This advisory—written by the Cybersecurity Security and Infrastructure Security Agency (CISA) with contributions from the Federal Bureau of Investigation (FBI)—highlights risks associated with Tor, along with technical details and recommendations for mitigation. Cyber threat actors can use Tor software and network infrastructure for anonymity and obfuscation purposes to clandestinely conduct malicious cyber operations.
Tor (aka The Onion Router) is software that allows users to browse the web anonymously by encrypting and routing requests through multiple relay layers or nodes. This software is maintained by the Tor Project, a nonprofit organization that provides internet anonymity and anti-censorship tools. While Tor can be used to promote democracy and free, anonymous use of the internet, it also provides an avenue for malicious actors to conceal their activity because identity and point of origin cannot be determined for a Tor software user. Using the Onion Routing Protocol, Tor software obfuscates a user’s identity from anyone seeking to monitor online activity (e.g., nation states, surveillance organizations, information security tools). This is possible because the online activity of someone using Tor software appears to originate from the Internet Protocol (IP) address of a Tor exit node, as opposed to the IP address of the user’s computer.
CISA and the FBI recommend that organizations assess their individual risk of compromise via Tor and take appropriate mitigations to block or closely monitor inbound and outbound traffic from known Tor nodes.
Click here for a PDF version of this report.
Risk Evaluation
Malicious cyber actors use Tor to mask their identity when engaging in malicious cyber activity impacting the confidentiality, integrity, and availability of an organization’s information systems and data. Examples of this activity include performing reconnaissance, penetrating systems, exfiltrating and manipulating data, and taking services offline through denial-of-service attacks and delivery of ransomware payloads. Threat actors have relayed their command and control (C2) server communications—used to control systems infected with malware—through Tor, obscuring the identity (location and ownership) of those servers.
The use of Tor in this context allows threat actors to remain anonymous, making it difficult for network defenders and authorities to perform system recovery and respond to cyberattacks. Organizations that do not take steps to block or monitor Tor traffic are at heightened risk of being targeted and exploited by threat actors hiding their identity and intentions using Tor.
The risk of being the target of malicious activity routed through Tor is unique to each organization. An organization should determine its individual risk by assessing the likelihood that a threat actor will target its systems or data and the probability of the threat actor’s success given current mitigations and controls. This assessment should consider legitimate reasons that non-malicious users may prefer to, or need to, use Tor for accessing the network. Organizations should evaluate their mitigation decisions against threats to their organization from advanced persistent threats (APTs), moderately sophisticated attackers, and low-skilled individual hackers, all of whom have leveraged Tor to carry out reconnaissance and attacks in the past.
Technical Details
Tor obfuscates the source and destination of a web request. This allows users to conceal information about their activities on the web—such as their location and network usage—from the recipients of that traffic, as well as third parties who may conduct network surveillance or traffic analysis. Tor encrypts a user’s traffic and routes the traffic through at least three Tor nodes, or relays, so that the user’s starting IP address and request is masked from network and traffic observers during transit. Once the request reaches its intended destination, it exits Tor through a public Tor exit node. Anyone conducting monitoring or analysis will only see the traffic coming from the Tor exit node and will not be able to determine the original IP address of the request.
