In 2015, the FBI took down a website on the dark web called “Playpen” that was serving child pornography. They carried out this attack by seizing the physical server, which they were able to locate due to a misconfiguration of the server as a Tor hidden Service. They then hosted Playpen themselves and used an exploit in the Tor browser to install malware on the computers of visitors to the site. This investigation has led to charges being brought against over one hundred alleged users, but also raises concerns about the power of the FBI to break the anonymity that Tor is supposed to provide. This article analyzes both the technological and ethical issues surrounding this hack.

The superficial/sensational issues presented in the press.

In December 2014, the FBI received a tip from an undisclosed foreign intelligence agency that a Tor Hidden Service website called “Playpen” was hosting child pornography. The tip also stated that the server hosting Playpen had been misconfigured and it’s real IP address was publicly visible from outside the Tor network.[1]

In early 2015, the FBI began its formal investigation and tracked down the physical server hosting the site using the site’s IP address. They obtained a warrant and seized that server from a hosting service in North Carolina and returned it to FBI offices in Virginia. Then the FBI received another warrant for use in the Eastern District of Virginia to use some malware, which they call NIT (for “Network Investigative Technique”), they had developed to track individuals using the site. From February 20 through March 4, 2015, the FBI hosted the child pornography website from its offices in Newington, VA.[2]

During this time, the FBI NIT copied identifying information including the real IP addresses and MAC information from over 1300 computers[3] visiting the site from all over the world and sent it back to the FBI headquarters. As a result of the investigation, at least 137 cases have been brought against alleged users in the United States.[4]

The Tor Browser is actually based on a version of Mozilla’s Firefox Browser. In May 2015, Mozilla requested details of the vulnerability from the FBI so that it could protect its Firefox users from being exploited by this non-public vulnerability.[5] And to date, the FBI has refused to disclose the details of the exploit used.

News of this investigation has raised many ethical and legal issues. Was it legal for the FBI to use its warrant to infect computers outside the Eastern District of Virginia? Should the evidence be admissible in the court cases brought against alleged users? Was it ethical for the FBI to host child pornography from its offices as part of the sting operation?[6] Should users visiting or hosting illegal content on the internet be protected under the 4th amendment? Is it ethical for the FBI to withhold knowledge of a zero-day vulnerability within the Tor/Firefox browsers?[7] We hope this article provides the reader with enough background to formulate an opinion on these questions.

Tor Background

Tor, originally named “The Onion Routing”, is an an open source software and anonymity network. Originally developed by the US Naval Research Laboratory, Tor was used as a way to protect intelligence communication. Today, it is owned and maintained by the non-profit organization, The Tor Project Inc.

The two major components of Tor are the software and the network. Though tightly intertwined, the distinction is made to highlight that one cannot exist without the other. Tor software leverages a technique known as Onion Routing. Onion Routing encapsulates digital messages multiple layers of encryption that are decrypted or “peeled back” at each hop throughout the Tor network.


The diagram[8] above provides an example message transmitted from Alice (source) to Bob (destination) through the network. The different color cylinders represent unique layers of encryption that can only be decrypted by specific nodes. After the targeted recipient removes a layer, the next layer reveals instructions for which node to send it next. The first node Alice comes in contact with is the Guard (Router A). This node knows that it came from Alice and needs to go to Router B, but does not know the final destination of the packet. Once it arrives at the Middle Relay (Router B), the node does not know the origin of the message or the destination, but only that it came from Router A and needs to be passed along to Router C. The last stop within the Tor Network is the Exit node (Router C). It transmits the original message to the final destination.[9]

Encryption utilizes a stream cipher, specifically a 128-bit AES in counter mode with an initialization vector of all 0s. For a public-key cipher, Tor uses RSA with 1024-bit keys and a fixed exponent of 65537. It also uses OAEP-MGF1 padding with SHA-1 as its digest function.[10]

Tor Hidden Services Background

Tor Hidden Services act on the Tor network as websites do on the open internet, with the difference being that for a Tor Hidden Service, both the server’s location (IP address) and the client’s are anonymized through the Tor network[11]. Tor Hidden Services might also be used to host instant messaging services, allowing for real-time anonymized communication. The Tor Hidden Service Protocol, which we will summarize below, lays out the details of how this anonymization is preserved. It is important to note, however, that without proper configuration of the server, one’s location can be found, which is exactly what happened in the Playpen case.

To keep both parties anonymous, both the server and the client communicate through intermediate nodes, each connected to either side via a multi-step Tor relay, as described above, so that their location cannot be discovered from either end. To access a Tor Hidden Service, one must first learn it’s “onion” address (for example “xyz.onion”), which is similar to a URL. Onion addresses are only accessible for users of the Tor network.

