[squid-users] Squid 3.5.21 "hangs" when trying to connect using unsupported cipher (complete DoS)

[Martin Tenev]

Hello,

I am having problems with squid & SSL. I have setup squid in reverse-proxy
configuration and overall it works fine, however for security reasons I had
to disable some of the ciphers. I have taken an example configuration from
http://www.rawiriblundell.com/?p=1442 and my https_port line looks pretty
much like this (this is the example from the website but the disabled
ciphers are the same for me as well):

https_port  443 accel defaultsite=someinternalhost vhost
cert=/etc/squid/CertAuth/supersecret.crt
key=/etc/squid/CertAuth/supersecret.key
options=NO_SSLv2,NO_SSLv3,CIPHER_SERVER_PREFERENCE
cipher=ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DES-CBC3-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4
dhparams=/etc/squid/CertAuth/dhparams.pem

During my build I have included the config options --enable-ssl and
--with-openssl=/usr

Using the proxy through a browser works fine, but if I try nmap --script
ssl-enum-ciphers -p 443 <host> or openssl s_client -cipher 'RC4-SHA'
-connect <host> these commands result in complete DoS for a few minutes. I
figured out that only the unsupported or disabled ciphers cause this
problem. Also when I do the openssl connection as shown above the proxy
will be unresponsive as long as openssl is trying to connect using the
disabled cipher. As soon as it finishes (eg times out unable to connect
using RC4) the proxy starts serving requests again. I should mention that I
am running squid inside a docker container if this matters at all.

The errors in my logs are :
"Error negotiating SSL connection on FD 22: error 1408A0C1:SSL
routines:SSL3_GET_CLIENT_HELLO: no shared cipher (1/-1)

"Error negotiating SSL connection on FD 25: error 1408A10B:SSL
routines:SSL3_GET_CLIENT_HELLO: wrong version number (1/-1)

P.S I also tried squid 4, and got exactly the same problem.

Any help will be much appreciated

Thanks!
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