CVE-2022-40735 in Key Agreement Protocol
Summary
by MITRE • 11/15/2022
Using long exponents in the Diffie-Hellman Key Agreement Protocol allows remote attackers (from the client side) to trigger unnecessarily expensive server-side DHE modular-exponentiation calculations. An attacker may cause asymmetric resource consumption with any common client application which uses a DHE implementation that applies short exponents. The attack may be more disruptive in cases where a client sends arbitrary numbers that are actually not DH public keys (aka the D(HE)ater attack) or 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. This can affect TLS, SSH, and IKE.
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Analysis
by VulDB Data Team • 12/18/2022
The vulnerability described in CVE-2022-40735 represents a significant security concern within cryptographic protocols that utilize the Diffie-Hellman Key Agreement Protocol. This issue specifically targets the implementation of Diffie-Hellman exponentiation calculations in server-side cryptographic operations, creating a potential denial-of-service vector that can be exploited by remote attackers. The flaw arises from the improper handling of exponent values during key agreement processes, where attackers can manipulate the mathematical operations to force servers into performing computationally intensive calculations that would not normally occur under standard operational conditions.
The technical implementation of this vulnerability stems from the mathematical properties of modular exponentiation within Diffie-Hellman cryptography. When a client presents excessively long exponents during the key agreement process, servers that implement Diffie-Hellman with insufficient input validation will proceed to perform the full modular exponentiation calculation using these extended values. This creates an asymmetric resource consumption pattern where client-side computation remains minimal while server-side processing becomes disproportionately expensive. The vulnerability is particularly concerning because it can be triggered through seemingly normal client applications that use standard DHE implementations, making detection more challenging for system administrators.
The operational impact of CVE-2022-40735 extends beyond simple resource exhaustion, potentially affecting the availability and performance of critical network services. The attack can be particularly disruptive in environments where servers must support legacy DHE implementations, as attackers can force the server to select its maximum supported key size, thereby maximizing computational overhead. This vulnerability affects multiple security protocols including TLS, SSH, and IKE, making it a widespread concern across different network security implementations. The D(HE)ater attack variant described in the vulnerability allows attackers to send arbitrary numbers that are not actual Diffie-Hellman public keys, further complicating detection and mitigation efforts.
Security practitioners should consider this vulnerability in the context of established frameworks such as CWE-327, which addresses weak cryptographic algorithms and improper implementation of cryptographic protocols. The attack pattern aligns with techniques found in the ATT&CK framework under the T1499.004 subtechnique for network denial of service, where attackers exploit protocol weaknesses to consume system resources. Mitigation strategies should focus on implementing proper input validation for exponent values, limiting the maximum allowable exponent length, and configuring servers to avoid unnecessary key size selection during handshake processes. Additionally, organizations should consider disabling DHE support where possible and transitioning to more secure elliptic curve cryptography implementations that are less susceptible to such attacks. The vulnerability highlights the importance of implementing robust cryptographic protocol implementations that account for potential adversarial input manipulation and resource consumption patterns.