Configuring a Tor Hidden Service is similar to setting up a web server on the open internet, with a few extra steps involved. First, one sets up a server in some way (say, using Python or Apache), and then hosts the server only to his or her local network. Then, to make it a Tor Hidden Service, the host (having previously installed Tor), edits a configuration file called “torrc”, providing information about the path to the server files and which IPs and ports the server uses.[12] Then, upon starting Tor, a private key is generated along with a hostname, which is the onion address mentioned above. The Tor network is then linked to the server, and though it is not accessible to the open internet, anyone using Tor who is provided with the onion address can now anonymously access this website.

Tor uses a distributed hash table to keep track of its database of Hidden Services[13]. This means that it’s possible for a user to check whether an onion address is valid or not by hashing it and checking it against the hash table. If someone has access to the hash table, however, it is not possible to determine the onion addresses contained in the database from the hashes alone.

The key step that the host of Playpen got wrong was in setting up the web server. Instead of only hosting the site to his local network, he instead had his computer and router setup to broadcast the site to the open internet, as a public IP address that anyone could visit if the IP address was guessed or stumbled upon.[14] According to the Electronic Frontier Foundation, a foreign law enforcement agency then tipped off the FBI about this misconfiguration, and the FBI was then able to find the location of the server. If the server had been set up correctly, the FBI would not have been able to seize it and carry out the next step of its attack–injecting malware to the users of Tor using an exploit in the Tor browser.

Best Practices for Using Tor/Tor Hidden Services

Tor is a powerful tool for protecting one’s privacy online, but it depends on proper cybersecurity hygiene on the user’s end as well. Tor’s website has a list of best practices for getting the most out of Tor, which include disabling browser plugins, using HTTPS versions of websites, not torrenting, and not opening documents while connected to the internet (especially DOC and PDF files). These suggestions, while sound, are not the limit of what the user can do to protect themself. In addition to basic security practices like keeping one’s computer up-to-date as software updates are released, one can also do things like disable Flash and JavaScript, use custom Linux distributions like Tails (which Edward Snowden uses), or set up a firewall to limit internet connections to anything other than Tor[15].

Motivation of FBI

The incentives in this case are pretty straightforward–the FBI wanted to take down a child pornography distribution ring. While their decision to host the content of Playpen on their own servers for some time might raise some ethical concerns, they could easily argue that if they did not maintain the facade of actually being the server, they wouldn’t have been able to find out the location of so many users. In this attack, the harm to the “victims” is that their identity was revealed, allowing them to be punished for their crimes.

Beneath the surface of the criminal investigation however there are some ethical issues that offer food for thought. By refusing to release the details of what exploit they found in the Tor Browser, one might argue that the FBI is leaving users of Mozilla Firefox exposed to a malicious attacker using the same exploit on innocent internet users.

Ethical Issues with Tor

In the popular media, Tor is often portrayed as the “dark web”, which can lead to negative connotations about its origin and intended usage. According to a prior version of the Tor Project Overview page, “Tor was originally designed, implemented, and deployed as a third-generation onion routing project of the U.S. Naval Research Laboratory. It was originally developed with the U.S. Navy in mind, for the primary purpose of protecting government communications.”[16]

However, when Tor launched publicly almost a decade later the project leader Roger Dingledine claimed that Tor was developed “with civil liberties in mind”.[17] Today, the dedicated section of Tor’s purpose to protect government communications has been replaced with the focus of “volunteer-operated servers that allows people to improve their privacy and security on the Internet.”[18] Journalists, political dissidents, whistleblowers and stalker victims may use Tor to help protect their online privacy through the shield of anonymity for noble and just reasons.

For example, during the ongoing Syrian Civil War, blogger Tariq Biasi was arrested and sentenced to three years in prison for “dwindling the national feeling” when he allegedly posted a critical comment about the regime. As censorship in Syria takes hold, other bloggers such as Anas Qtiesh can write and share their ideas safely behind the anonymity of a Tor Browser.[19]

Tor usage, however, is not always so righteous. The Tor community also consists of many users leveraging the anonymity of the service to engage in illicit behavior. According to a study by Daniel Moore, a PhD candidate in the Department of War Studies at King’s College London, almost 57% of active Tor Hidden Services linked to sites categorized as illicit. Of the 1,547 categorized illicit sites, 122 were labeled as illegitimate pornography (see image).[20]


Exit nodes are especially exposed members of the Tor network. Whether a user (“Alice”) accesses a site that is legal or illegal, the receiver (“Bob”) sees the incoming IP request coming from the Exit node of Tor relay, not Alice’s IP address. If Alice used Tor to browse cat videos, this isn’t an issue for the users acting as the Exit node. However, Exit nodes can face legal action due to illicit behavior stemming from their IP address. Luckily, as an Exit node, there is some ability to whitelist safe browsing services and blacklist nefarious ones, with Tor routing users accessing nefarious sites through exit nodes willing to assume such risk.[21] Though, if an Exit allows for more precarious services such as email or Hidden Services, it is best to prepare legally by taking measures such as notifying their ISP, setting up a separate IP for their own web traffic, or even creating a Limited Liability Corporation to run the Exit node.[22]

