CVE-2018-0732 in Fujitsu M10-1
Summary
by MITRE
During key agreement in a TLS handshake using a DH(E) based ciphersuite a malicious server can send a very large prime value to the client. This will cause the client to spend an unreasonably long period of time generating a key for this prime resulting in a hang until the client has finished. This could be exploited in a Denial Of Service attack. Fixed in OpenSSL 1.1.0i-dev (Affected 1.1.0-1.1.0h). Fixed in OpenSSL 1.0.2p-dev (Affected 1.0.2-1.0.2o).
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Analysis
by VulDB Data Team • 07/28/2024
The vulnerability described in CVE-2018-0732 represents a significant denial of service weakness in OpenSSL implementations that handle Diffie-Hellman key exchange operations within TLS handshakes. This flaw specifically impacts systems using DH(E) based ciphersuites where the server provides a prime number for key generation. When a malicious server deliberately sends an excessively large prime value to a client, the client's cryptographic library must perform computationally intensive operations to generate the corresponding key material, resulting in extended processing times that can cause the client to become unresponsive or hang indefinitely.
The technical root cause of this vulnerability lies in the insufficient validation of cryptographic parameters during the TLS handshake process. According to CWE-327, this represents a weakness in cryptographic implementation that allows for improper handling of parameters that should be constrained to reasonable values. The vulnerability specifically targets the Diffie-Hellman key exchange mechanism where the security and performance characteristics depend on the size and properties of the prime numbers used in the calculations. When an attacker manipulates the prime value to an extremely large size, the computational complexity of the modular exponentiation operations increases exponentially, creating a resource exhaustion scenario.
From an operational standpoint, this vulnerability presents a severe threat to systems that rely on OpenSSL for secure communications, particularly web servers, load balancers, and any infrastructure that processes TLS handshakes from untrusted clients. The attack vector is straightforward yet effective, requiring only that a malicious server or man-in-the-middle attacker send an oversized prime value during the key exchange phase. The impact manifests as complete service unavailability for clients attempting to establish TLS connections with the compromised server, making this a critical vulnerability for any organization relying on TLS-based services.
The mitigation strategy involves updating OpenSSL implementations to versions that include proper parameter validation and size constraints for Diffie-Hellman primes. OpenSSL versions 1.1.0i-dev and 1.0.2p-dev contain the necessary fixes that implement reasonable upper limits for prime values and include additional validation checks during the key exchange process. Organizations should also consider implementing additional network-level protections such as connection timeouts and rate limiting to minimize the impact of potential attacks. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 (Endpoint Denial of Service) and represents a classic example of how cryptographic weaknesses can be exploited to create system-level denial of service conditions that affect availability and operational continuity.