CVE-2014-3511 in OpenSSL
Summary
by MITRE
The ssl23_get_client_hello function in s23_srvr.c in OpenSSL 1.0.1 before 1.0.1i allows man-in-the-middle attackers to force the use of TLS 1.0 by triggering ClientHello message fragmentation in communication between a client and server that both support later TLS versions, related to a "protocol downgrade" issue.
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Analysis
by VulDB Data Team • 02/10/2022
The vulnerability described in CVE-2014-3511 represents a critical protocol downgrade attack vector within the OpenSSL implementation that affects versions prior to 1.0.1i. This flaw specifically targets the ssl23_get_client_hello function located in the s23_srvr.c source file, which serves as a crucial component in the SSL/TLS handshake process. The vulnerability enables malicious actors to manipulate the negotiation process between client and server, forcing a downgrade to TLS 1.0 even when both parties support more secure later versions of the protocol. This represents a fundamental weakness in the cryptographic handshake mechanism that undermines the security assurances typically provided by modern TLS implementations.
The technical execution of this vulnerability relies on a specific condition involving ClientHello message fragmentation that occurs during the initial handshake phase. When a client and server both support TLS 1.0 or later versions, an attacker positioned in the network path can manipulate the communication to fragment the ClientHello message in a particular way. This fragmentation triggers the ssl23_get_client_hello function to incorrectly interpret the handshake, leading to a forced downgrade to TLS 1.0. The attacker essentially exploits a design flaw in how OpenSSL handles the transition between different SSL/TLS protocol versions during the handshake process. This issue is categorized under CWE-310 as a Cryptographic Vulnerability, specifically involving protocol-level weaknesses in cryptographic implementations.
The operational impact of this vulnerability extends beyond simple security degradation, as it exposes systems to a range of sophisticated attacks that were previously mitigated by stronger TLS implementations. When TLS 1.0 is forced, systems become vulnerable to attacks such as POODLE (Padding Oracle On Downgraded Legacy Encryption) which specifically targets weaknesses in TLS 1.0's encryption mechanisms. The vulnerability affects any system using OpenSSL versions before 1.0.1i and can be exploited by man-in-the-middle attackers without requiring special privileges or authentication. This creates a significant risk for web servers, email servers, and any network service that relies on OpenSSL for secure communications, potentially allowing attackers to intercept and modify sensitive data transmitted over what should be secure connections. The attack vector is particularly concerning because it can be executed automatically and transparently to end users, making it difficult to detect and prevent.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to OpenSSL 1.0.1i or later versions, which contain the necessary patches to address the protocol downgrade issue. Additionally, administrators should consider implementing strict TLS version policies that disable support for TLS 1.0 entirely, as recommended by industry best practices and standards such as those outlined in NIST SP 800-52. Network monitoring should be enhanced to detect unusual handshake patterns that might indicate exploitation attempts. The vulnerability also highlights the importance of implementing proper protocol version negotiation and the necessity of robust cryptographic libraries that properly handle version compatibility checks. Security teams should conduct comprehensive audits of all systems using OpenSSL to ensure proper patching and configuration, as this vulnerability can be exploited silently and without user awareness. The ATT&CK framework categorizes this as a protocol downgrade attack, which falls under the technique of "T1072 - Web Shell" and "T1566 - Phishing with Malicious Attachments" when combined with other attack vectors, making it a critical component of layered security defense strategies.