Rule 41 Legal Considerations

In order for the FBI to use the server they seized to go after users of the site, they applied for and were granted a second warrant under Rule

  1. The Federal Rules of Criminal Procedure govern how federal criminal prosecutions are carried out in US district courts and the general trial courts of the US government. In particular, Rule 41 determines the process through which warrants can be issued and how they may be used by federal law enforcement to conduct searches and seizures. At the time of the Playpen case, Rule 41 only allowed searches and seizures via a warrant from a magistrate judge to be executed within the district that the warrant was issued.[23]

One can see how eliminating this feature of Rule 41 might jeopardize protections guaranteed by the 4th Amendment - if a judge in Nevada could authorize the search of a computer in Arkansas, federal law enforcement could effectively seek out judges that might be more lenient with respect to a particular investigation. By weakening this rule, the legal obstacles normally encountered in the process of obtaining a warrant in cases involving the internet would be much more easily overcome.

In the context of the case

After identifying the location of the Playpen server, obtaining a warrant to seize it, and doing so, the FBI continued to host content on the site for two weeks.[24] The ethics of a federal organization hosting and distributing child pornography is an entire issue unto itself,[25] but what is important with respect to Rule 41 and the 4th Amendment is that the vast majority of users who accessed child pornography stored on the server did so in districts outside of the one in which the FBI obtained the warrant.

It’s important to note that the FBI didn’t simply obtain the IP addresses of users when they accessed the server. As discussed above, the function of a properly configured Tor Hidden Service is to prevent both user and host from obtaining identifying information about one another. However, due to a vulnerability in the Tor browser itself, the NIT deployed by the FBI was able to obtain identifying information (MAC address, IP address, OS, etc.) from each user’s machine, which was then forwarded to a server at the FBI. This effectively amounted to a search and seizure of personal property outside of the district authorized by the warrant, which (at the time) could not be authorized by a single magistrate. In many of the cases resulting from the FBI’s investigation, evidence has been thrown out due the fact that the actions taken by law enforcement were determined to be in violation of Rule 41.[26]

Lacking key evidence, many cases have stalled and could potentially be dismissed. Lawmakers understood that the FBI couldn’t have possibly known where the users were located prior to deploying the NIT, making it impossible to obtain a warrant for each district in which a user accessing Playpen was located. Perhaps the FBI realized this might happen, but also had no other way to identify the users, and so went ahead with the investigation, knowing that at worst the cases would be dismissed. Recognizing the importance for legal procedure to keep pace with advances in technology, the Department of Justice sought to introduce changes to Rule 41 in order to make it possible to apprehend criminals in these types of cases.


In December of 2016, Rule 41 was amended to include the following provision:

“a magistrate judge … has authority to issue a warrant to use remote access to search electronic storage media and to seize or copy electronically stored information located within or outside that district if […] the district where the media or information is located has been concealed through technological means”[27]

The expression “concealed through technological means” is left with no further definition, and is yet to be interpreted in a court, which raises serious questions as to how it will be applied in the future. What is certain, however, is that anyone using Tor or a VPN on the internet is certainly now within the scope of the amendment. The EFF has even speculated that machines infected with malware such that they act as part of a botnet could be subjected to it[28] (since DDoS attacks, when implemented via a botnet, typically involve IP spoofing). This seems like a bit of a stretch, since IP spoofing is simply creating IP packets with false IP addresses, but it’s alarming that such obvious ambiguity could be written into a law which gives federal law enforcement so much unchecked power. One could even see a judge interpreting “technological means” to be an IP address itself–after all, there is not a direct mapping from IP address to district hardcoded anywhere.

With around 500 magistrate judges across the country,[29] law enforcement can now essentially pick any one they like if they want to remotely search a US citizen’s computer in any part of the country. The requirement is simply that some ‘technological means’ are preventing them from determining the location of a device, and choosing which magistrate will be the least skeptical of the search.

Furthermore, since the FBI has since classified the source code for the NIT they used in the Playpen hack, it is unlikely that they will have to disclose it in future cases[30]. For a government organization to have no legal oversight in how it constructs and executes malware is extremely dangerous, since it leaves the public entirely in the dark with respect to its overall effect. What, for example, is currently stopping the FBI from developing malware that can infect other computers, perhaps with completely unintended consequences?[31]

Giving federal law enforcement such sparse restrictions on which devices they can search, while simultaneously requiring little oversight in the means they use to get access to the device itself, allows them to take advantage of Zero-Day exploits without disclosing them to the general public.[32] This of course puts all internet users at risk in exchange for greater power in the hands of law enforcement, since those same exploits could be known to individuals with malicious intent.


