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Insecure TLS Ciphers supported

Insecure TLS Ciphers supported

Description

Transport Layer Security (TLS) starts with the client requesting a secure connection and presenting a list of supported cipher suites.

A cipher suite is a list of authentication, encryption, message authentication code (MAC), and key exchange algorithms. Each algorithm serves a specific role during the protocol negotiation. For example, using a week algorithm might critically impact the security of the whole channel.

It was identified that the endpoint supports a combination of cipher suites and Secure Sockets Layer / Transport Layer Security (SSL/TLS) protocols that suffer from known cryptographic weaknesses. Therefore they should not be relied upon for effective transport layer security. An attacker who can eavesdrop on the connection could influence or decrypt the traffic passing by.

Cryptographic weaknesses have been demonstrated for all the following configurations:

  • Support for deprecated SSL protocol (v2 or v3)
  • Support cipher suites offering keys smaller than 128bit
  • Support of cipher suites offering CBC mode in combination with TLS protocol lower than version 1.1 vulnerable to the BEAST attack
  • Use of compression within TLS vulnerable to the CRIME attack
  • Support of cipher suites offering RC4 as a cipher
  • Support of cipher suites offering DES as a cipher
  • Support of cipher suites offering MD5 as a signature algorithm

Recommendation

The recommended TLS configuration should enforce the following recommendations:

  • Restrict to TLSv1 and above, with TLS1.2 being preferred
  • If SSLv3 is required, it is advised to implement the TLS Fallback SCSV feature to prevent protocol downgrade attacks
  • Disable Anonymous Diffie-Hellman (ADH)
  • Disable aNULL and eNull cipher suites
  • Disable Export key exchange suites
  • Remove RC4 support
  • Remove DES support
  • Remove MD5 support
  • Prefere SHA256 over SHA1
  • Prefer AES128 over AES256 as 256 offers little security advantages and is less robust to timing attacks
  • Disable Client-Initiated Renegotiation
  • Disable TLS compression
  • Offer only ciphers with a key length of greater than 128bit
  • Offer cipher suites with Perfect-Forward Secrecy protocol properties (DHE, ECDHE)
  • Use custom Diffie-Hellman group to protect against Logjam attack
  • Implement the HTTP Strict Transport Security header field
  • Implement OSCP stapling

Sample of a secure TLS configuration for Nginx generated with Mozilla SSL Configuration Generator

    server {
        listen 443 ssl;

        # certs sent to the client in SERVER HELLO are concatenated in ssl_certificate
        ssl_certificate /path/to/signed_cert_plus_intermediates;
        ssl_certificate_key /path/to/private_key;
        ssl_session_timeout 1d;
        ssl_session_cache shared:SSL:50m;
        ssl_session_tickets off;

        # Diffie-Hellman parameter for DHE ciphersuites, recommended 2048 bits
        ssl_dhparam /path/to/dhparam.pem;

        # modern configuration. tweak to your needs.
        ssl_protocols TLSv1.1 TLSv1.2;
        ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!3DES:!MD5:!PSK';
        ssl_prefer_server_ciphers on;

        # HSTS (ngx_http_headers_module is required) (15768000 seconds = 6 months)
        add_header Strict-Transport-Security max-age=15768000;

        # OCSP Stapling ---
        # fetch OCSP records from URL in ssl_certificate and cache them
        ssl_stapling on;
        ssl_stapling_verify on;

        ## verify chain of trust of OCSP response using Root CA and Intermediate certs
        ssl_trusted_certificate /path/to/root_CA_cert_plus_intermediates;

        resolver <IP DNS resolver>;

        ....
    }

Standards