Headline
CVE-2022-40735: Stop recommending DHE, because of "dheater" vulnerability · Issue #162 · mozilla/ssl-config-generator
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.
It’s a well known property of DHE that it’s more computationally intensive than RSA and ECDHE key exchanges.
This is not about the (well known) performance difference under normal usage. This novel technique lets an SSL client trivially occupy 100% CPU on the SSL server by sending rogue data, ie. without requiring a corresponding computational effort from the client. This is an easy DoS attack, which can be mitigated by disabling DHE, or by rate limiting new connections per client (which not all SSL servers can do).
That’s the case for all DH requests from the client, be it FFDHE or ECDHE, no ClientHello (not even in TLS 1.3) includes proof of work or proof of freshness of the key share, at the same time the server has to generate a fresh key share and calculate the shared keys (the processes that use up the CPU time).
Even on a very fast CPU, you can’t expect more than ~2k ECDH key exchanges per second per core with a P-384 curve. With the massive 379 byte large TLS 1.3 Client Hello from OpenSSL 1.1.1 that translates to something like 1MiB/s of traffic (after including connection establishment) to fully saturate a core. That’s residential broadband levels of upload.
Does that qualify as a DoS too? At what point we can’t add support for a stronger, but slower algorithm because that would make the server vulnerable?