NTLM HTTP Authentication
(and possibly other connection-oriented
HTTP authentication and authorization protocols)
is insecure by design
NTLM Authentication and HTTP proxies
Amit Klein, July 2005
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
HTTP Response Splitting
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).
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
(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
I tested this security issue with Microsoft IIS/6.0 (as the web
server that requires NTLM authentication =96 "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
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
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
*) 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 -
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 =96 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.
*) Proxy vendors =96 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 =96 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 =96 abandon NTLM authentication in favor of other
authentication/authorization options (e.g. HTTP digest
authentication =96 see RFC 2617 =96
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
Another alternative is to configure the web server not to use
persistent HTTP connections for resources that are protected by NTLM
*) Proxy owners =96 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 =96 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 =96 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...