SSL/TLS Security

Downgrade Attack prevention

 openssl s_client –tls1 -fallback_scsv -connect

If your server supports something better than SSLv3 and checks for the presence of the TLS_FALLBACK_SCSV cipher, it should abort the connection with an error like the following:

 tlsv1 alert inappropriate fallback:s3_pkt.c:1262:SSL alert number 86

Cipher Suites

DES Cipher (Connection should fail):

 openssl s_client -cipher DES -connect

3DES Cipher (Connection should fail)

 openssl s_client -cipher 3DES -connect

Export Cipher (Connection should fail):

 openssl s_client -cipher EXPORT -connect

Low Cipher (Connection should fail):

 openssl s_client -cipher LOW -connect

RC4 Cipher (Connection should fail):

 openssl s_client -cipher RC4 -connect

NULL Cipher (Connection should fail):

 openssl s_client -cipher NULL -connect

Perfect Forward Secrecy Cipher (Connection should NOT fail):

 openssl s_client -cipher EECDH, EDH NULL -connect


Secure Renegotiation

 openssl s_client -connect [host]:[port]

Testing this should return the following

 Secure Renegotiation IS NOT supported

Client-initiated Renegotiation

Once the connection is established, the server will wait for us to type the next command. We can write the following two lines in order to initiate a renegotiation by specifying R in the second line, followed by enter or return.

 openssl s_client -connect [host]:[port]
 HEAD / HTTP/1.0
 <Enter or Return key>

A system that does not support client-initiated renegotiation will return an error and end the connection, or the connection will time out. Please note that below is just one of the many different error messages that you can encounter when your renegotiation is blocked.



 openssl s_client -connect [host]:[port] -cipher "EDH"

The DH parameter size used is displayed in the output next to β€œServer Temp Key”. Please note that you’ll need at least OpenSSL 1.0.2 to display the β€˜Sever Temp Key’ parameter.

 No client certificate CA names sent
 Peer signing digest: SHA512
 Server Temp Key: DH, 2048 bits
 SSL handshake has read 6641 bytes and written 455 bytes
 New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-GCM-SHA384

TLS Service Supports Anonymous DH Key Exchange

$ sslscan --xml=sslscan-output.xml [host]:[port]
$ cat sslscan-output.xml | grep -i accept | grep ADH

TLS Insecure Renegotiation Supported

$ openssl s_client -connect [host]:[port]

Check the output for the following string: "Secure Renegotiation is NOT supported"

Type "R" with a SINGLE carriage return:


If the connection stays open, issue an HTTP request with DOUBLE carriage returns:


If the server replies with some data, it is affected by this issue

Determine the server's preferred cipher suite

Using OpenSSL, we can connect presenting the list of all ciphers:

openssl s_client -connect [host]:[port] -cipher 'ALL:COMPLEMENTOFALL'


   Protocol  : TLSv1
   Cipher    : AES128-SHA

TLS Weak Ciphers Supported

sslscan [host]:[port] | grep Accept

Any cipher with key length shorter than 128 bit is to be considered weak.

NULL TLS Ciphers Supported

sslscan [host]:[port] > sslscan.out
cat sslscan.out | grep -i null
openssl s_client -connect [host]:[port] -cipher NULL

CRIME Attack

openssl s_client -connect [host]:[port]
Compression: zlib compression
Expansion: zlib comprression
Compression: 1 (zlib compression)

or the one-liner:

echo -ne "\\n\\n! | openssl s_client -connect [host]:[port] | grep "Compression\\|Expansion"
openssl s_client -nextprotoneg NULL [host]:[port]
Protocols advertised by server: h2, spdy/3.1, http/1.1


 openssl s_client -connect [host]:[port]

Submitting the following will allow us to see if HTTP compression is supported by the server.

 GET / HTTP/1.1
 Host: [host]
 Accept-Encoding: compress, gzip

If the response contains encoded data, similar to the following response, it indicates that HTTP compression is supported; therefore the remote host is vulnerable.

 HTTP/1.1 200 OK
 Server: nginx/1.1.19
 Date: Sun, 19 Mar 2015 20:48:31 GMT
 Content-Type: text/html
 Last-Modified: Thu, 19 Mar 2015 23:34:28 GMT
 Transfer-Encoding: chunked
 Connection: keep-alive
 Content-Encoding: gzip

A system which does not support deflate or compression will ignore the compress header request and respond with uncompressed data, indicating that it is not vulnerable.

HeartBleed Attack

python [host]:[port]

Metasploit has a dedicate module that can be used to exploit the vulnerability:

use auxiliary/scanner/ssl/openssl_heartbleed
set RHOST ip_address
set RPORT 443

Also nmap:

nmap -p 443 --script ssl-heartbleed [host]

FREAK Attack

nmap --script ssl-enum-ciphers -p 443 |grep EXPORT

A specific tool can be downloaded from

SSL Certificate Expired

$openssl s_client -connect [host]:[port]
 Not Before: Oct 26:00:00:00 2011 GMT
      Not After: Sep 30 23:59:59 2013 GMT

SSL Certificate Signed Using Weak Hashing Algorithm

$openssl s_client -connect [host]:[port]
 Signature Algorithm: sha1WithRSAEncryption

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