Headline
CVE-2002-20001: GitHub - Balasys/dheater: D(HE)ater is a security tool can perform DoS attack by enforcing the DHE key exchange.
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.
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, D(HE)ater can initiate a denial-of-service (DoS) attack.
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
Mitigation****Configuration
Diffie-Hellman (DHE) key exchange should be disabled.
TLS****Apache****NGINX****Postfix
Diffie-Hellman key exchange algorithms can be removed by setting the tls_medium_cipherlist configuration option.
tls_medium_cipherlist ...:!kDHEMaximal 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
Others
See moz://a SSL Configuration Generator for configuration syntax.
SSH****OpenSSH
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-sha512Maximum number of concurrent unauthenticated connections can be controlled by configuration option MaxStartups configuration option.
MaxStartups 10:30:100
Fail2Ban****TLS****Apache
There are no relevant filters.
apache-ssl.confinfail2bandirectory should be copied to thefilter.ddirectory under the fail2ban configuration directorythe followings should be added to the
jail.localfile in the fail2ban configuration directory[apache-ssl]
port = https logpath = %(apache_error_log)s maxretry = 1
Postfix
There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.
Dovecot
There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.
[dovecot] mode = aggressive
or a specific filter can be used without changing the mode of dovecot.
dovecot-ssl.confinfail2bandirectory should be copied to thefilter.ddirectory under the fail2ban configuration directorythe followings should be added to
jail.localin tge fail2ban configuration directory[dovecot-ssl]
port = pop3,pop3s,imap,imaps,submission,465,sieve logpath = %(dovecot_log)s backend = %(dovecot_backend)s maxretry = 1
SSH****OpenSSH
There is a relevant filter, but it is applied only in ddos mode. The followings should be added to jail.local.
License
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.