smuggling) or inject unexpected data (CRLF) through the application
> into the HTTP response stream (HTTP Response Splitting). In contrast,
> this write-up discusses a completely different problem, one which is
> inherent to the situation of a connection-oriented authentication/
> authorization protocol (e.g. NTLM authentication) used with a proxy
> server that shares TCP connections among several clients. Exploiting
> this vulnerability can be performed with 100% RFC compliant HTTP
> requests, and without attacking the application (i.e. without sending
> malicious data to the application).
>
>
> Theory
> ======
>
> In connection oriented security, the authentication is associated
> with the TCP connection, rather than to the individual HTTP requests
> it transports. As a result, a proxy server that shares a TCP
> connection to the server among 2 clients may jeopardize the security
> of the web application by sending a first request (or a set of
> requests) with authentication/authorization credentials from the
> first client, followed by a request with no credentials from the
> second client, and have the web server associate the privileges of
> the first request with the second request.
>
> NTLM authentication is an example to such connection-oriented
> security scheme.
> From http://curl.haxx.se/rfc/ntlm.html#ntlmHttpAuthentication
> (lacking official Microsoft specification, this resource is one of
> the most comprehensive descriptions of NTLM authentication):
>
> This [HTTP NTLM authentication] scheme differs from most "normal"
> HTTP authentication mechanisms, in that subsequent requests over
> the authenticated connection are not themselves authenticated;
> NTLM is connection-oriented, rather than request-oriented. So a
> second request for "/index.html" would not carry any
> authentication information, and the server would request none.
>
> This attack is possible because:
>
> 1. Proxy servers share the same TCP connection to the server, among
> several clients. This enables several attacks (on top of the one
> described here), as discussed in "Meanwhile, on the other side of
> the web server".
>
> 2. Connection-oriented security is an insecure concept because
> there's no guarantee in the HTTP RFC that a single connection will
> be used by a single entity. As can be seen, this simply doesn't
> hold. Note that SSL is not connection-oriented security since each
> request is encrypted with a secret, shared key, making this protocol
> implicitly request-oriented.
>
>
> Results
> =======
>
> I tested this security issue with Microsoft IIS/6.0 (as the web
> server that requires NTLM authentication – "Integrated Windows
> Authentication" in Microsoft's IIS GUI terminology) and Sun
> Microsystems Sun Java System Web Proxy 4 (as the proxy server that
> shares TCP connections to the same server).
>
> There are some tricky points in making this attack work:
>
> 1. Microsoft IE 6.0 refuses to conduct NTLM authentication when it
> is configured to use a forward proxy. Therefore, the setup used was
> with the Sun Proxy as a reverse proxy.
>
> 2. Microsoft IIS/6.0 does not induce the authentication level of a
> request to the whole connection, if the HTTP request contains the
> Via header. The Sun Proxy server sends this header by default (is
> there a way to turn this off?), and so, in order to strip it off, an
> Apache 2.0.54 reverse proxy server (with ProxyVia Block directive)
> was introduced between the Sun Proxy server and the IIS server.
>
> After these tweaks, both IE 6.0 and Mozilla 1.4 were used to
> demonstrate the attack:
>
> In the first step, a browser was used to authenticate to the IIS/6.0
> (through the Sun Proxy and the Apache proxy). The authentication was
> done in NTLM. Since the Apache proxy removed the Via header, the
> IIS/6.0 induced the authentication credentials on the whole TCP
> connection.
>
> In the second step, a different client was used to access a
> restricted resource on the IIS/6.0 through the Sun proxy (and the
> Apache proxy). The Sun Proxy used the same TCP connection to the
> Apache as it used for the first request, and likewise, the Apache
> used the same connection to the IIS/6.0 as it used for the first
> request, and therefore the credentials of the first request were
> successfully induced onto the second request, although it arrived
> from a different client on a different TCP connection (from the
> client to the Sun Proxy).
>
>
> Scope of the attack
> ===================
>
> *) Not all proxy servers honor NTLM authentication. Squid, for one,
> deliberately doesn't support NTLM
> (http://www.squid-cache.org/Doc/FAQ/FAQ-11.html#ss11.14). Indeed,
> Squid seems to strip off the WWW-Authenticate header if it contains
> NTLM or Negotiate, thereby effectively disabling NTLM authentication
> between the client and the web server. But as mentioned above, there
> are some proxy servers that do support NTLM authentication, such as
> Sun Proxy 4.
>
> *) Not all proxy servers share TCP connection to the server. Many
> do,some don't (e.g. Apache 2.0 mod_proxy).
>
> *) If IE is to be tricked, then it mustn't be configured with a
> forward proxy server. That means that the attack is effective for IE
> (only) with transparent proxy servers (such as ones used by many
> ISPs), and reverse proxy servers (as demonstrated above). The
> Mozilla browser has no such inhibitions, and therefore, a Mozilla
> shop (e.g. some universities and open source organizations) may be
> more vulnerable.
