CVE-2016-8610 in OpenSSL
Summary
by MITRE
A denial of service flaw was found in OpenSSL 0.9.8, 1.0.1, 1.0.2 through 1.0.2h, and 1.1.0 in the way the TLS/SSL protocol defined processing of ALERT packets during a connection handshake. A remote attacker could use this flaw to make a TLS/SSL server consume an excessive amount of CPU and fail to accept connections from other clients.
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Analysis
by VulDB Data Team • 09/28/2022
The vulnerability identified as CVE-2016-8610 represents a critical denial of service weakness within the OpenSSL cryptographic library that affects multiple major versions including 0.9.8, 1.0.1, 1.0.2 through 1.0.2h, and 1.1.0. This flaw specifically targets the TLS/SSL protocol implementation during the connection handshake process, where the protocol defines how ALERT packets should be processed. The issue manifests when a remote attacker can manipulate the TLS protocol state machine in a way that causes the server to enter an infinite loop or excessive CPU consumption during alert handling. This vulnerability operates at the protocol level rather than at the application layer, making it particularly dangerous as it can affect any service relying on OpenSSL for secure communications including web servers, email servers, and database connections. The flaw essentially creates a condition where the server's processing of malformed or specially crafted ALERT packets causes it to consume disproportionate computational resources, leading to service degradation or complete unavailability. The vulnerability is classified under CWE-400 as an Uncontrolled Resource Consumption flaw, which directly maps to the resource exhaustion characteristics of this denial of service condition.
The technical implementation of this vulnerability stems from how OpenSSL handles the TLS protocol's ALERT message processing during the handshake phase. When a TLS server receives an ALERT packet that triggers a specific sequence of state transitions, the processing logic enters a loop where it continuously reprocesses the same alert condition without proper bounds checking or termination conditions. This occurs because the protocol state machine does not adequately validate the sequence of operations when handling certain alert types, particularly those related to protocol version negotiation and connection termination. The flaw is particularly insidious because it can be triggered by a single malicious connection attempt without requiring any authentication or privileged access, making it extremely easy to exploit at scale. The attacker need only establish a TLS connection and send a specially crafted ALERT packet that causes the server to enter the problematic processing loop, consuming CPU cycles indefinitely. This behavior aligns with ATT&CK technique T1499.004 for Network Denial of Service, where an adversary leverages protocol implementation weaknesses to exhaust system resources.
The operational impact of CVE-2016-8610 extends far beyond simple service disruption as it can effectively render TLS services completely unavailable to legitimate users while consuming significant computational resources. Organizations running affected OpenSSL versions face the risk of complete service outages, particularly in high-traffic environments where multiple concurrent connections are common. The vulnerability can be exploited in various attack scenarios including distributed denial of service attacks where multiple attackers coordinate to overwhelm server resources, or as a single attacker causing sustained disruption to critical services. The CPU consumption pattern typically shows sustained high utilization levels that can persist for extended periods, making it difficult for system administrators to identify the root cause of performance degradation. Additionally, the vulnerability affects all TLS implementations that rely on OpenSSL, including web servers like Apache and Nginx, mail servers, database servers, and any application that uses OpenSSL for secure communication. The attack vector is particularly concerning because it operates over standard network protocols and requires no specialized tools or knowledge beyond basic understanding of TLS protocol mechanics. Security teams must also consider that this vulnerability could be combined with other attack vectors to create more complex exploitation scenarios, potentially leading to broader system compromise beyond simple denial of service.
Mitigation strategies for CVE-2016-8610 require immediate action to upgrade affected OpenSSL installations to patched versions that properly handle the ALERT packet processing without entering infinite loops. Organizations should prioritize updating their OpenSSL libraries to versions 1.0.2i, 1.1.0b, or later releases that contain the necessary protocol state machine fixes. System administrators should implement monitoring solutions that can detect unusual CPU consumption patterns and alert on sustained high utilization that may indicate exploitation attempts. Network-level protections such as rate limiting and connection tracking can help reduce the impact of exploitation attempts, though these measures do not prevent the vulnerability itself. Additionally, organizations should consider implementing connection pooling and load balancing strategies to distribute the risk across multiple servers and reduce the impact of individual server compromises. The vulnerability highlights the importance of maintaining up-to-date cryptographic libraries and demonstrates how protocol-level flaws can have catastrophic effects on service availability. Security teams should also implement regular vulnerability scanning procedures to identify other potential OpenSSL-related issues and maintain comprehensive incident response plans that include specific procedures for handling TLS protocol-based attacks. The remediation process should include thorough testing of patched versions in staging environments before deployment to production systems to ensure that the updates do not introduce compatibility issues with existing applications.