D(HE)ater is a proof of concept implementation of the D(HE)at attack (CVE-2002-20001) through which denial-of-service can be performed by enforcing the Diffie-Hellman key exchange.
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D(HE)ater is an attacking tool based on CPU heating in that it forces the ephemeral variant of Diffie-Hellman key exchange (DHE) in given cryptography protocols (e.g. TLS, SSH). It is performed without calculating a cryptographically correct ephemeral key on the client-side, but with a significant amount of calculation on the server-side. Based on this, a denial-of-service (DoS) attack can be initiated, called D(HE)at attack (CVE-2002-20001).

Quick start

D(HE)ater can be installed directly via pip from PyPi

pip install dheater
dheat --protocol tls ecc256.badssl.com
dheat --protocol ssh ecc256.badssl.com

or can be used via Docker from Docker Hub

docker pull balasys/dheater
docker run --tty --rm balasys/dheater --protocol tls ecc256.badssl.com
docker run --tty --rm balasys/dheater --protocol ssh ecc256.badssl.com

You can increase load by string extra threads.

dheat --thread-num 4 --protocol tls ecc256.badssl.com
docker run --tty --rm balasys/dheater --thread-num 4 --protocol tls ecc256.badssl.com
docker run --tty --rm balasys/dheater --thread-num 4 --protocol ssh ecc256.badssl.com


Without attacking a server or accessing its configuration it is still possible to determine whether Diffie-Hellman (DH) key exchange is enabled and if so what DH parameters (prime, genrator, key size) are used. Command line tools such as CryptoLyzer (TLS, SSH KEX/GEX), testssl.sh (TLS only), or ssh-audit (SSH KEX only) can do that work.


cryptolyze tls1_2 dhparams example.com
cryptolyze tls1_3 dhparams example.com

testssl.sh --fs example.com


cryptolyze ssh2 dhparams example.com

ssh-audit example.com



Diffie-Hellman (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.


Elliptic-curve (named group) setting is necessary only if the underlying cryptographic library supports negotiation Diffie-Hellman groups by implementing RFC7919 in TLS 1.2 or supporting the Finite Field Diffie-Hellman parameter groups named groups in TLS 1.3.

Library Version FFDHE goups
in TLS 1.2
FFDHE groups
in TLS 1.3
OpenSSL < 3.0 no no
OpenSSL 3.0 no yes
GnuTLS 3.5.6 yes no
GnuTLS 3.6.3 yes yes
SSLCipherSuite ...:!kDHE
SSLOpenSSLConfCmd Groups x25519:secp256r1:x448:secp521r1:secp384r1
ssl_ciphers ...:!kDHE;
ssl_ecdh_curve x25519:secp256r1:x448:secp521r1:secp384r1;
  1. Diffie-Hellman key exchange algorithms can be removed by setting the tls_medium_cipherlist configuration option.

    tls_medium_cipherlist ...:!kDHE

  2. Maximal number of new TLS sessions that a remote SMTP client is allowed to negotiate can be controlled by configuration option smtpd_client_new_tls_session_rate_limit configuration option.

    smtpd_client_new_tls_session_rate_limit 100


See moz://a SSL Configuration Generator for configuration syntax.

DH parameter files

If DH key exchange need to be supported recommended private key length value should be set to ensure the best performance of DH key exchange this option value should be set appropriately to achieve the best performance without a security risk.

You can check whether you DH parameter file contains the recommended private key value by the following command:

tools/dh_param_priv_key_size_setter /path/to/dh/parameter/file.pem

The result looks like the following. If the original private key size is None it some cryptographic libraries use the public size for private key size unless the application server overrides this behaviour. This will cause much lower performance than small private keys would be used.

Original private key size: None
Set private key size: None

To set the recommended private key size in a DH parameter file use the following commmand:

tools/dh_param_priv_key_size_setter --private-key-size KEY_SIZE /path/to/dh/parameter/file.pem

For appropriately private key sizes see Table 2 of NIST SP 800-57 Part 1. Alternatively you can download the well-know DH parameters where the recommended private key size is set according to OpenSSL default values from data directory.


  1. Diffie-Hellman key exchange algorithms can be removed by setting the KexAlgorithms configuration option.

    KexAlgorithms -diffie-hellman-group1-sha1,diffie-hellman-group1-sha256,diffie-hellman-group14-sha1,diffie-hellman-group14-sha256,diffie-hellman-group15-sha256,diffie-hellman-group15-sha512,diffie-hellman-group16-sha256,diffie-hellman-group16-sha512,diffie-hellman-group17-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha512

  2. Maximum number of concurrent unauthenticated connections can be controlled by some configuration options




There are no relevant filters.

  1. apache-ssl.conf in fail2ban directory should be copied to the filter.d directory under the fail2ban configuration directory

  2. the followings should be added to the jail.local file in the fail2ban configuration directory

    port    = https
    logpath = %(apache_error_log)s
    maxretry = 1

There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.

mode = ddos

There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.

mode = aggressive

or a specific filter can be used without changing the mode of dovecot.

  1. dovecot-ssl.conf in fail2ban directory should be copied to the filter.d directory under the fail2ban configuration directory

  2. the followings should be added to jail.local in tge fail2ban configuration directory

    port    = pop3,pop3s,imap,imaps,submission,465,sieve
    logpath = %(dovecot_log)s
    backend = %(dovecot_backend)s
    maxretry = 1



There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.

mode = ddos


The code is available under the terms of Apache License Version 2.0. A non-comprehensive, but straightforward description and also the full license text can be found at Choose an open source license website.

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