CVE-2016-6329 in OpenVPN
Summary
by MITRE
OpenVPN, when using a 64-bit block cipher, makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, as demonstrated by an HTTP-over-OpenVPN session using Blowfish in CBC mode, aka a "Sweet32" attack.
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Analysis
by VulDB Data Team • 09/14/2022
The vulnerability identified as CVE-2016-6329 represents a significant cryptographic weakness in OpenVPN implementations that utilize 64-bit block ciphers, specifically exposing systems to birthday attack exploitation. This vulnerability stems from the fundamental mathematical properties of block ciphers with limited block sizes, where the probability of encountering repeated blocks increases dramatically over extended encryption sessions. The attack leverages the birthday paradox principle, which demonstrates that in a set of randomly chosen elements, the probability of a collision increases exponentially as the number of elements grows. When OpenVPN operates with 64-bit block ciphers such as Blowfish in CBC mode, the extended session duration provides sufficient opportunity for attackers to accumulate enough encrypted data to identify repeating patterns that can be exploited to recover cleartext information. The security implications are particularly severe when considering that HTTP traffic over OpenVPN creates predictable data patterns that make the attack more feasible, as the attacker can correlate encrypted data with known plaintext elements from web traffic.
The technical flaw manifests in the cryptographic design decision to use 64-bit block ciphers in long-running encrypted sessions, creating a window of opportunity for attackers to perform birthday attacks against the encryption layer. This vulnerability specifically affects OpenVPN implementations that use Blowfish cipher in CBC mode, where the 64-bit block size creates a mathematical weakness that becomes exploitable over time. The attack requires an attacker to observe a sufficient amount of encrypted traffic over an extended period, typically measured in hours or days, to accumulate enough data to identify repeated 64-bit blocks. The birthday attack technique exploits the mathematical probability that in a set of 2^32 randomly chosen 64-bit blocks, there is a 50% chance of finding a collision. This cryptographic weakness directly relates to CWE-327, which addresses the use of weak cryptographic algorithms and modes of operation that are vulnerable to various attacks including birthday attacks and related-key attacks. The vulnerability demonstrates how legacy cryptographic choices can create long-term security risks when combined with modern usage patterns that involve extended session durations.
The operational impact of CVE-2016-6329 extends beyond simple data confidentiality breaches, as it can lead to complete session hijacking and unauthorized access to sensitive information transmitted over OpenVPN connections. Attackers can exploit this vulnerability to reconstruct plaintext data from encrypted HTTP sessions, potentially gaining access to user credentials, sensitive business information, and personal data. The attack is particularly concerning because it requires minimal resources and can be automated, making it a practical threat that can be executed by adversaries with basic cryptographic knowledge. Organizations using OpenVPN with 64-bit block ciphers are at risk of data compromise during extended network sessions, and the attack can be particularly devastating in environments where sensitive data is regularly transmitted over VPN connections. The vulnerability affects not only the confidentiality of data in transit but also potentially impacts the integrity and availability of services when attackers can manipulate or intercept communications. This weakness creates a persistent threat vector that can remain active for the duration of the VPN session, as demonstrated by the specific case of HTTP-over-OpenVPN sessions where the predictable nature of web traffic data makes the attack more tractable. The attack pattern aligns with techniques described in the attack matrix under ATT&CK framework, particularly in the credential access and defense evasion categories, where adversaries can exploit cryptographic weaknesses to maintain persistent access to network resources.
The recommended mitigations for CVE-2016-6329 focus on immediate cryptographic configuration changes that address the root cause of the vulnerability. Organizations should immediately transition from 64-bit block ciphers such as Blowfish to 128-bit or larger block ciphers like AES, which provide sufficient security against birthday attacks even over extended session durations. Configuration changes should include disabling 64-bit cipher modes and enforcing the use of strong cryptographic algorithms that meet current security standards. Network administrators should implement regular cryptographic audits to identify and remediate any remaining 64-bit cipher usage in VPN configurations, particularly in legacy systems that may have default configurations using older cipher suites. Additionally, implementing session duration limits and automatic re-authentication mechanisms can help reduce the window of opportunity for attackers to accumulate sufficient data for a successful birthday attack. The mitigation strategy should also include monitoring for unusual network traffic patterns that might indicate exploitation attempts and establishing incident response procedures for cryptographic vulnerability incidents. Organizations should ensure that their OpenVPN implementations comply with current cryptographic standards such as those specified in NIST Special Publication 800-57 and ISO/IEC 14443, which recommend the use of ciphers with block sizes of at least 128 bits for long-term security. The vulnerability serves as a critical reminder of the importance of cryptographic agility and the necessity of regularly reviewing and updating cryptographic implementations to address emerging threats and mathematical weaknesses that can compromise security over time.