CVE-2017-8821 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, an attacker can cause a denial of service (application hang) via crafted PEM input that signifies a public key requiring a password, which triggers an attempt by the OpenSSL library to ask the user for the password, aka TROVE-2017-011.
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Analysis
by VulDB Data Team • 12/11/2019
The vulnerability identified as CVE-2017-8821 represents a critical denial of service weakness in the Tor anonymization network that affected multiple versions of the software from 0.2.5.16 through 0.3.1.8. This flaw specifically targets the handling of PEM (Privacy-Enhanced Mail) formatted cryptographic materials within the Tor client and server implementations, creating a scenario where maliciously crafted input can cause the application to become unresponsive or hang indefinitely. The vulnerability stems from improper validation and handling of public key files that require password protection, creating a dangerous interaction with the underlying OpenSSL cryptographic library that Tor depends upon for its security infrastructure. The issue manifests when Tor encounters a PEM file containing a public key that is marked as requiring a password, triggering OpenSSL's interactive password prompting mechanism within the context of a non-interactive Tor process, resulting in an application hang that can persist indefinitely.
The technical root cause of this vulnerability aligns with CWE-209, which describes improper handling of password-protected keys in cryptographic libraries, and demonstrates a classic example of how cryptographic library interactions can lead to denial of service conditions in network security software. When the OpenSSL library attempts to prompt for a password in a context where user interaction is impossible, such as in automated Tor processes or server environments, the library blocks execution waiting for input that will never arrive. This creates a deadlock condition where the Tor application becomes unresponsive and effectively unusable, as the process remains in a waiting state indefinitely. The vulnerability specifically affects versions where the OpenSSL integration was not properly configured to handle password-protected keys gracefully, particularly in scenarios where the software operates without user interaction capabilities, which is fundamental to Tor's operation as a network of distributed nodes.
The operational impact of CVE-2017-8821 extends beyond simple application unresponsiveness to create significant risks for network availability and service integrity within the Tor ecosystem. Attackers can exploit this vulnerability by crafting malicious PEM files and distributing them to Tor nodes or clients, potentially causing widespread disruption across the anonymization network. The vulnerability particularly affects Tor relay operators who may inadvertently process compromised PEM files, leading to service degradation or complete unavailability of their nodes. From an ATT&CK framework perspective, this represents a denial of service attack vector that can be categorized under T1499, which covers network denial of service attacks, and demonstrates how cryptographic library weaknesses can be leveraged to compromise availability in security infrastructure. The impact is particularly severe because Tor nodes are critical components of the network infrastructure, and their unavailability can fragment the anonymity network, affecting user privacy and access to the service.
Mitigation strategies for CVE-2017-8821 focus on both immediate patching and architectural improvements to prevent similar vulnerabilities in the future. The most effective immediate solution involves upgrading to patched versions of Tor software, specifically versions 0.2.8.17, 0.2.9.14, 0.3.0.13, and 0.3.1.9, which contain the necessary code modifications to properly handle password-protected keys without triggering OpenSSL's interactive prompting behavior. Organizations should also implement input validation measures to prevent processing of untrusted PEM files, particularly in environments where Tor is used as a service. From a security architecture standpoint, this vulnerability highlights the importance of proper cryptographic library integration and the need for non-interactive applications to handle password-protected keys gracefully through either pre-processing validation or by configuring cryptographic libraries to fail gracefully when password prompts would be required. Additionally, implementing monitoring systems to detect application hangs or unresponsive behavior can help identify exploitation attempts and provide early warning of potential attacks against Tor infrastructure.