CVE-2009-2426 in Tor
Summary
by MITRE
The connection_edge_process_relay_cell_not_open function in src/or/relay.c in Tor 0.2.x before 0.2.0.35 and 0.1.x before 0.1.2.8-beta allows exit relays to have an unspecified impact by causing controllers to accept DNS responses that redirect to an internal IP address via unknown vectors. NOTE: some of these details are obtained from third party information.
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Analysis
by VulDB Data Team • 08/20/2017
The vulnerability described in CVE-2009-2426 resides within the Tor anonymity network's relay component, specifically in the connection_edge_process_relay_cell_not_open function located in src/or/relay.c. This flaw affects Tor versions 0.2.x prior to 0.2.0.35 and 0.1.x prior to 0.1.2.8-beta, representing a critical security issue that undermines the network's fundamental promise of anonymity and privacy. The vulnerability stems from insufficient validation of DNS responses received through the Tor network, creating a pathway for malicious actors to manipulate the system's behavior in ways that could compromise user security.
The technical implementation of this vulnerability involves the improper handling of relay cells that are not open, allowing exit relays to potentially influence how DNS responses are processed and accepted by Tor controllers. When an exit relay encounters a DNS response that redirects to an internal IP address, the flawed function fails to properly validate or reject such responses, enabling what is known as DNS rebinding attacks. This occurs because the Tor network's controller components accept these potentially malicious DNS responses without adequate scrutiny, creating a scenario where internal network addresses could be accessed through the anonymizing proxy. The unspecified impact mentioned in the description indicates that the vulnerability could be leveraged through various unknown vectors, making it particularly dangerous as attackers can exploit multiple pathways to achieve their objectives.
The operational impact of this vulnerability extends beyond simple privacy concerns to potentially enable significant reconnaissance and exploitation activities. Attackers could use this flaw to map internal network topologies, identify vulnerable internal services, or even gain access to sensitive systems that would normally be protected by the Tor network's routing mechanisms. This represents a fundamental breakdown in the trust model of the Tor network, where exit relays that should only act as intermediaries in the anonymization process can be manipulated to perform malicious actions. The vulnerability essentially allows for the circumvention of network isolation boundaries, enabling attackers to potentially bridge the gap between the anonymizing network and internal systems. This aligns with ATT&CK technique T1090.004 for proxy evasion and potentially connects to CWE-20 for improper input validation, where the system fails to properly validate the legitimacy of DNS responses.
Mitigation strategies for this vulnerability involve upgrading to patched versions of Tor where the connection_edge_process_relay_cell_not_open function properly validates DNS responses and rejects those that redirect to internal IP addresses. Network administrators should ensure they are running Tor versions 0.2.0.35 or later for the 0.2.x series, and 0.1.2.8-beta or later for the 0.1.x series. Additionally, implementing proper network segmentation and access controls can help limit the potential impact of successful exploitation attempts. Organizations should also consider monitoring for unusual DNS resolution patterns and implementing additional layers of security to detect and prevent unauthorized access attempts through the Tor network. The fix implemented in the patched versions likely includes enhanced input validation and proper rejection of DNS responses that could lead to internal address resolution, addressing the core weakness in the relay cell processing logic that allowed this vulnerability to exist.