CVE-2017-8819

Vulnerability

In Tor before versions 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, the replay-cache protection mechanism for v2 onion services is ineffective [1]. This allows an attacker to send many INTRODUCE2 cells, which are supposed to be uniquely identified and rejected if replayed, but the cache fails to detect duplicates. The issue is tracked as TROVE-2017-009 [1].

Exploitation

An attacker does not require any special privileges or authentication beyond being able to communicate with the Tor network. The attacker sends multiple INTRODUCE2 cells to a v2 onion service's introduction point. Because the replay cache is ineffective, these duplicate cells are processed, potentially causing the service to build multiple circuits to the attacker's rendezvous point. The attack requires no user interaction on the victim's part and can be executed from any position on the network that can relay cells to the introduction point [1].

Impact

Successful exploitation of this vulnerability allows an attacker to degrade the anonymity of v2 onion services. By replaying INTRODUCE2 cells, an attacker can cause the onion service to create multiple rendezvous circuits, which can be used for traffic confirmation attacks or to link the service to its introduction point [1]. The attack does not directly lead to remote code execution or data disclosure, but it undermines the security guarantees of Tor's hidden service protocol.

Mitigation

The vulnerability is fixed in Tor versions 0.2.5.16, 0.2.8.17, 0.2.9.14, 0.3.0.13, and 0.3.1.9, which were released on December 1, 2017 [1]. All users should upgrade to one of these patched releases, or to 0.3.2.6-alpha. No workaround exists for unpatched versions. The Tor Project has not listed this CVE on the Known Exploited Vulnerabilities (KEV) catalog as of this writing.