Hanno's blog

tl;dr PrivDog will send webpage URLs you surf to a server owned by AdTrustMedia. This happened unencrypted in cleartext HTTP. This is true for both the version that is shipped with some Comodo products and the standalone version from the PrivDog webpage.

On Sunday I wrote here that the software PrivDog had a severe security issue that compromised the security of HTTPS connections. In the meantime PrivDog has published an advisory and an update for their software. I had a look at the updated version. While I haven't found any further obvious issues in the TLS certificate validation I found others that I find worrying.

Let me quickly recap what PrivDog is all about. The webpage claims: "PrivDog protects your privacy while browsing the web and more!" What PrivDog does technically is to detect ads it considers as bad and replace them with ads delivered by AdTrustMedia, the company behind PrivDog.

I had a look at the network traffic from a system using PrivDog. It sent some JSON-encoded data to the url http://ads.adtrustmedia.com/safecheck.php. The sent data looks like this:

{"method": "register_url", "url": "https:\/\/blog.hboeck.de\/serendipity_admin.php?serendipity[adminModule]=logout", "user_guid": "686F27D9580CF2CDA8F6D4843DC79BA1", "referrer": "https://blog.hboeck.de/serendipity_admin.php", "af": 661013, "bi": 661, "pv": "3.0.105.0", "ts": 1424914287827}
{"method": "register_url", "url": "https:\/\/blog.hboeck.de\/serendipity_admin.php", "user_guid": "686F27D9580CF2CDA8F6D4843DC79BA1", "referrer": "https://blog.hboeck.de/serendipity_admin.php", "af": 661013, "bi": 661, "pv": "3.0.105.0", "ts": 1424914313848}
{"method": "register_url", "url": "https:\/\/blog.hboeck.de\/serendipity_admin.php?serendipity[adminModule]=entries&serendipity[adminAction]=editSelect", "user_guid": "686F27D9580CF2CDA8F6D4843DC79BA1", "referrer": "https://blog.hboeck.de/serendipity_admin.php", "af": 661013, "bi": 661, "pv": "3.0.105.0", "ts": 1424914316235}

And from another try with the browser plugin variant shipped with Comodo Internet Security:

{"method":"register_url","url":"https:\\/\\/www.facebook.com\\/?_rdr","user_guid":"686F27D9580CF2CDA8F6D4843DC79BA1","referrer":""}
{"method":"register_url","url":"https:\\/\\/www.facebook.com\\/login.php?login_attempt=1","user_guid":"686F27D9580CF2CDA8F6D4843DC79BA1","referrer":"https:\\/\\/www.facebook.com\\/?_rdr"}

On a linux router or host system this could be tested with a command like tcpdump -A dst ads.adtrustmedia.com|grep register_url. (I was unable to do the same on the affected system with the windows version of tcpdump, I'm not sure why.)

Now here is the troubling part: The URLs I surf to are all sent to a server owned by AdTrustMedia. As you can see in this example these are HTTPS-protected URLs, some of them from the internal backend of my blog. In my tests all URLs the user surfed to were sent, sometimes with some delay, but not URLs of objects like iframes or images.

This is worrying for various reasons. First of all with this data AdTrustMedia could create a profile of users including all the webpages the user surfs to. Given that the company advertises this product as a privacy tool this is especially troubling, because quite obviously this harms your privacy.

This communication happened in clear text, even for URLs that are HTTPS. HTTPS does not protect metadata and a passive observer of the network traffic can always see which domains a user surfs to. But what HTTPS does encrypt is the exact URL a user is calling. Sometimes the URL can contain security sensitive data like session ids or security tokens. With PrivDog installed the HTTPS URL was no longer protected, because it was sent in cleartext through the net.

The TLS certificate validation issue was only present in the standalone version of PrivDog and not the version that is bundled with Comodo Internet Security as part of the Chromodo browser. However this new issue of sending URLs to an AdTrustMedia server was present in both the standalone and the bundled version.

I have asked PrivDog for a statement: "In accordance with our privacy policy all data sent is anonymous and we do not store any personally identifiable information. The API is utilized to help us prevent fraud of various types including click fraud which is a serious problem on the Internet. This allows us to identify automated bots and other threats. The data is also used to improve user experience and enables the system to deliver users an improved and more appropriate ad experience." They also said that they will update the configuration of clients to use HTTPS instead of HTTP to transmit the data.

PrivDog made further HTTP calls. Sometimes it fetched Javascript and iframes from the server trustedads.adtrustmedia.com. By manipulating these I was able to inject Javascript into webpages. However I have only experienced this with HTTP webpages. This by itself doesn't open up security issues, because an attacker able to control network traffic is already able to manipulate the content of HTTP webpages and can therefore inject JavaScript anyway. There are also other unencrypted HTTP requests to AdTrustMedia servers transmitting JSON data where I don't know what their exact meaning is.

tl;dr There is a software called Privdog. It totally breaks HTTPS security in a similar way as Superfish.

In case you haven't heard it the past days an Adware called Superfish made headlines. It was preinstalled on Lenovo laptops and it is bad: It totally breaks the security of HTTPS connections. The story became bigger when it became clear that a lot of other software packages were using the same technology Komodia with the same security risk.

What Superfish and other tools do is that it intercepts encrypted HTTPS traffic to insert Advertising on webpages. It does so by breaking the HTTPS encryption with a Man-in-the-Middle-attack, which is possible because it installs its own certificate into the operating system.

A number of people gathered in a chatroom and we noted a thread on Hacker News where someone asked whether a tool called PrivDog is like Superfish. PrivDog's functionality is to replace advertising in web pages with it's own advertising "from trusted sources". That by itself already sounds weird even without any security issues.

A quick analysis shows that it doesn't have the same flaw as Superfish, but it has another one which arguably is even bigger. While Superfish used the same certificate and key on all hosts PrivDog recreates a key/cert on every installation. However here comes the big flaw: PrivDog will intercept every certificate and replace it with one signed by its root key. And that means also certificates that weren't valid in the first place. It will turn your Browser into one that just accepts every HTTPS certificate out there, whether it's been signed by a certificate authority or not. We're still trying to figure out the details, but it looks pretty bad. (with some trickery you can do something similar on Superfish/Komodia, too)

There are some things that are completely weird. When one surfs to a webpage that has a self-signed certificate (really self-signed, not signed by an unknown CA) it adds another self-signed cert with 512 bit RSA into the root certificate store of Windows. All other certs get replaced by 1024 bit RSA certs signed by a locally created PrivDog CA.

US-CERT writes: "Adtrustmedia PrivDog is promoted by the Comodo Group, which is an organization that offers SSL certificates and authentication solutions." A variant of PrivDog that is not affected by this issue is shipped with products produced by Comodo (see below). This makes this case especially interesting because Comodo itself is a certificate authority (they had issues before). As ACLU technologist Christopher Soghoian points out on Twitter the founder of PrivDog is the CEO of Comodo. (See this blog post.)

We will try to collect information on this and other simliar software in a Wiki on Github. Discussions also happen on irc.ringoflightning.net #kekmodia.)

Thanks to Filippo, slipstream / raylee and others for all the analysis that has happened on this issue.

Update/Clarification: The dangerous TLS interception behaviour is part of the latest version of PrivDog 3.0.96.0, which can be downloaded from the PrivDog webpage. Comodo Internet Security bundles an earlier version of PrivDog that works with a browser extension, so it is not directly vulnerable to this threat. According to online sources PrivDog 3.0.96.0 was released in December 2014 and changed the TLS interception technology.

Update 2: Privdog published an Advisory.