>
> *) The web server (IIS/6.0) must receive a Via-less request. The
> Microsoft implementation assumes that the Via header is always sent
> by a proxy server, and this is indeed mandated by the HTTP/1.1 RFC
> 2616 (http://www.ietf.org/rfc/rfc2616.txt), section 14.45:
> The Via general-header field MUST be used by gateways and proxies
> to indicate the intermediate protocols and recipients between the
> user agent and the server on requests [...]
> However, it seems that not all servers adhere to this standard. For
> example, Apache 2.0.54 mod_proxy does not generate a Via header by
> default (see the ProxyVia directive -
> http://httpd.apache.org/docs-2.0/mod/mod_proxy.html#proxyvia, yet
> the default httpd.conf file contains a commented-out "ProxyVia On"
> directive, so it's possible that many Apache proxy deployments do
> send the Via header). That isn't to say that Apache 2.0.54 mod_proxy
> facilitates this attack – as mentioned above, it does not, because
> it does not share the connection to the server among several clients.
> Anyway, there are many "anonymous" proxy servers in the Internet,
> which deliberately do not send the Via header, ironically with the
> intention to increase the privacy of their users. And there are many
> other devices and configurations that may remove the Via header if
> it exists (in the above example, I introduced the Apache proxy
> server to do just that).
>
> *) Last but not least - NTLM authentication should be used, and over
> HTTP (not over HTTPS). This is the default configuration of
> Microsoft Outlook Web Access 2000/2003.
>
>
> Recommendations
> ===============
>
> *) Proxy vendors – do not to share TCP connections to the server
> among several clients. Yes, it improves performance, but it's also
> insecure and enables/aids 3 different attacks (the one described
> here, HTTP Request Smuggling and HTTP Response Splitting).
> Also, comply to the RFC and send the HTTP Via request header by
> default (Apache Group - please take note).
>
> *) Designers of protocols past, present and future – do not rely on
> TCP connection being used by a single logical entity. As a special
> case, NTLM should be withdrawn or redesigned (OK, this won't
> happen...). Also, do not rely on the Via header (or any other
> header) to indicate that the client is a proxy server. Design the
> protocol such that it will be indifferent to whether the client is a
> proxy server or a browser.
>
> *) Site owners – abandon NTLM authentication in favor of other
> authentication/authorization options (e.g. HTTP digest
> authentication – see RFC 2617 –
> http://www.ietf.org/rfc/rfc2617.txt).
> Alternatively, use NTLM over HTTPS (SSL) to avoid this
> vulnerability, but make sure that the SSL is terminated on the web
> server, not some SSL accelerator (which may in itself facilitate the
> attack, e.g. if it shares a TCP connection to the server among
> several clients).
> Another alternative is to configure the web server not to use
> persistent HTTP connections for resources that are protected by NTLM
> autnehtication.
>
> *) Proxy owners – in order to protect your clients and your clients'
> privacy: do not turn off generating the "Via" HTTP request header by
> the proxy server. True, it indicates that the request comes from a
> proxy server, but in the case of NTLM authentication, it increases
> the likelihood of the client not to be subject to the attack
> described here. If possible, turn off TCP connection sharing in your
> proxy server. If none of this is possible, consider actively
> disrupting NTLM authentication, in order to force your clients to
> use other (hopefully more secure) authentication methods.
>
>
> A note about detection/prevention
> =================================
>
> Since the attacker's request is practically identical to the request
> sent by the authenticated user, it's quite a problem for an external
> product (such as IDS/IPS/WAF) to detect this attack.
>
> Of course, if the IDS/IPS/WAF is between the web-server and the proxy,
> it stands very little chance to detect that something's wrong, since
> the attacker's request is practically identical to the valid user's
> requests. However, it can block the attack simply by (gracefully, if
> possible) closing the TCP connection after a successful response (i.e.
> not 401) for a request containing NTLM authentication.
>
> If the proxy server is on site, and the IDS/IPS/WAF is in front
> of it, then protection becomes harder – the IDS/IPS/WAF would have to
> replace the NTLM authentication of the server with its own, and
> practically replicate the logic from the web-server to itself, in
> order to ensure that a request without credentials is made only to a
> resource which is public.
>
> It's also not too trivial to automatically scan for this kind of
> vulnerability. A scanner would have to be positioned in front of the
> proxy server (which may be away from the site), and would have to
> simulate the attack using two TCP connections.
>
>
> A note about basic authentication in IIS/5.0
> ============================================
>
> If memory serves, and peculiarly enough, awhile ago Ronen Heled,
> Chaim Linhart and me bumped into an implementation quirk of IIS/5.0
> wherein HTTP basic authentication seems to be also connection
> oriented, that is, if the TCP connection had already transmitted an
> HTTP request with valid Authorization header, the credentials are
> used for the next requests (on this TCP connection) even if these do
> not contain the Authorization header. Here too, the presence of the
> Via HTTP request header turns off the connection-orientedness.
> Again – this is something we noted awhile ago as a byproduct of a
> research in a different direction, and since I have no solid
> evidence, I am reluctant to point at it as a vulnerability. If
> someone can verify this on IIS/5.0 (I didn't manage to replicate it
> on IIS/6.0), please step foreward...
Received on Wed Jul 20 2005 - 01:26:06 MDT