CVE-2015-1794 in OpenSSL
Summary
by MITRE
The ssl3_get_key_exchange function in ssl/s3_clnt.c in OpenSSL 1.0.2 before 1.0.2e allows remote servers to cause a denial of service (segmentation fault) via a zero p value in an anonymous Diffie-Hellman (DH) ServerKeyExchange message.
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Analysis
by VulDB Data Team • 06/28/2022
The vulnerability identified as CVE-2015-1794 represents a critical denial of service flaw within OpenSSL's SSL/TLS implementation that specifically affects versions prior to 1.0.2e. This issue resides in the ssl3_get_key_exchange function located in the ssl/s3_clnt.c source file, which handles the processing of key exchange messages during SSL 3.0 handshakes. The flaw manifests when a remote server sends a specially crafted anonymous Diffie-Hellman ServerKeyExchange message containing a zero p value, causing client applications to experience segmentation faults and subsequent crashes.
The technical root cause of this vulnerability stems from inadequate input validation within the Diffie-Hellman key exchange processing logic. When OpenSSL processes a ServerKeyExchange message, it expects certain mathematical parameters to adhere to specific constraints for secure cryptographic operations. The zero p value represents an invalid mathematical parameter that should be rejected during validation, but the vulnerable code fails to properly check for this condition before proceeding with cryptographic operations. This failure creates a path where the application attempts to perform mathematical operations with invalid parameters, leading to memory corruption and ultimately segmentation faults. The vulnerability specifically targets anonymous Diffie-Hellman exchanges where the server does not provide authentication certificates, making it particularly dangerous in scenarios where clients might encounter untrusted servers.
The operational impact of CVE-2015-1794 extends beyond simple service disruption to potentially compromise the availability of critical network services. Any OpenSSL-based client application that processes SSL 3.0 handshakes becomes vulnerable to this attack, including web browsers, email clients, and any software that relies on OpenSSL for secure communications. The segmentation fault resulting from this vulnerability can cause applications to terminate abruptly, leading to service outages that may be difficult to distinguish from other system failures. In high-availability environments, this could result in cascading failures affecting multiple dependent services. The vulnerability is particularly concerning because it can be exploited by remote attackers without requiring authentication or special privileges, making it an attractive vector for denial of service attacks against SSL/TLS clients. According to CWE classification, this vulnerability maps to CWE-129: Improper Validation of Array Index, as the flaw involves improper validation of cryptographic parameters that leads to memory access violations.
Mitigation strategies for CVE-2015-1794 primarily focus on upgrading affected OpenSSL installations to version 1.0.2e or later, where the vulnerability has been addressed through proper input validation. System administrators should prioritize patching all affected systems, particularly those running SSL/TLS clients that may encounter untrusted servers. Additional protective measures include implementing network-level filtering to block suspicious SSL 3.0 handshakes, disabling anonymous Diffie-Hellman cipher suites where possible, and monitoring for unusual segmentation fault patterns in application logs. From an ATT&CK framework perspective, this vulnerability aligns with T1499.004: Endpoint Denial of Service, as it specifically targets the availability of client endpoints through deliberate application crashes. Organizations should also consider implementing intrusion detection systems that can identify and alert on anomalous SSL handshake patterns that may indicate exploitation attempts. The fix implemented in OpenSSL 1.0.2e includes enhanced validation of Diffie-Hellman parameters, ensuring that invalid values such as zero p parameters are properly rejected before cryptographic operations are attempted, thereby preventing the memory corruption that led to segmentation faults.