How JavaScript Restrictor prevents other parties from sniffing on your local applications?

We recently found a blog post about ThreatMetrix Inc. (a part of LexisNexis) scanning locally open ports for about 30,000 web sites, including eBay. The figure below shows that a browser tries to connect to ports commonly used for remote access to the computer (e.g., RDesktop, VNC, TeamViewer) and other applications.

A screenshot of the browser being used as a proxy to scan locally open ports

The obvious question is, what is the reason for such behaviour? The simple answer is security. See additional links to Security Boulevard, Avast, and The register.

One possibility is that ThreatMetrix creates a fingerprint, and locally running applications are a part of the fingerprint. Consequently, the authentication algorithm stores attributes about your device(s) and compare them during each log in with the previous values. Seeing that you are logging in using a previously seen device, the algorithm can let you in with just a password without additional proves. However, should you use a new device, the algorithm might decide that additional authentication steps are required and send you an SMS.

Another option is that ThreatMetrix knows that many fraudulent activities occur on devices with specific ports open. Recall that the ports being checked concern remote desktop access. Having a remote desktop port open means that the computer may be used by an adversary that does not sit near the computer but is connected remotely. Consequently, the authentication algorithm might decide that additional proves about the user identity should be checked.

We do not know what the real reason behind the scanning is. It might be one of the above, both, or a similar reason.

Although it could be that the underlying intentions are benign and users actually do benefit from the scanning, the scanning raises some ethical issues.

Very often, security and privacy are interconnected. But sometimes, one might increase security by revealing something private. In this case, ThreatMetrix learns information about the running device that is not obvious to the device owner (a user or a company). Typically, the owner of the device does not even know that such information can leak. If the information stays with ThreatMetrix, then the benefits could appear to be greater than the disadvantages. However, adversaries could stole information from ThreatMetrix (see for example the Ecquifax breach) or the company can start to sell the information or even share with others.

So is the scanning and data collecting legal? As we are based in the EU, we will dig into the EU perspective. You might want to consult your local laws if you are outside the EU. Moreover, as we are not lawyers, you might want to consult one even in the EU.

EU ePrivacy Directive applies. However, as WP29 clarified (use case 7.5), user-centric security can be viewed as strictly necessary to provide the service. So it seems likely that port scanning for security reasons would trigger the ePrivacy exception and user consent is not necessary.

As the port scanning is a part of the login mechanism, open ports are personal data without doubts. So GDPR also applies. GDPR also list security as a possible legitimate interest of a data controller (e.g. eBay), see recital 49. Nevertheless, if such a scan is proportionate is an open question; it is possible that the legitimate interests of data controllers (such as eBay) are overriden by the interests or fundamental rights and freedoms of the data subject (you), see Article(6)(1)(f). The Court of Justice of EU (CJEU) decided several issues that concerned legitimate interests and the necessity of processing, e.g. C-13/16, point 30 that also points to other related cases or C-708/18 points 40–45. It might be possible that it is strictly necessary for eBay to perform local port scanning.

Nevertheless, Article 12-14 of GDPR lists requirements on the information that a data controller should reveal to each data subject before the data processing starts or in a reasonable time afterwards. Hence, each controller employing ThreatMetrix should reveal, for example, in the privacy policy, what categories of data it is using and for which purposes. From the linked articles, it seems that ThreatMetrix and eBay are secretive about data being collected.

Another GDPR issue might be data transfers to third countries. Data transfers of open ports may not be compatible with GDPR in the light of the CJEU C-311/18 decision if the information leaves EEA.

Why is not my browser protecting me from remote servers accessing local information?

OK, so even though the scanning could be legal, one can disagree that others should be allowed to sniff on local applications. So why does a browser leak the information?

Well, the browser employs so called same origin policy (SOP) that in abstract theory should prevent websites from the scans in question. As your local computer is of a different origin from the remote website, your computer should be protected by SOP. Nevertheless, SOP has its limitations. First of all, some cross-origin resource sharing is beneficial, so the browser cannot block outgoing requests to other origins. Such behaviour opens possibilities for side-channels to be identified. So even though the web page cannot communicate with applications on your computer (or in your network) without the cooperation of these applications, it can observe the behaviour and make some conclusions based on the observed errors, timing, etc.

An (ad) blocker can protect you from the activity. As the blockers typically leverage blocklists, such a port scanning script URL needs to match a rule in a block list. Once information about a misbehaving script becomes public, a rule can be added to a block list. However, blockers do not protect in the time span between the deployment and rule update at your computer. Moreover, additional techniques like DNS de-cloaking need to be applied in this case. Not all blockers (can) use DNS de-cloacking.

Network Boundary Shield to the rescue

JSR contains a Network Boundary Shield (NBS) that blocks outgoing browser requests based on the observed behaviour, i.e. a page hosted on public internet tries to access local URLs. NBS just works and cannot be fooled by changes in the URL path, DNS cloaking or other techniques.

JSR blocks the scan

Firefox contains DNS API, so NBS works flawlessly. In Chromium-based browsers, the exact blocking behaviour depends on how quickly a scanning script can fire the requests and the precise destination (IP address or a domain name). Depending on the interaction with DNS, NBS can be side-stepped on Chrome. In this case, ThreatMetrix does not try any evasion technique, so NBS just works in the case of eBay and ThreatMetrix.