During security audits, such as a PCI-DSS compliance audit, it is very commonplace to test the cipher mechanism that a website / server uses and supports to ensure that weak / outdated cipher methods are not used.
Weak ciphers allow for an increased risk in encryption compromise, man-in-the-middle attacks and other related attack vectors.
Due to historic export restrictions of high grade cryptography, legacy and new web servers are often able and configured to handle weak cryptographic options.
Even if high grade ciphers are normally used and installed, some server misconfiguration could be used to force the use of a weaker cipher to gain access to the supposed secure communication channel.
Testing SSL / TLS cipher specifications and requirements for site
The http clear-text protocol is normally secured via an SSL or TLS tunnel, resulting in https traffic. In addition to providing encryption of data in transit, https allows the identification of servers (and, optionally, of clients) by means of digital certificates.
Historically, there have been limitations set in place by the U.S. government to allow cryptosystems to be exported only for key sizes of, at most, 40 bits, a key length which could be broken and would allow the decryption of communications. Since then, cryptographic export regulations have been relaxed (though some constraints still hold); however, it is important to check the SSL configuration being used to avoid putting in place cryptographic support which could be easily defeated. SSL-based services should not offer the possibility to choose weak ciphers.
Testing for weak ciphers : examples
In order to detect possible support of weak ciphers, the ports associated to SSL/TLS wrapped services must be identified. These typically include port 443, which is the standard https port; however, this may change because a) https services may be configured to run on non-standard ports, and b) there may be additional SSL/TLS wrapped services related to the web application. In general, a service discovery is required to identify such ports.
The nmap scanner, via the “–sV” scan option, is able to identify SSL services. Vulnerability Scanners, in addition to performing service discovery, may include checks against weak ciphers (for example, the Nessus scanner has the capability of checking SSL services on arbitrary ports, and will report weak ciphers).
Example 1. SSL service recognition via nmap.
[root@test]# nmap -F -sV localhost Starting nmap 3.75 ( http://www.insecure.org/nmap/ ) at 2005-07-27 14:41 CEST Interesting ports on localhost.localdomain (127.0.0.1): (The 1205 ports scanned but not shown below are in state: closed) PORT STATE SERVICE VERSION 443/tcp open ssl OpenSSL 901/tcp open http Samba SWAT administration server 8080/tcp open http Apache httpd 2.0.54 ((Unix) mod_ssl/2.0.54 OpenSSL/0.9.7g PHP/4.3.11) 8081/tcp open http Apache Tomcat/Coyote JSP engine 1.0 Nmap run completed -- 1 IP address (1 host up) scanned in 27.881 seconds [root@test]#
Example 2. Identifying weak ciphers with Nessus. The following is an anonymized excerpt of a report generated by the Nessus scanner, corresponding to the identification of a server certificate allowing weak ciphers
https (443/tcp) Description Here is the SSLv2 server certificate: Certificate: Data: Version: 3 (0x2) Serial Number: 1 (0x1) Signature Algorithm: md5WithRSAEncryption Issuer: C=**, ST=******, L=******, O=******, OU=******, CN=****** Validity Not Before: Oct 17 07:12:16 2002 GMT Not After : Oct 16 07:12:16 2004 GMT Subject: C=**, ST=******, L=******, O=******, CN=****** Subject Public Key Info: Public Key Algorithm: rsaEncryption RSA Public Key: (1024 bit) Modulus (1024 bit): 00:98:4f:24:16:cb:0f:74:e8:9c:55:ce:62:14:4e: 6b:84:c5:81:43:59:c1:2e:ac:ba:af:92:51:f3:0b: ad:e1:4b:22:ba:5a:9a:1e:0f:0b:fb:3d:5d:e6:fc: ef:b8:8c:dc:78:28:97:8b:f0:1f:17:9f:69:3f:0e: 72:51:24:1b:9c:3d:85:52:1d:df:da:5a:b8:2e:d2: 09:00:76:24:43:bc:08:67:6b:dd:6b:e9:d2:f5:67: e1:90:2a:b4:3b:b4:3c:b3:71:4e:88:08:74:b9:a8: 2d:c4:8c:65:93:08:e6:2f:fd:e0:fa:dc:6d:d7:a2: 3d:0a:75:26:cf:dc:47:74:29 Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Basic Constraints: CA:FALSE Netscape Comment: OpenSSL Generated Certificate Page 10 Network Vulnerability Assessment Report 25.05.2005 X509v3 Subject Key Identifier: 10:00:38:4C:45:F0:7C:E4:C6:A7:A4:E2:C9:F0:E4:2B:A8:F9:63:A8 X509v3 Authority Key Identifier: keyid:CE:E5:F9:41:7B:D9:0E:5E:5D:DF:5E:B9:F3:E6:4A:12:19:02:76:CE DirName:/C=**/ST=******/L=******/O=******/OU=******/CN=****** serial:00 Signature Algorithm: md5WithRSAEncryption 7b:14:bd:c7:3c:0c:01:8d:69:91:95:46:5c:e6:1e:25:9b:aa: 8b:f5:0d:de:e3:2e:82:1e:68:be:97:3b:39:4a:83:ae:fd:15: 2e:50:c8:a7:16:6e:c9:4e:76:cc:fd:69:ae:4f:12:b8:e7:01: b6:58:7e:39:d1:fa:8d:49:bd:ff:6b:a8:dd:ae:83:ed:bc:b2: 40:e3:a5:e0:fd:ae:3f:57:4d:ec:f3:21:34:b1:84:97:06:6f: f4:7d:f4:1c:84:cc:bb:1c:1c:e7:7a:7d:2d:e9:49:60:93:12: 0d:9f:05:8c:8e:f9:cf:e8:9f:fc:15:c0:6e:e2:fe:e5:07:81: 82:fc Here is the list of available SSLv2 ciphers: RC4-MD5 EXP-RC4-MD5 RC2-CBC-MD5 EXP-RC2-CBC-MD5 DES-CBC-MD5 DES-CBC3-MD5 RC4-64-MD5 The SSLv2 server offers 5 strong ciphers, but also 0 medium strength and 2 weak "export class" ciphers. The weak/medium ciphers may be chosen by an export-grade or badly configured client software. They only offer a limited protection against a brute force attack Solution: disable those ciphers and upgrade your client software if necessary. See http://support.microsoft.com/default.aspx?scid=kben-us216482 or http://httpd.apache.org/docs-2.0/mod/mod_ssl.html#sslciphersuite This SSLv2 server also accepts SSLv3 connections. This SSLv2 server also accepts TLSv1 connections. Vulnerable hosts (list of vulnerable hosts follows)
Example 3. Manually audit weak SSL cipher levels with OpenSSL. The following will attempt to connect to Google.com with SSLv2.
