Description

Tor before 0.4.9.11 is prone to a race condition where in just the right circumstances a rendezvous point could man-in-the-middle (impersonate) the onion service that the client was trying to reach.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.9",
  "pubDate": "2026-08-20T21:17:11.097Z",
  "pubdate": "2026-08-20T21:17:11.097Z",
  "executiveSummary": "Tor before version 0.4.9.11 is susceptible to a race condition vulnerability within the rendezvous protocol implementation.\nThis flaw allows a compromised or malicious rendezvous point to successfully man-in-the-middle and impersonate the targeted onion service that a client intends to establish a connection with.\nThe affected product is Tor, specifically all software versions preceding 0.4.9.11.\nThe risk implications are severe, as successful exploitation breaks the end-to-end cryptographic integrity and anonymity guarantees of Tor onion services, potentially exposing sensitive client traffic to interception, inspection, and manipulation by malicious intermediary nodes.\nAttacker capabilities require positioning a malicious entity as the rendezvous point during circuit establishment under specific timing and synchronization circumstances.\nThe exploitation requirement relies on triggering the race condition precisely during the cryptographic handshake and circuit extension phase of the rendezvous protocol.",
  "technicalDetails": "The root cause of the vulnerability resides in a race condition within the Tor rendezvous circuit handling logic in Tor before version 0.4.9.11.\nDuring the standard Tor onion service connection process, a client and an onion service establish a connection through an intermediate rendezvous point to preserve anonymity.\nUnder specific and precise timing circumstances, the rendezvous point can manipulate the circuit handshake messages to impersonate the destination onion service to the connecting client.\nThe vulnerability affects the rendezvous point component responsible for facilitating the cryptographic binding between the client and the onion service circuits.\nNo authentication or elevated privileges are required for a malicious node acting as a rendezvous point to attempt the race condition, provided it is selected by the protocol routing mechanics.\nThe network exposure is inherent to the Tor overlay network, where untrusted nodes can be selected as rendezvous points.\nDuring exploitation, the malicious rendezvous point exploits the race condition to inject or spoof handshake parameters, deceiving the client into believing it has established a direct end-to-end encrypted circuit with the intended onion service, while the rendezvous point acts as an active man-in-the-middle proxy.\nThe post-exploitation impact includes the complete compromise of confidentiality and integrity for the communications between the client and the onion service, enabling traffic decryption, modification, or session hijacking despite the underlying onion routing architecture."
}