CVE-2026-77638: Tor race condition in onion service rendezvous

CVE-2026-77638 · Severity: high · CVSS 8.9 · Published 2026-08-20

Tor, a widely-used anonymity network and software, contains a race condition in its onion service (hidden service) protocol that could allow a rendezvous point to impersonate the intended destination and intercept communications. An attacker controlling a rendezvous point under specific timing conditions could perform a man-in-the-middle attack, compromising the privacy and security that users rely on Tor to provide. This affects users connecting to onion services, potentially exposing their communications to eavesdropping.

Technologies: Torproject Tor. Vendors: Torproject.

Quick answers

Is CVE-2026-77638 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-77638 being exploited in the wild.
How severe is CVE-2026-77638?
High severity, with a CVSS score of 8.9, published 20 August 2026.
Which products and versions are affected?
Tor Project Tor before 0.4.9.11.
Is there a fix for CVE-2026-77638?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-77638 attacked?
It is attacked over the network.
Am I affected by CVE-2026-77638?
Check whether you run Torproject Tor. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

This vulnerability is a race condition in Tor's v3 onion service rendezvous protocol. The flaw occurs when a rendezvous point (the meeting point between client and service) processes connection establishment in a way that allows insufficient validation of the service's identity under concurrent conditions. An attacker controlling or monitoring a rendezvous point can exploit this timing window to impersonate the target onion service before proper authentication is completed. The attack requires network-level access to the rendezvous point and knowledge of the target service address, but does not require authentication. The vulnerability is fixed in Tor 0.4.9.11 and later versions.

Affected products

  • Tor Project Tor before 0.4.9.11

Timeline

  • 2026-08-20: disclosed

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.