CVE-2016-6305 in OpenSSL
Summary
by MITRE
The ssl3_read_bytes function in record/rec_layer_s3.c in OpenSSL 1.1.0 before 1.1.0a allows remote attackers to cause a denial of service (infinite loop) by triggering a zero-length record in an SSL_peek call.
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Analysis
by VulDB Data Team • 09/21/2022
The vulnerability identified as CVE-2016-6305 resides within the OpenSSL cryptographic library implementation, specifically affecting versions 1.1.0 through 1.1.0a. This issue manifests in the ssl3_read_bytes function located in the record/rec_layer_s3.c file, which handles the processing of SSL/TLS record layers. The flaw represents a classic denial of service vulnerability that can be exploited by remote attackers to disrupt service availability. The vulnerability occurs when a malicious actor triggers a zero-length record during an SSL_peek operation, a function commonly used to examine data without removing it from the buffer.
The technical root cause of this vulnerability stems from inadequate input validation within the SSL record processing logic. When the ssl3_read_bytes function encounters a zero-length record during an SSL_peek call, the function fails to properly handle this edge case, leading to a condition where the processing loop becomes infinite. This occurs because the function does not adequately check for zero-length records or properly terminate the loop when such records are encountered. The flaw creates a scenario where the application enters a continuous processing cycle without making progress, consuming system resources and effectively rendering the service unavailable to legitimate users. This type of vulnerability falls under CWE-835, which specifically addresses the issue of infinite loops in software implementations.
The operational impact of CVE-2016-6305 extends beyond simple service disruption to potentially compromise system stability and availability. When exploited, this vulnerability can cause applications using affected OpenSSL versions to become unresponsive, leading to extended downtime and potential business disruption. The infinite loop condition consumes CPU cycles and memory resources continuously, making it particularly dangerous in high-traffic environments where multiple connections might be simultaneously affected. Network services that rely on OpenSSL for secure communications, including web servers, email servers, and database connections, become vulnerable to this attack vector. The vulnerability is particularly concerning because it can be triggered through legitimate SSL/TLS operations without requiring authentication or special privileges, making it accessible to any remote attacker with network access to the target service.
Mitigation strategies for CVE-2016-6305 primarily focus on upgrading to the patched version of OpenSSL 1.1.0a or later, which contains the necessary code modifications to properly handle zero-length records during SSL_peek operations. System administrators should prioritize patching affected systems and verify that all applications using OpenSSL are updated to versions that address this vulnerability. Additionally, network monitoring should be implemented to detect unusual CPU utilization patterns that might indicate exploitation attempts. Organizations should also consider implementing rate limiting and connection pooling mechanisms to reduce the impact of potential denial of service attacks. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, and the mitigation approaches align with defensive measures outlined in the MITRE ATT&CK matrix for preventing service disruption attacks. The vulnerability demonstrates the importance of proper input validation and error handling in cryptographic implementations, as highlighted by industry best practices for secure coding and the NIST cybersecurity framework requirements for robust system design.