[root@test]# openssl s_client -no_tls1 -no_ssl3 -connect www.google.com:443 CONNECTED(00000003) depth=0 /C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com verify error:num=20:unable to get local issuer certificate verify return:1 depth=0 /C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com verify error:num=27:certificate not trusted verify return:1 depth=0 /C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com verify error:num=21:unable to verify the first certificate verify return:1 --- Server certificate -----BEGIN CERTIFICATE----- MIIDYzCCAsygAwIBAgIQYFbAC3yUC8RFj9MS7lfBkzANBgkqhkiG9w0BAQQFADCB zjELMAkGA1UEBhMCWkExFTATBgNVBAgTDFdlc3Rlcm4gQ2FwZTESMBAGA1UEBxMJ Q2FwZSBUb3duMR0wGwYDVQQKExRUaGF3dGUgQ29uc3VsdGluZyBjYzEoMCYGA1UE CxMfQ2VydGlmaWNhdGlvbiBTZXJ2aWNlcyBEaXZpc2lvbjEhMB8GA1UEAxMYVGhh d3RlIFByZW1pdW0gU2VydmVyIENBMSgwJgYJKoZIhvcNAQkBFhlwcmVtaXVtLXNl cnZlckB0aGF3dGUuY29tMB4XDTA2MDQyMTAxMDc0NVoXDTA3MDQyMTAxMDc0NVow aDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3JuaWExFjAUBgNVBAcTDU1v dW50YWluIFZpZXcxEzARBgNVBAoTCkdvb2dsZSBJbmMxFzAVBgNVBAMTDnd3dy5n b29nbGUuY29tMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC/e2Vs8U33fRDk 5NNpNgkB1zKw4rqTozmfwty7eTEI8PVH1Bf6nthocQ9d9SgJAI2WOBP4grPj7MqO dXMTFWGDfiTnwes16G7NZlyh6peT68r7ifrwSsVLisJp6pUf31M5Z3D88b+Yy4PE D7BJaTxq6NNmP1vYUJeXsGSGrV6FUQIDAQABo4GmMIGjMB0GA1UdJQQWMBQGCCsG AQUFBwMBBggrBgEFBQcDAjBABgNVHR8EOTA3MDWgM6Axhi9odHRwOi8vY3JsLnRo YXd0ZS5jb20vVGhhd3RlUHJlbWl1bVNlcnZlckNBLmNybDAyBggrBgEFBQcBAQQm MCQwIgYIKwYBBQUHMAGGFmh0dHA6Ly9vY3NwLnRoYXd0ZS5jb20wDAYDVR0TAQH/ BAIwADANBgkqhkiG9w0BAQQFAAOBgQADlTbBdVY6LD1nHWkhTadmzuWq2rWE0KO3 Ay+7EleYWPOo+EST315QLpU6pQgblgobGoI5x/fUg2U8WiYj1I1cbavhX2h1hda3 FJWnB3SiXaiuDTsGxQ267EwCVWD5bCrSWa64ilSJTgiUmzAv0a2W8YHXdG08+nYc X/dVk5WRTw== -----END CERTIFICATE----- subject=/C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com issuer=/C=ZA/ST=Western Cape/L=Cape Town/O=Thawte Consulting cc/OU=Certification Services Division/CN=Thawte Premium Server CA/emailAddressfirstname.lastname@example.org --- No client certificate CA names sent --- Ciphers common between both SSL endpoints: RC4-MD5 EXP-RC4-MD5 RC2-CBC-MD5 EXP-RC2-CBC-MD5 DES-CBC-MD5 DES-CBC3-MD5 RC4-64-MD5 --- SSL handshake has read 1023 bytes and written 333 bytes --- New, SSLv2, Cipher is DES-CBC3-MD5 Server public key is 1024 bit Compression: NONE Expansion: NONE SSL-Session: Protocol : SSLv2 Cipher : DES-CBC3-MD5 Session-ID: 709F48E4D567C70A2E49886E4C697CDE Session-ID-ctx: Master-Key: 649E68F8CF936E69642286AC40A80F433602E3C36FD288C3 Key-Arg : E8CB6FEB9ECF3033 Start Time: 1156977226 Timeout : 300 (sec) Verify return code: 21 (unable to verify the first certificate) --- closed
These tests usually provide a very in-depth and reliable method for ensuring weak and vulnerable ciphers are not used in order to comply with said audits.
Personally, I prefer the nessus audit scans. Usually the default “free” plugins are enough to complete these types of one-off audits. There are, however, commercial nessus plugins designed just for PCI-DSS compliance audits and are available for purchase from the nessus site.