filtering HTTPS

Sorry for the lengthy message. I could not address the issue with less words.

Filtering HTTPS is more difficult than filtering HTTPS because CONNECT is used
by various applications (browsers, Chat applications, tunnels, VPNs etc).
So CONNECT is used for 'regular' HTTPS sites who use SSL+HTTP and sites
that use other protocols, e.g. SSL+XMPP, Skype, SSH, and many others.

To filter CONNECT with Squid one can use ICAP or the URL rewritor.
I have written and published ufdbGuard, a free URL rewritor, and
I am developing a new ICAP-based filter which is not yet published.
The problem with filtering CONNECT/HTTPS exists for both filtering solutions
and differ only in a few details.

The Squid administrator can choose to use the sslBump feature.
Although sslBump enables filters to look at content and see complete URLs
there is still a major issue: applications that use CONNECT to
access sites that do not use SSL+HTTP (https).

Issue 1: one cannot block CONNECT in an elegant way. I.e. a CONNECT
to an undesired site cannot be redirected or anything since the
application (possibly browser) want do a SSL handshake and it it fails
it displays the 'vague error' "cannot connect to site www.example.com"
which is indeed vague for an end user who usually only understands
messages like "you are not authorised to go to www.example.com".

For true SSL+HTTP (https) sites, issue 1 can be resolved by *not* blocking
the CONNECT and wait for the next "GET https://www.example.com/index.html"
and block/redirect this object. Lets call this a 'postponed SSL+HTTP block'.
But for sites which do not use SSL+HTTP there is not a good solution since
Squid and the URL redirector only see a CONNECT and never see a GET/HEAD/POST.

Issue 2: Skype does not work any more with sslBump. SSH tunnels, VPNs and other
chat applications also stop working with sslBump since the sslBump feature
does its SSL certificate checking and if this fails, the CONNECT fails.
Using the options 'sslproxy_cert_error allow all' and
'sslproxy_flags DONT_VERIFY_PEER' is not considered useful since thay are
truely very unsafe and I recommend never to use them.

More background information:
The URL redirector ufdbGuard has a feature to probe HTTPS connections.
It does a SSL-handshake if this works it is followed by "GET / HTTP/1.0"
If the SSL-handshake does not work it probes for SSH, Skype and other
chat protocols to find out where the the application CONNECTs to.
ufdbGuard can block CONNECT to IP addresses but make exceptions
for the CONNECTs which are used by allowed chat protocols.
SSH and VPNs are blocked by ufdbGuard if the administrator has configured
to block proxies.

HTTPS is used more and more. Even Google uses it for their search engine.
It is necessary to have a safe HTTPS proxy and content filtering in an
absolutely safe and efficient way.

Proposal:
To have a good combination of web proxy and content filtering combination
I propose the following:
A) Squid's behaviour is modified for sslBump: after an unsuccessful SSL
handshake, the CONNECT does not fail any more by default. This is to ensure
that Skype et al. remains functional.
B) Squid gets a new option to define its behaviour in case the SSL handshake
fails. The options could be called sslBumpForNoneSSL with values
prohibitNoneSSL (terminate connection), passNoneSSL (always allow),
filterNoneSSL (default value - let ICAP or URL rewritor decide).
C) Squid notifies the URL rewritor and ICAP server about the result of
the SSL handshake. This is to optimise the filters and not do things twice.
Web servers do no like probes and may temporarily block sites that use Squid
if they receive too many probes, so the least number of probes the better.
I.e. the line sent to the URL redirector is extended with a new flag
like "SSLhandshake=(verified|noSSL)". This should not break existing URL
redirectors since it already has the variable length urlgroup and most URL
redirectors will consider the new flag part of the urlgroup.
Probably a few URL redirectors need a minor modification.
For ICAP Squid could send a new header called X-Squid-SSLhandshakeResult.
D) squid.conf.documented, wiki and other documentation is updated that
'sslproxy_flags DONT_VERIFY_PEER' and 'sslproxy_cert_error allow all' are
unsafe and not recommended.
E) the option 'squid-uses-ssl-bump' is introduced to ufdbGuard. If set to
'yes' it will not verify the use of proper SSL certificates.
If Squid can send the new flag SSLhandshake (URL redirector) or
X-Squid-SSLhandshakeResult (ICAP server), the URL redirector and ICAP
servers can be optimised further.

Marcus Kool
Received on Tue Mar 13 2012 - 13:34:01 MDT