CVE-2002-20001 in Key Agreement Protocol
Summary
by MITRE • 11/11/2021
The Diffie-Hellman Key Agreement Protocol allows remote attackers (from the client side) to send arbitrary numbers that are actually not public keys, and trigger expensive server-side DHE modular-exponentiation calculations, aka a D(HE)ater attack. The client needs very little CPU resources and network bandwidth. The attack may be more disruptive in cases where a client can require a server to select its largest supported key size. The basic attack scenario is that the client must claim that it can only communicate with DHE, and the server must be configured to allow DHE.
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Analysis
by VulDB Data Team • 04/15/2025
The vulnerability described in CVE-2002-20001 represents a significant security weakness in the Diffie-Hellman key agreement protocol implementation within cryptographic systems. This flaw specifically targets the Diffie-Hellman Ephemeral (DHE) key exchange mechanism, which is designed to provide forward secrecy in secure communications. The attack exploits the fundamental assumption that all parties in the key exchange process will provide valid public keys. When a client sends arbitrary numbers instead of legitimate public keys, the server must still perform the computationally expensive modular exponentiation operations that are characteristic of the Diffie-Hellman algorithm. This vulnerability falls under the category of cryptographic weakness and specifically relates to improper implementation of key exchange protocols as classified by CWE-326. The attack demonstrates how an attacker can leverage the mathematical properties of the Diffie-Hellman algorithm against the server's computational resources rather than targeting the cryptographic strength of the algorithm itself.
The technical execution of this D(HE)ater attack relies on the server's acceptance of malformed public keys from clients. When a client claims to support DHE key exchange but sends invalid public keys, the server processes these values through the expensive modular exponentiation calculations required for the Diffie-Hellman protocol. This computational overhead occurs because the server cannot easily distinguish between valid public keys and the maliciously crafted numbers that the attacker sends. The attack is particularly effective when the server is configured to support large key sizes, as this increases the computational cost of the modular exponentiation operations. From an operational perspective, this vulnerability represents a resource exhaustion attack that can be executed with minimal client-side resources, making it particularly dangerous in environments where servers must handle numerous concurrent connections. The attack demonstrates the importance of proper input validation and the potential for cryptographic protocols to be exploited through implementation flaws rather than mathematical weaknesses.
The impact of this vulnerability extends beyond simple resource exhaustion to potentially compromise the availability of cryptographic services. Servers configured to support DHE key exchange become vulnerable to this attack when they accept client requests that specify DHE as the preferred key exchange mechanism. The computational burden placed on the server can lead to significant performance degradation or complete service unavailability, especially when multiple clients simultaneously initiate the attack. This type of attack aligns with the ATT&CK technique T1496 for resource exhaustion and can be classified under the broader category of denial of service attacks. The vulnerability particularly affects systems where the server must perform key exchange calculations in response to client requests without adequate validation of the inputs. The attack is amplified when servers are configured to support maximum key sizes, as this increases the computational complexity of the modular exponentiation operations and makes the attack more effective.
Effective mitigation strategies for this vulnerability involve implementing proper input validation and limiting the maximum key sizes that servers will accept from clients. Organizations should configure their cryptographic systems to reject invalid public keys and implement rate limiting to prevent abuse of the key exchange process. The most effective long-term solution involves disabling DHE key exchange support when it is not strictly required, or implementing proper key validation mechanisms that can detect and reject malformed public keys before processing. Additionally, servers should be configured to use fixed key sizes that are appropriate for their computational capabilities rather than allowing clients to dictate the key size. This vulnerability highlights the critical importance of proper cryptographic protocol implementation and the necessity of validating all inputs to cryptographic operations. Security practitioners should also consider implementing monitoring mechanisms to detect unusual patterns of key exchange requests that may indicate an ongoing attack. The vulnerability serves as a reminder that even well-established cryptographic protocols can be compromised through implementation flaws, and that proper validation and resource management are essential components of secure system design.