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At work my company uses internet monitoring software (websense). I know if I visit a https ssl-encrypted site (such as https://secure.logmein.com) they can't see what I'm doing on the site since all the traffic is encrypted. But do they see, that I visited https://secure.logmein.com ?

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8 Answers

up vote 29 down vote accepted

An encrypted connection is established first before any HTTP requests are performed (e.g. GET, POST, HEAD, etc.), but the hostname and port are visible.

There are many other ways to detect which sites you’re visiting as well, for example:

  • your DNS queries (i.e. they’ll see the IP request for secure.logmein.com)
  • via network monitoring (e.g. netflow, IP to IP sessions, sniffing, etc.)
  • if the device you are working on is owned by the company and they have administrator access/privileges to view anything on the device (e.g. view your browser caches)

A popular way to evade a websense proxy is to first establish a connection (via HTTPS:) to an outside proxy (e.g. https://proxy.org/) and make your request from there.

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Strictly speaking, the encrypted connection is created by sending a CONNECT http request, but that's not a "regular" HTTP request... –  AviD Apr 17 '11 at 6:34
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Simple Proxy Servers

Even a simple proxy will see and log the names of the servers. For example visiting https://example.com/some/address.html will create a request like this from the browser to the proxy server:

CONNECT example.org:443 HTTP/1.1
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:2.0b13pre) ...
Proxy-Connection: keep-alive
Host: example.org

The rest of the connection is encrypted and a simple proxy just forwards it.

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Complex Proxy Servers

There are, however, more complex proxy servers, that are able to see the complete traffic in plain text. These kinds of proxy servers, however, require that you have a root certificate installed for which they can create server certificates on the fly.

Looking at the certificate chain in the browser usually reveals this kind of man in the middle attack. At least in the common case of it being done by your own company and not state agencies:

Proxy in the Middle with custom root certificate

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It is possible, but it requires some setup. Here is how it is done, and how you can tell.

On a corporate computer, where software updates are pushed from a central location, it is possible to send to your computer a "trusted" certificate that will be stored next to the trusted certificate of say, Verising or Entrust.

Your company's proxy will hold the private key of that certificate.

When you visite a HTTPS web site, like https://mybank.com/, the proxy will put itself in the middle. It will establish a HTTPS connection with your browser generating on the fly a certificate for mybank.com. It will replay (and possibly monitor or log) all you traffic on a new connexion, from the proxy to mybank.com.

You can tell if this is the case by looking at the padlock icon. If you see that the certificate for mybank.com was issued by acmesprockets.com (your company), then you know they can see your "encrypted" traffic. You can also check with a computer on another network (at home) and compare the results.

I know that BlueCoat supports that configuration, and I'm sure others do, too.

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The host name can be seen by any proxy without special setup. –  Hendrik Brummermann Apr 6 '11 at 6:28
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Why acmesprockets.com? Why not call themselves VeriSign? ;) –  Calmarius Jul 27 '12 at 14:08
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Yes, indeed ! You would have to compare fingerprint and such wich is very very unlikely to happen. –  ixe013 Jul 30 '12 at 11:53
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If websense is configured to log it, then yes, they will be able to see where you went, all the URLs you visit.

Content is less likely to be viewed - it depends on how websense/proxy is set up - but it can be done. It depends whether the SSL session is from your browser to the server or if it is just to the proxy (effectively running a man in the middle attack)

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i don't know how my company has websense setup but i doubt they can read the contents of traffic sent if i was in SSL-encrypted gmail, correct? –  IAmARegisteredUser Apr 5 '11 at 21:15
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@IAmARegisteredUser - As Rory said in his answer, it depends on how it is set up. It's unlikely, but it is indeed possible. –  Xander Apr 5 '11 at 22:31
    
If websense is set up to run a man-in-the-middle attack on you, won't you see certificate errors when you connect to the site? If you don't see a certificate error, it seems plausible to me that websense can't see the URLs or content you view. Is that inaccurate? –  D.W. Apr 6 '11 at 1:33
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@D.W. it depends, some corporate web proxying setups (eg, as @atdre mentions below, Bluecoat) involve CA certs being installed n the users browsers, with the proxy then creating new certs as needed so it can do content inspection on SSL enabled connections. –  Rоry McCune Apr 6 '11 at 6:52
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They can see all of your SSL traffic unencrypted if they use a BlueCoat proxy or similar. Many large Enterprises do this.

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... and if they have pre-installed their own CA into your browser - which makes this workable in a corporate setting, where the IT is in control over client machines. (otherwise the users would be seeing SSL errors left, right, and center, which is a strong indication of a MITM attack in progress) –  Piskvor May 22 '11 at 22:17
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Following on Guillaume's answer (sorry, not enough reputation to comment), another way to check for foul play is to use the "Perspectives" Firefox add-on. When visiting an https site it checks (through Internet "notaries") that the public key you receive (via the web server certificate) does indeed belong to the site you're visiting.

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Of course, this exposes you to the possibility that your visits to secure sites will be tracked by yet another party, even though Perspectives promises to not do so, or even record your IP addr. –  nealmcb Apr 15 '11 at 15:56
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With HTTPS, the SSL/TLS tunnel is established first, and HTTP traffic happens only within that tunnel. Some information still leaks:

  • If the client uses a proxy, the connection to the proxy looks like: CONNECT www.example.com:443 with the target server name. Alternatively, the client could send the target server IP address, but this is only marginally less revealing; and, to know the server IP address, the client must do some name resolution, using the DNS servers provided by the company itself.

  • Recent enough clients will send the target server name as part of the initial SSL handshake (that's the Server Name Indication extension).

  • The server responds by sending its certificate back, which includes, in plain view and by definition, the server name.

From these we can conclude that the target server name is definitely not a secret. You may assume that your company learns it.

The rest of the communication is encrypted so it is nominally inaccessible from outsiders. However, the length of the data packets which are sent and received by the client can still be inferred by any eavesdropper (with single byte accuracy if a RC4 cipher suite is used), and this can also reveal a lot of information, depending on context.

If your company is serious about security then it may have installed a more advanced proxy like Blue Coat's ProxySG. Such systems perform a Man-in-the-Middle attack by dynamically generating a fake certificate for the target server. This gives them access to the complete data, as if there was no SSL.

Note that, however, such interception is possible only if the company could add to the trust store of your desktop system the root CA certificate that the proxy uses to issue the fake certificates. This is a rather intrusive action. Therefore, if they could do that, why would they stop there ? They may have inserted, just as easily, a handful of spying software which will plug in your Web browser, your keyboard and your display; and everything you do on the machine is known to them.

Alternatively, if you can make sure that your machine is free from any interference from your company (e.g. it is your own device and you installed no company-provided software on it), then MitM-proxy cannot decrypt your SSL connections.

A very simple way to hide your traffic from your company is not to use their facilities at all. Bring your own laptop with a 3G key (or tethered to your smartphone). By paying for your own Internet, you can evade network-based detection and spend your days roaming the Web instead of doing the work you are paid to do (but, of course, detection of slackers has never been restricted to only using computerized gizmos).

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When in doubt, use the smart-phone bypass. Freedom often has costs that are worth paying. –  Fiasco Labs Apr 25 '13 at 6:05
    
Another great answer from Thomas Pornin! This should be the accepted answer now. –  Rahil Arora Feb 7 at 16:08
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Not just from certificate but also from handshake messages servername information can be get. As I tested %80 of the traffic contains server_name extension in client hello message(first message send by client to server in https protocol) . But this extension is optional and sometimes it does not exist .In this case certificate can be checked. In certificate there exist server name again .

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