CVE-2016-2181 in OpenSSL
Summary
by MITRE
The Anti-Replay feature in the DTLS implementation in OpenSSL before 1.1.0 mishandles early use of a new epoch number in conjunction with a large sequence number, which allows remote attackers to cause a denial of service (false-positive packet drops) via spoofed DTLS records, related to rec_layer_d1.c and ssl3_record.c.
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Analysis
by VulDB Data Team • 08/29/2024
The vulnerability described in CVE-2016-2181 represents a critical flaw in the Datagram Transport Layer Security (DTLS) implementation within OpenSSL versions prior to 1.1.0. This issue specifically affects the anti-replay mechanism that is designed to protect against replay attacks by ensuring that duplicate packets are not processed multiple times. The flaw manifests when the system encounters early use of a new epoch number combined with large sequence numbers, creating a scenario where legitimate packets are incorrectly identified as duplicates and subsequently dropped. This vulnerability falls under the category of improper input validation and weak cryptographic protocol implementation, with direct implications for the integrity and availability of DTLS communications.
The technical root cause of this vulnerability lies in the improper handling of epoch transitions within the DTLS protocol implementation. When a new epoch number is introduced, the system should properly manage sequence number tracking to prevent false positives in the anti-replay mechanism. However, in affected OpenSSL versions, the code in rec_layer_d1.c and ssl3_record.c fails to correctly account for large sequence numbers during epoch transitions, leading to a condition where legitimate packets are flagged as replayed and discarded. This occurs because the anti-replay window management logic does not adequately handle the mathematical relationship between epoch numbers and sequence numbers, particularly when sequence numbers approach maximum values. The flaw creates a race condition in packet processing where the system's state management becomes inconsistent during epoch changes.
From an operational impact perspective, this vulnerability enables remote attackers to perform denial of service attacks against DTLS implementations by sending spoofed packets with carefully crafted epoch numbers and large sequence numbers. The false-positive packet drops effectively disrupt legitimate communications without requiring the attacker to possess cryptographic keys or perform complex attacks. This makes the vulnerability particularly dangerous in environments where DTLS is used for critical communications such as VoIP, IoT device management, or secure messaging applications. The attack can be executed remotely with minimal resources, making it a significant threat to network availability and service integrity. According to CWE classification, this represents a weakness in the implementation of cryptographic protocols, specifically CWE-295 for improper certificate validation and CWE-310 for cryptographic issues.
The attack vector for CVE-2016-2181 aligns with techniques described in the MITRE ATT&CK framework under the T1498 category for network denial of service, specifically targeting protocol manipulation. Attackers can exploit this vulnerability by crafting DTLS packets with epoch numbers that trigger the flawed anti-replay logic, causing legitimate traffic to be dropped and disrupting service availability. The vulnerability affects all applications and systems that rely on OpenSSL's DTLS implementation, including but not limited to VPN services, secure web communications, and IoT device management protocols. Organizations using affected OpenSSL versions are particularly vulnerable during high-traffic periods when the likelihood of epoch transitions and large sequence numbers occurring simultaneously increases.
Mitigation strategies for this vulnerability primarily focus on upgrading to OpenSSL 1.1.0 or later versions where the flaw has been addressed through improved epoch and sequence number handling in the anti-replay mechanism. System administrators should also implement network monitoring to detect unusual packet drop patterns that might indicate exploitation attempts. Additional defensive measures include configuring DTLS implementations to minimize epoch transitions, implementing rate limiting on incoming DTLS packets, and deploying intrusion detection systems that can identify suspicious DTLS packet patterns. The fix implemented in OpenSSL 1.1.0 addresses the core issue by properly managing the relationship between epoch numbers and sequence numbers during transitions, ensuring that legitimate packets are not incorrectly flagged as duplicates. Organizations should also consider implementing redundant DTLS endpoints and failover mechanisms to maintain service availability during potential exploitation attempts, as the vulnerability can be leveraged to cause sustained disruption of DTLS services.