CVE-2017-8819 in Tor
Summary
by MITRE
In Tor before 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 is ineffective for v2 onion services, aka TROVE-2017-009. An attacker can send many INTRODUCE2 cells to trigger this issue.
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Analysis
by VulDB Data Team • 12/11/2019
The vulnerability described in CVE-2017-8819 represents a critical flaw in the Tor anonymity network's v2 onion service protocol that undermines fundamental security protections designed to prevent replay attacks. This weakness affects multiple versions of the Tor software across different release branches, specifically targeting the replay-cache mechanism that should protect against duplicate or maliciously repeated communication attempts. The vulnerability operates at the protocol level within the Tor network's onion service infrastructure, where the replay-cache protection mechanism fails to properly validate or track repeated INTRODUCE2 cells that are essential for establishing hidden service connections.
The technical flaw manifests through the ineffective replay-cache protection mechanism that is supposed to detect and reject duplicate or maliciously repeated INTRODUCE2 cells sent by attackers to v2 onion services. This vulnerability stems from the failure of the Tor software to properly maintain and verify cache entries that should prevent attackers from overwhelming onion services with repeated connection attempts. The attack vector involves sending numerous INTRODUCE2 cells to a target v2 onion service, exploiting the weakness in the cache validation logic that should have prevented such repeated attempts from being processed. This mechanism failure creates a potential denial of service condition where legitimate service operations can be disrupted or where attackers could potentially exploit the service in unexpected ways.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it compromises the integrity of the onion service protocol and potentially allows for more sophisticated attacks against hidden services. Attackers can leverage this weakness to consume excessive resources on v2 onion services, potentially causing service degradation or complete unavailability of the targeted hidden services. The vulnerability affects all v2 onion services running on affected Tor versions, making it particularly concerning given the widespread use of these services for legitimate privacy-preserving communications. The TROVE-2017-009 designation indicates this was recognized as a significant security issue within the Tor community, highlighting the potential for abuse in real-world deployments.
Security professionals should immediately upgrade to patched versions of Tor software to address this vulnerability, as the affected versions span multiple release branches and have been widely deployed across the Tor network. The fix implemented in subsequent releases addresses the replay-cache mechanism to properly validate and track INTRODUCE2 cells, ensuring that duplicate or malicious attempts are correctly identified and rejected. Organizations maintaining v2 onion services should conduct immediate vulnerability assessments and implement monitoring to detect potential exploitation attempts. This vulnerability aligns with CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and CWE-284 (Improper Access Control) categories, reflecting both cryptographic weakness and access control failures in the Tor protocol implementation.
The attack pattern described in this vulnerability corresponds to the ATT&CK technique T1499.004 (Endpoint Denial of Service) and T1566.002 (Phishing with Malicious Attachment) as attackers could potentially leverage this weakness to disrupt services or create conditions favorable for more sophisticated attacks. The vulnerability demonstrates how protocol-level weaknesses can create cascading security issues in anonymity networks where proper validation mechanisms are essential for maintaining service integrity. Network administrators and security teams should also consider the broader implications for Tor network security and implement appropriate monitoring for unusual patterns of INTRODUCE2 cell traffic that might indicate exploitation attempts. The remediation process requires careful attention to ensure that all v2 onion service deployments are updated and that proper testing is conducted to verify the effectiveness of the replay-cache protection mechanism in the patched versions.