CVE-2015-2689 in Tor
Summary
by MITRE
Tor before 0.2.4.26 and 0.2.5.x before 0.2.5.11 does not properly handle pending-connection resolve states during periods of high DNS load, which allows remote attackers to cause a denial of service (assertion failure and daemon exit) via crafted packets.
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Analysis
by VulDB Data Team • 11/27/2024
The vulnerability described in CVE-2015-2689 represents a critical flaw in the Tor anonymization network that affects versions prior to 0.2.4.26 and 0.2.5.x versions before 0.2.5.11. This issue specifically targets the Tor daemon's handling of DNS resolution states during periods of elevated network load conditions. The flaw manifests when the system encounters high DNS query volumes and fails to properly manage pending connection resolution states, creating a condition that can be exploited by remote attackers to trigger system instability.
The technical root cause of this vulnerability lies in the improper management of connection state handling within the Tor daemon's network resolution subsystem. When DNS servers experience high load or respond slowly to queries, the Tor client maintains pending connection states that should be properly managed and timed out. However, under stress conditions, the system fails to handle these pending states correctly, leading to assertion failures within the codebase. This assertion failure occurs when the system encounters an unexpected state condition that violates internal assumptions, ultimately causing the Tor daemon to terminate unexpectedly. The vulnerability operates at the protocol level where DNS resolution failures and timeouts are not properly handled, creating a cascade of failures that can be triggered by sending specifically crafted network packets to the vulnerable Tor instance.
The operational impact of CVE-2015-2689 is severe for users relying on Tor for anonymous communication and network privacy. Remote attackers can exploit this vulnerability to perform denial of service attacks against Tor nodes, potentially disrupting the anonymity network's availability and integrity. When the daemon exits due to assertion failure, it creates a temporary disruption in service that can affect both the compromised node and potentially impact the broader Tor network topology. The attack requires minimal resources from the attacker, as they only need to send specific network packets that trigger the DNS resolution stress conditions, making this a particularly dangerous vulnerability for network administrators and privacy advocates who depend on Tor infrastructure. The vulnerability can be classified under CWE-617 as a reachable assertion, where an assertion failure leads to program termination and denial of service conditions.
Mitigation strategies for this vulnerability involve immediate upgrade to patched versions of the Tor software, specifically versions 0.2.4.26 or 0.2.5.11 and later. Network administrators should implement proper monitoring and alerting systems to detect unusual daemon termination patterns that may indicate exploitation attempts. The fix addresses the underlying state management issue by implementing proper timeout handling for pending DNS resolution requests and ensuring that assertion checks do not lead to complete daemon termination. Organizations using Tor should also consider implementing rate limiting and connection pooling mechanisms to reduce the likelihood of triggering the vulnerable code paths. This vulnerability aligns with ATT&CK technique T1499.004 for network denial of service attacks and represents a critical security gap that affects the availability and reliability of anonymous communication infrastructure. The patch implemented by the Tor project specifically addresses the race condition and state management issues that occur during high DNS load scenarios, ensuring that the daemon can gracefully handle DNS resolution timeouts and failures without crashing.