CVE-2026-77638 in Tor
Summary
by MITRE • 08/21/2026
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.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified in Tor versions prior to 0.4.9.11 represents a critical security flaw within the core protocol logic governing circuit establishment and identity verification during anonymous communication sessions. This race condition specifically targets the rendezvous point mechanism, which is fundamental to how clients connect to hidden services on the Tor network. In normal operation, when a client wishes to communicate with an onion service, it establishes circuits through multiple nodes, including a designated rendezvous point that facilitates the handshake between the client and the service without revealing their direct IP addresses or identities. The flaw arises from a timing discrepancy in how these cryptographic handshakes are processed and validated by the Tor daemon software. Under specific network conditions where packet arrival times vary significantly due to latency or congestion, an attacker positioned at the rendezvous point can exploit this window of inconsistency to intercept and manipulate the connection parameters before they are fully authenticated.
From a technical perspective, the race condition allows for a man-in-the-middle attack scenario wherein the malicious node impersonates the intended onion service. This is achieved by manipulating the sequence of messages exchanged during the circuit extension phase. Because the Tor implementation did not strictly enforce atomicity in certain state transitions related to key exchange and identity confirmation, an adversary can inject fraudulent data or delay legitimate responses just enough to cause the client to accept a false identity certificate or session key associated with the attacker rather than the genuine service provider. This undermines the core promise of anonymity provided by Tor, as it breaks the guarantee that the communication endpoint is indeed the intended hidden service and not an intermediary controlled by a malicious actor. The exploit relies on precise timing manipulation, making it particularly dangerous in environments where network latency can be artificially induced or naturally fluctuates unpredictably.
The operational impact of this vulnerability is severe for users relying on Tor for sensitive communications, such as whistleblowing, journalism, political activism, or accessing censored information. If an attacker successfully executes this man-in-the-middle attack, they can not only eavesdrop on the encrypted traffic but also modify it in transit without detection by standard cryptographic checks that assume a trusted endpoint identity. This compromises both confidentiality and integrity of data transmitted through Tor circuits. Furthermore, because the vulnerability affects the fundamental trust model of hidden services, users may inadvertently interact with malicious infrastructure disguised as legitimate onion services, leading to potential phishing attacks, malware distribution, or theft of credentials if such information is exchanged during the session. The risk is amplified by the fact that many applications built on top of Tor assume the underlying transport layer provides strong identity guarantees which are now compromised in this specific edge case.
Mitigation strategies primarily involve upgrading the Tor software to version 0.4.9.11 or later, where the race condition has been addressed through stricter synchronization mechanisms and improved validation logic during the rendezvous process. System administrators should ensure that all nodes within their infrastructure are updated promptly to prevent them from being exploited as part of a larger attack chain against end-users. Additionally, users employing Tor for high-security activities should consider implementing additional layers of application-level encryption and authentication where possible, such as using HTTPS with strict certificate validation or specialized secure messaging protocols that do not rely solely on the anonymity network's identity guarantees. Monitoring network latency patterns can also help in detecting potential attempts to exploit timing-based vulnerabilities, although software patching remains the most effective defense against this specific class of race condition attacks within the Tor protocol stack.