TLS Security (Final) – Assessment

In this blog, we will go over some of the methods and tools to assess the TLS security posture during an engagement. Please remember to document all of the information that you found that could bring values to customers.

I. Enumeration Supported Protocols and Cipher Suites

Nmap has an extensive scripts that do various security checks, fortunately there is a script that helps us enumerate the cipher cipher of the TLS.

nmap --script ssl-enum-ciphers -p 443 domain.com

Note: The default TLS port in web is 443 but this could be changed so it’s recommended to perform a port scanning first.

II. Weak Cipher Suites

Based on the output returned from Nmap or other tools, we need to know what the weak cipher suites are so we can analyze and compare.

1. Anonymous DH suites

Static DH running in anonymous mode which lacks authentication and impersonation which is vulnerable via MITM.

2. Suites using null cipher

Most null cipher suites (e.g., TLS_RSA_WITH_NULL_SHA) perform key exchange and authentication but send material in plaintext.

3. Export-grade suites

Cipher suites deems as export-grade use bulk symmetric encryption algorithms with 40 and 56 bit keys. Data is encrypted, but not the short key lengths permits decrypted via brute-force attack.

4. Suites with weak encryption algorithms

DES, 3DES, IDEA, RC2, and RC4 ciphers used to provide bulk symmetric encryption have known weaknesses. Although byte bias attacks against RC4 are practically cumbersome to undertake, most big tech companies have removed RC4 support from their product which means you will hardly see it.

III. Enumerating Supported Features and Extensions

1. Session resumption

TLS endpoints support resumption via session IDs or RFC 5077 tickets. Handshake flooding can result in denial of service, thus many TLS servers limit the number of session IDs cached for a particular source.

sslyze --resum www.domain.com:443

2. Session renegotiation

sslyze --reneg www.domain.com:443

sslyze --reneg --startls=smtp aspmx.1.google.com:25

3. Listing supported TLS extensions using openssl

openssl s_client -tlsextdebug -connect www.google.com:443

openssl s_client -tlsextdebug -connect www.google.com:443

4. Compression support

sslyze --compression www.google.com:443

5. Fallback support

openssl s_client -connect www.google.com:443 -no_tls1_2 -fallback_scsv

IV. TLS Hardening

Consider going through the following list to hardening your TLS endpoint as:

• Upgrade software to the latest version
• Disable support for SSL 3.0
• Disable weak encryption algorithms
• Disable support for client-initiated renegotiation
• Enforce minimum key lengths
• Ensure that private keys are generated, handled, and stored in a secure fashion
• Use a reputable CA to sign your certificates

V. Summary

Throughout the series, we have gone over some of the important points when it comes to TLS as:

• Identify the TLS library and version
• Enumerate supported protocols and cipher suites
• List supported extensions and features
• Manually qualify vulnerabilities
• TLS Hardening

TLS Security (Part I), SECURITY FEATURES on MacOSX, SHODAN For PENETRATION TESTER, Introduction to IDS and IPS, Data Security