CVE-2017-8823 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, there is a use-after-free in onion service v2 during intro-point expiration because the expiring list is mismanaged in certain error cases, aka TROVE-2017-013.
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Analysis
by VulDB Data Team • 12/11/2019
The vulnerability described in CVE-2017-8823 represents a critical use-after-free flaw in the Tor anonymity network that affects multiple versions of the Tor software. This issue specifically impacts onion service version two implementations where the software fails to properly manage the expiration list for introduction points, creating a scenario where memory that has been freed is subsequently accessed. The vulnerability was categorized as TROVE-2017-013 by the Tor Project and falls under the broader category of memory safety issues that can lead to arbitrary code execution. The flaw occurs during the normal operation of onion services when introduction points are being expired, indicating a fundamental problem in how the Tor software handles memory cleanup and management during error conditions.
The technical implementation of this vulnerability stems from improper handling of the introduction point expiration process within the Tor onion service v2 protocol. When certain error conditions occur during the expiration of introduction points, the software fails to correctly remove entries from the expiration list, leaving pointers to freed memory. This mismanagement creates a window where an attacker could potentially manipulate the system to cause the freed memory to be reallocated and then accessed, leading to a use-after-free condition. The flaw demonstrates a classic memory safety issue that can be exploited to execute arbitrary code on the victim's system, particularly when the Tor client or relay is processing onion service traffic. This vulnerability specifically aligns with CWE-416, which describes the use of freed memory condition, and represents a failure in proper memory lifecycle management within the Tor codebase.
The operational impact of CVE-2017-8823 extends beyond simple memory corruption, as it creates potential for remote code execution and system compromise. An attacker who can influence the expiration of introduction points in an onion service could potentially trigger this vulnerability and gain control over the Tor client or relay system. This represents a significant threat to the anonymity and security guarantees that Tor provides, as it could allow adversaries to compromise the infrastructure that protects users. The vulnerability affects a wide range of Tor versions, indicating it was a persistent issue that required multiple patches across different release branches. The timing of the expiration process makes this particularly dangerous, as it occurs during normal operation when the system is handling legitimate onion service traffic, meaning legitimate users could be affected without any indication of compromise.
Mitigation strategies for this vulnerability require immediate patching of affected Tor versions to the recommended secure releases, including 0.2.5.16, 0.2.8.17, 0.2.9.14, 0.3.0.13, and 0.3.1.9. Organizations running Tor infrastructure should prioritize updating their systems to prevent exploitation, as the vulnerability could be leveraged to compromise entire Tor networks. The fix implemented by the Tor Project addresses the improper list management during error conditions, ensuring that introduction point entries are correctly removed from the expiration list before memory is freed. Security monitoring should include detection of unusual Tor service behavior that might indicate exploitation attempts, particularly around introduction point expiration events. This vulnerability also highlights the importance of memory safety practices in network security software and demonstrates the need for comprehensive testing of error handling paths in anonymity systems where security is paramount. The ATT&CK framework would categorize this vulnerability under T1059 for command and scripting interpreter and T1071 for application layer protocol, as exploitation could enable attackers to establish persistent access through compromised Tor infrastructure.