CVE-2015-1787 in OpenSSL
Summary
by MITRE
The ssl3_get_client_key_exchange function in s3_srvr.c in OpenSSL 1.0.2 before 1.0.2a, when client authentication and an ephemeral Diffie-Hellman ciphersuite are enabled, allows remote attackers to cause a denial of service (daemon crash) via a ClientKeyExchange message with a length of zero.
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Analysis
by VulDB Data Team • 04/15/2022
The vulnerability described in CVE-2015-1787 represents a critical denial of service flaw within the OpenSSL cryptographic library that specifically affects versions prior to 1.0.2a. This vulnerability resides within the ssl3_get_client_key_exchange function located in the s3_srvr.c source file, which handles the secure socket layer protocol version 3 implementation. The flaw becomes exploitable when a server is configured with client authentication enabled alongside ephemeral Diffie-Hellman ciphersuites, creating a specific operational context where the vulnerability can be triggered. The attack vector involves sending a malformed ClientKeyExchange message with a zero-length payload, which causes the OpenSSL daemon to crash and subsequently become unavailable to legitimate users. This vulnerability directly impacts the availability aspect of the security triad and can be classified under CWE-122 as an improper restriction of operations within a recognized security boundary.
The technical mechanism behind this vulnerability stems from inadequate input validation within the SSL/TLS handshake process. When a client connects to a server using client authentication and ephemeral Diffie-Hellman ciphersuites, the server expects a properly formatted ClientKeyExchange message containing the client's public key for key exchange operations. However, the ssl3_get_client_key_exchange function fails to properly validate the length of the incoming ClientKeyExchange message before processing it. A malicious attacker can exploit this by sending a ClientKeyExchange message with zero bytes, bypassing the expected validation checks and causing the server to attempt operations on invalid data structures. The function does not perform sufficient boundary checking or length validation, leading to an unhandled exception that results in the daemon crashing. This behavior aligns with ATT&CK technique T1499.004 for network denial of service and demonstrates a classic buffer over-read vulnerability pattern that has been documented in various security frameworks.
The operational impact of CVE-2015-1787 extends beyond simple service disruption, as it can be leveraged by attackers to create sustained availability issues within systems relying on OpenSSL for secure communications. Servers configured with client authentication and ephemeral Diffie-Hellman ciphersuites represent high-value targets since they are commonly used in enterprise environments, web servers, and secure communication platforms. The vulnerability affects a wide range of applications including web servers, mail servers, and any system utilizing OpenSSL for TLS/SSL connections with client certificate authentication. When exploited, this vulnerability can cause cascading failures in services that depend on these secure communication channels, potentially leading to significant business disruption. The crash occurs during the TLS handshake phase, meaning that legitimate clients attempting to establish secure connections would be unable to complete the authentication process, effectively preventing access to protected services. Organizations using affected OpenSSL versions may experience service degradation or complete outages depending on their configuration and the frequency of attacks.
Mitigation strategies for CVE-2015-1787 primarily focus on immediate version upgrades to OpenSSL 1.0.2a or later, which contain the necessary patches to address the input validation flaw. System administrators should prioritize updating their OpenSSL installations across all affected servers and applications, particularly those configured with client authentication and ephemeral Diffie-Hellman ciphersuites. Additionally, network administrators can implement temporary workarounds such as disabling ephemeral Diffie-Hellman ciphersuites or restricting client authentication where possible. Monitoring and intrusion detection systems should be configured to identify suspicious ClientKeyExchange messages with zero-length payloads, providing early warning of potential exploitation attempts. The vulnerability highlights the importance of thorough input validation in cryptographic implementations and demonstrates how seemingly minor validation gaps can lead to severe operational consequences. Security teams should also consider implementing rate limiting and connection throttling mechanisms to reduce the impact of potential denial of service attacks. Regular security assessments and vulnerability scanning should be conducted to identify systems running outdated OpenSSL versions, ensuring comprehensive protection against similar vulnerabilities in the future. This vulnerability serves as a reminder of the critical importance of maintaining up-to-date cryptographic libraries and the potential consequences of failing to address security patches promptly in enterprise environments.