CVE-2013-7295 in Tor
Summary
by MITRE
Tor before 0.2.4.20, when OpenSSL 1.x is used in conjunction with a certain HardwareAccel setting on Intel Sandy Bridge and Ivy Bridge platforms, does not properly generate random numbers for (1) relay identity keys and (2) hidden-service identity keys, which might make it easier for remote attackers to bypass cryptographic protection mechanisms via unspecified vectors.
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Analysis
by VulDB Data Team • 12/22/2024
The vulnerability described in CVE-2013-7295 represents a critical weakness in the Tor anonymity network that specifically affects versions prior to 0.2.4.20. This flaw manifests when Tor operates with OpenSSL 1.x on Intel Sandy Bridge and Ivy Bridge processors that have HardwareAccel settings enabled, creating a significant risk to the cryptographic integrity of the network's security mechanisms. The vulnerability stems from improper random number generation during the creation of critical cryptographic keys that are fundamental to Tor's operation.
The technical root cause of this vulnerability lies in the interaction between Tor's cryptographic key generation process and the hardware acceleration features of Intel's Sandy Bridge and Ivy Bridge processors. When the HardwareAccel setting is enabled, the system's random number generator fails to produce sufficiently unpredictable values for generating relay identity keys and hidden-service identity keys. This weakness creates a predictable pattern in the cryptographic keys that should otherwise be completely random, making them susceptible to analysis and potential exploitation by adversaries who can then bypass the cryptographic protections that Tor relies upon to maintain user anonymity.
The operational impact of this vulnerability extends beyond simple cryptographic weakness to encompass the fundamental trust model of the Tor network. Relay identity keys are crucial for establishing trust relationships between Tor nodes, while hidden-service identity keys protect the anonymity of services operating within the Tor network. When these keys are generated with insufficient entropy, attackers can potentially predict or reconstruct these keys, enabling them to impersonate legitimate Tor relays or gain unauthorized access to hidden services. This compromise undermines the entire purpose of the Tor network, which depends on the unpredictability of cryptographic keys to protect user identities and communications.
This vulnerability aligns with CWE-330, which addresses the use of insufficiently random values in security contexts, and demonstrates how hardware-software interactions can create unexpected security weaknesses in cryptographic implementations. The attack surface is particularly concerning because it leverages legitimate hardware acceleration features that users might enable for performance reasons, making the vulnerability more likely to be present in real-world deployments. The unspecified vectors mentioned in the description suggest that attackers could potentially exploit this weakness through various means including statistical analysis of the predictable key patterns or by leveraging the reduced entropy to perform more sophisticated cryptographic attacks against the Tor network's security infrastructure.
The mitigation strategy for this vulnerability requires immediate upgrading to Tor version 0.2.4.20 or later, which contains fixes for the random number generation issues. System administrators should disable HardwareAccel settings on affected Intel processors when running Tor, particularly in environments where security is paramount. Additionally, organizations should implement monitoring to detect any unusual patterns in Tor relay operations that might indicate key compromise. The fix addresses the core issue by ensuring that cryptographic key generation utilizes proper random number sources that are not affected by the hardware acceleration features of specific Intel processor generations. This vulnerability serves as a reminder of the importance of thorough testing when integrating hardware acceleration features with security-critical applications, particularly those that rely on unpredictable cryptographic operations to maintain their security guarantees.