The 9th item on the OWASP Top 10 is A9 - Insufficient Transport Layer Protection. This is mostly a browser to server and server to server issue.
This is the risk rating from OWASP:
|Consider anyone who can monitor the network traffic of your users. If the application is on the internet, who knows how your users access it. Don't forget back end connections.
|Monitoring users' network traffic can be difficult, but is sometimes easy. The primary difficulty lies in monitoring the proper network's traffic while users are accessing the vulnerable site.
|Applications frequently do not protect network traffic. They may use SSL/TLS during authentication, but not elsewhere, exposing data and session IDs to interception. Expired or improperly configured certificates may also be used.
Detecting basic flaws is easy. Just observe the site's network traffic. More subtle flaws require inspecting the design of the application and the server configuration.
|Such flaws expose individual users' data and can lead to account theft. If an admin account was compromised, the entire site could be exposed. Poor SSL setup can also facilitate phishing and MITM attacks.
|Consider the business value of the data exposed on the communications channel in terms of its confidentiality and integrity needs, and the need to authenticate both participants.
Whenever you are handling private data that needs protection, you should employ SSL or TLS. Ideally you should use SSL/TLS not only between your server and the browser, but also between the server and any backend services. The idea is to prevent disclosure or changes to data at an unauthorized network hops.
Any cookie used for authentication (like session cookies) need to be protected from disclosure over unsecured links. By setting the Secure flag in the Set-Cookie header when delivering the cookie over HTTPS, the server instructs the browser not to send the cookie when making insecure HTTP-requests. This will make sure the application does not accidentally disclose the cookie.
Default installations of SSL/TLS are often quite insecure. They support low security key lengths and also support insecure renegotiation etc., making them vulnerable to key cracking, BEAST or other Man-in-the-middle attacks. A good tool for checking your SSL/TLS configuration is https://www.ssllabs.com/ from Qualys. This tool will only work for endpoints available directly on the internet, so you may also want to look at SSLScan for internal endpoints.