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Multiple Tor security issues disclosed, more to come


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Image: Tor Project

Over the past week, a security researcher has published technical details about two vulnerabilities impacting the Tor network and the Tor browser.

In blog posts last week and today, Dr. Neal Krawetz said he was going public with details on two alleged zero-days after the Tor Project has repeatedly failed to address multiple security issues he reported throughout the past years.

The researcher also promised to reveal at least three more Tor zero-days, including one that can reveal the real-world IP address of Tor servers.

Approached for comment on Dr. Krawetz’s intentions, the Tor Project did not reply to a request for comment and provide additional details on its stance on the matter.

The first Tor security issue

Dr. Krawetz, who operates multiple Tor nodes himself and has a long history of finding and reporting Tor bugs, disclosed the first Tor security issue last week.

In a blog post dated July 23, the researcher described how companies and internet service providers could block users from connecting to the Tor network by scanning network connections for “a distinct packet signature” that is unique to Tor traffic.

The packet could be used as a way to block Tor connections from initiating and effectively ban Tor altogether — an issue that oppressive regimes are very likely to abuse.

The second Tor security issue

Earlier today, in a blog post shared with ZDNet, Dr. Krawetz disclosed a second issue. This one, like the first, allows network operators to detect Tor traffic.

However, while the first issue could be used to detect direct connections to the Tor network (to Tor guard nodes), the second one can be used to detect indirect connections.

These are connections that users make to Tor bridges, a special type of entry points into the Tor network that can be used when companies and ISPs block direct access to the Tor network.

Tor bridges act as proxy points and relay connections from the user to the Tor network itself. Because they are sensitive Tor servers, the list of Tor bridges is being constantly updated to make it difficult for ISPs to block it.

But Dr. Krawetz says connections to Tor bridges can be easily detected, as well, using a similar technique of tracking specific TCP packets.

“Between my previous blog entry and this one, you now have everything you need to enforce the policy [of blocking Tor on a network] with a real-time stateful packet inspection system. You can stop all of your users from connecting to the Tor network, whether they connect directly or use a bridge,” Dr. Krawetz said.

Both issues are specifically concerning for Tor users residing in countries with oppressive regimes.

Dissatisfaction towards the Tor Project’s security stance

The reason why Dr. Krawetz is publishing these issues in Tor is that he believes the Tor Project does not take the security of its networks, tools, and users seriously enough.

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The security researcher cites previous incidents when he tried to report bugs to the Tor Project only to be told that they were aware of the issue, working on a fix, but never actually deploying said fix. This includes:

  • A bug that allows websites to detect and fingerprint Tor browser users by the width of their scrollbar, which the Tor Project has known about since at least June 2017.
  • A bug that allows network adversaries to detect Tor bridge servers using their OR (Onion routing) port, reported eight years ago.
  • A bug that lets attackers identify the SSL library used by Tor servers, reported on December 27, 2017.

All of these issues are still not fixed, which has led Dr. Krawetz in early June 2020 to abandon his collaboration with the Tor Project and take the current approach of publicly shaming the company into taking action.


Updated at 20:30 ET, July 30:

The Tor Project has responded to Dr. Krawetz’ two blog posts in a public tweet. In summary, the Tor Project’s reply is that they are aware of the issues the researcher reported, but they differ on the threats they pose to users, claiming they can’t be enforced at scale. The Tor Project also disagreed with Dr. Krawetz’ classification of the issues he detailed on the blog as zero-days. The title has been updated accordingly.

Updated at 18:50 ET, August 2:

Dr. Krawetz published a counter-reply to the Tor Project’s response.



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I'm a 50 year old PLC programmer from Burnley, UK. I severed my time as an electrician in the baking industry and soon got involved with the up and coming technology of PLC's. Initially this was all based in the Uk but as the years went by I have gradually worked my way around the globe. At first it was mainly Mitsubishi with a bit of Modicon thrown in but these days the industry leaders seem to be the Allen Bradley range of PLC and HMI’s.

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