Test ID:	1.3.6.1.4.1.25623.1.0.117840
Category:	SSL and TLS
Title:	Diffie-Hellman Ephemeral Key Exchange DoS Vulnerability (SSL/TLS, D(HE)ater)
Summary:	The remote SSL/TLS server is supporting Diffie-Hellman ephemeral; (DHE) Key Exchange algorithms and thus could be prone to a denial of service (DoS); vulnerability.
Description:	Summary: The remote SSL/TLS server is supporting Diffie-Hellman ephemeral (DHE) Key Exchange algorithms and thus could be prone to a denial of service (DoS) vulnerability. Vulnerability Insight: - CVE-2002-20001: 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. - CVE-2022-40735: The Diffie-Hellman Key Agreement Protocol allows use of long exponents that arguably make certain calculations unnecessarily expensive, because the 1996 van Oorschot and Wiener paper found that '(appropriately) short exponents' can be used when there are adequate subgroup constraints, and these short exponents can lead to less expensive calculations than for long exponents. This issue is different from CVE-2002-20001 because it is based on an observation about exponent size, rather than an observation about numbers that are not public keys. The specific situations in which calculation expense would constitute a server-side vulnerability depend on the protocol (e.g., TLS, SSH, or IKE) and the DHE implementation details. In general, there might be an availability concern because of server-side resource consumption from DHE modular-exponentiation calculations. Finally, it is possible for an attacker to exploit this vulnerability and CVE-2002-20001 together. - CVE-2024-41996: Validating the order of the public keys in the Diffie-Hellman Key Agreement Protocol, when an approved safe prime is used, allows remote attackers (from the client side) to trigger unnecessarily expensive server-side DHE modular-exponentiation calculations. The client may cause asymmetric resource consumption. 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 and validate the order of the public key. Vulnerability Impact: This vulnerability 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, also known as a D(HE)ater attack. There could be an increase in CPU usage in the affected component. For OpenSSH, users may observe issues such as a slowdown in SSH connections. Solution: - DHE key exchange should be disabled if no other mitigation mechanism can be used and either elliptic-curve variant of Diffie-Hellman (ECDHE) or RSA key exchange is supported by the clients. The fact that RSA key exchange is not forward secret should be considered. - Limit the maximum number of concurrent connections in e.g. the configuration of the remote server. For Postfix this limit can be configured via 'smtpd_client_new_tls_session_rate_limit' option, for other products please refer to the manual of the product in question on configuration possibilities. CVSS Score: 5.0 CVSS Vector: AV:N/AC:L/Au:N/C:N/I:N/A:P
Cross-Ref:	Common Vulnerability Exposure (CVE) ID: CVE-2002-20001 https://dheatattack.gitlab.io/ https://ieeexplore.ieee.org/document/10374117 https://cert-portal.siemens.com/productcert/pdf/ssa-506569.pdf https://dheatattack.com https://github.com/Balasys/dheater https://github.com/mozilla/ssl-config-generator/issues/162 https://gitlab.com/dheatattack/dheater https://support.f5.com/csp/article/K83120834 https://www.arubanetworks.com/assets/alert/ARUBA-PSA-2022-004.txt https://www.openssl.org/blog/blog/2022/10/21/tls-groups-configuration/ https://www.reddit.com/r/netsec/comments/qdoosy/server_overload_by_enforcing_dhe_key_exchange/ https://www.researchgate.net/profile/Anton-Stiglic-2/publication/2401745_Security_Issues_in_the_Diffie-Hellman_Key_Agreement_Protocol https://www.suse.com/support/kb/doc/?id=000020510 Common Vulnerability Exposure (CVE) ID: CVE-2022-40735 https://gist.github.com/c0r0n3r/9455ddcab985c50fd1912eabf26e058b https://link.springer.com/content/pdf/10.1007/3-540-68339-9_29.pdf https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r5.pdf https://raw.githubusercontent.com/CVEProject/cvelist/9d7fbbcabd3f44cfedc9e8807757d31ece85a2c6/2022/40xxx/CVE-2022-40735.json https://www.researchgate.net/profile/Anton-Stiglic-2/publication/2401745_Security_Issues_in_the_Diffie-Hellman_Key_Agreement_Protocol/links/546c144f0cf20dedafd53e7e/Security-Issues-in-the-Diffie-Hellman-Key-Agreement-Protocol.pdf https://www.rfc-editor.org/rfc/rfc3526 https://www.rfc-editor.org/rfc/rfc4419 https://www.rfc-editor.org/rfc/rfc5114#section-4 https://www.rfc-editor.org/rfc/rfc7919#section-5.2 Common Vulnerability Exposure (CVE) ID: CVE-2024-41996
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