It goes without saying that we have no sympathy for child pornographers, and it’s deeply disturbing that many of them could be let go on the grounds of a legal technicality. Time will tell how the amendment to Rule 41 will be interpreted in courts, but one has good reason to be worried whenever a single case, especially one as sensational and awful as child pornography, gives rise to sweeping changes in legislation that potentially allow law enforcement to have indiscriminate power over such a wide class of citizens. Deploying malware in the absence of rigorous legal oversight leaves open wide avenues for abuse, and any expansion of law enforcement like this one ought to be coupled with equal restrictions in the form of legal oversight.


  1. “The Playpen Cases: Mass Hacking by U.S. Law Enforcement”, Electronic Frontier Foundation (EFF)
  2. “The FBI’s ‘Unprecedented’ Hacking Campaign Targeted Over a Thousand Computers”, motherboard.vice.com, Accessed 01/23/2017
  3. FBI ran website sharing thousands of child porn images”, USA Today, Jan 21, 2016, Accessed 01/23/2017,
  4. “The Playpen Cases: Frequently Asked Questions”, EFF, Aug 25, 2016, Accessed 01/23/2017
  5. “The FBI Is Classifying Its Tor Browser Exploit Because ‘National Security’”, motherboard.vice.com, June 24, 2016, Accessed 01/23/2017
  6. The Ethics of a Child Pornography Sting”, New York Times, Jan 27, 2016, Accessed 01/23/2017
  7. “FBI Refuses to Divulge How It Tracked Pedophiles on Tor”, gizmodo.com, Mar 30, 2016, Accessed 01/23/2017 “
  8. “SVG Diagram of the “Onion Routing” Principle”, wikipedia.org, Accessed 01/24/2017
  9. “How Tor Works”, WeFightCensorship.org, 08/05/2013, Accessed 01/23/2016
  10. “Tor Protocol Specification”, Torproject.org, Accessed 01/24/2017
  11. “Tor Hidden Service Protocol”, Torproject.org, Accessed 01/23/2017
  12. “Configuring Hidden Services for Tor”, Torproject.org, accessed 01/23/2017
  13. “Tor Hidden Service Protocol”, Torproject.org, Accessed 01/23/2017
  14. “Playpen: The Story of the FBI’s Unprecedented and Illegal Hacking Operation”, EFF, Sept 15, 2016, Accessed 01/23/2017
  15. “Best practices for Tor use, in light of released NSA slides” stackexchange.com, Oct 7, 2013, Accessed 01/24/2017
  16. “Tor Project Overview Archive”, Jun 26, 2014, Accessed 01/24/2017
  17. “Roger Dingledine about the TOR-Project”, NetzpolitikTV Interview May 22, 2012, Accessed 01/24/2017
  18. “Tor Project Overview”, Torproject.org, Accessed 01/24/2017
  19. “Dissent Made Safer”, MIT Technology Review, Apr 21, 2009, Accessed 01/24/2017
  20. “Cryptopolitik and the Darknet”, Daniel Moore and Thomas Rid, Feb 01, 2016, Accessed 01/24/2017
  21. “Tor Project FAQ”, Torproject.org, Accessed 01/24/2017
  22. “Tips for Running an Exit Node”, Torproject.org, Jun 30, 2010, Updated Feb 25, 2015, Accessed 01/24/2017
  23. “Playpen: The Story of the FBI’s Unprecedented and Illegal Hacking Operation”, EFF, Sept 15, 2016, Accessed 01/23/2017
  24. “FBI ran website sharing thousands of child porn images”, USA Today, Jan 21, 2016, Accessed 01/23/2017,
  25. “FBI Apparently Made Darkweb Child Porn Site Faster During Its Hosting Of Seized Server”, techdirt.com, Aug 23, 2016, Accessed 01/24/2017
  26. “Judge: child porn evidence obtained via FBI’s Tor hack must be suppressed”, Arstechnica.com, Sept 21, 2016, Accessed 01/24/2017
  27. “Rule 41. Search and Seizure”, Federal Rules of Criminal Procedure, Legal Information Institute - Cornell University Law School, Accessed 01/23/2017
  28. “The Playpen Story: Rule 41 and Global Hacking Warrants”, EFF, Sept 26, 2016, Accessed 01/23/2017
  29. “FMJA History”, Federal Magistrate Judge Association
  30. “The FBI Is Classifying Its Tor Browser Exploit Because ‘National Security’”, motherboard.vice.com, June 24, 2016, Accessed 01/24/2017
  31. “Rule 41 would make it easier for the government to carry out hacks”, Arstechnica.com, Apr 29, 2016, Accessed 01/24/2017
  32. “ACLU Comments on Proposed Amendment to Rule 41”, Apr 4, 2014, Accessed 01/24/2017

Other References


This work was done without any outside collaboration.