On 8 May 2006 at 16:01, Zaninotti, Thiago wrote:
Folks,
During some specific tests with our upcoming Web App Security Scanner tool,
we have found that Apache would kindly accept HTML injection through
"Expect" header. Originally meant to be a protocol flow control that would
give web client the capacity of sending the HTTP headers for server's
pre-analysis before submitting the rest of the payload (body), Expect header
is not usually used by common user's client-side software (for more details
see section 8.2.6 from RFC 2616 - http://www.ietf.org/rfc/rfc2616.txt).
Dear Lists,
Thiago Zaninotti (the author of the original message - ) and me worked a bit
more on
some aspects of
this phenomena. Here are our results:
In Apache 2.0.x and 2.2.x, the 417 response will be sent by the server
only upon
connection termination. Such termination can be forced by the client (e.g.
closing the browser), or by
the server (after the timeout has elapsed, see the Timeout directive in
the httpd.conf,
typically its value is few minutes). So if you're testing this with
Apache, please be
patient, the response will take time to appear.
Things to keep in mind:
Regarding XSS
This phenomenon is not XSS per-se. Unless someone can show me how it is
possible to force a
browser to send the Expect header to the target site (other than via a
rogue browser plug-
in, which seems to me a bigger problem than XSS...). We did verify that
the XmlHttpRequest
object can send the "Expect" header simply via the setRequestHeader
method. Yet there is a
bootstrap problem using XHR since it is subject to the same origin
policy.
Regarding proxy servers
Proxy servers are not supposed to forward a request with "unexpected"
Expect value - see
RFC 2616 section 14.20. Indeed, Apache mod_proxy does not forward this
request. Yet Squid-
2.5.STABLE10 does seem to forward this request (but it probably doesn't
cache the response,
see below).
Regarding Cacheability
The 417 response is not supposed to be cached. Apache sends the response
without any cache-
related headers, and thus caching this response is in violation of the RFC
(RFC 2616
section 13.4). Indeed, IE 6.0 SP2 and FireFox 1.5.0.2 do not cache this
response, even when
a META tag is injected to introduce cache-related headers. Still, it's
possible that there
are cache servers out there that will cache this response.
Combining the above two notes, if there's a proxy cache server that
forwards the invalid
Expect header, AND caches the response, then this can be used to poison
this cache server
(e.g. an attacker sending a request to /index.html + Expect header, to the
vulnerable
Apache server through the proxy server). At the moment we're not aware of
a proxy server
that behaves this way.
Even without the above vector, this behavior "doesn't feel right",
security-wise.
- Thiago and Amit
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