F5 Networks Response to US-CERT Alert (TA17-075A) HTTPS Interception Weakens TLS Security Summary When properly configured, the F5 BIG-IP addresses nearly all the concerns, and avoids nearly all the risks, specified in US-CERT Alert (TA17-075A). This is the key point; on BIG-IP most of the functionality, and the level of security, is determined by the customer created configuration. Whether certificates are validated, and the level of validation, is determined by the configuration, as are the specific cipher suites and protocols that are supported, etc. There are a couple of corner cases, as explained below, where the BIG-IP does not fully meet the recommendations; F5 Networks is continuously improving our products and working to address these areas in future releases. In short, when properly configured, the F5 BIG-IP or SSL Orchestrator can perform SSL Interception without compromising overall security. Detailed Examination A great deal of interest has been generated by the recent publication of US-CERT Alert (TA17-075A) HTTPS Interception Weakens TLS Security. This is of interest to F5 customers as we do support SSL Intercept on BIG-IP, as well as commonly being used for SSL/TLS termination. While the US-CERT Alert does not name F5 Networks, a primary reference for the alert is a two year old blog post, The Risks of SSL Inspection, which does. It is important to acknowledge upfront that the concern, in general, is not unwarranted. Any point where encrypted communications are decrypted, such as for SSL Intercept, is a potential weak link in the chain, a point of increased vulnerability. But it is equally important to acknowledge that such interception may be necessary as part of a comprehensive security solution. Interception allows for traffic inspection, such as to protect networks from malware or phishing attempts. It is also used for scanning inbound & outbound traffic for evidence of data extraction or an existing intrusion. Without the use of SSL/TLS Interception at an ingress or egress chokepoint, the same level of protection could only be achieved by monitoring every end-point device on the network. End-point protection is much more resource intensive, and it may not be practical to do so on appliances such as routers, switches, or even networked printers. With the growth of the Internet of Things (IoT) endpoint-based protection is increasingly unfeasible. It is not possible to run detection software on every connected thermostat or lightbulb. Malware is increasingly using encrypted communication to hide from conventional detection systems, and interception is required to level the playing field. In response to the question of ‘why use or support SSL/TLS interception at all?’ consider an even larger question: If not SSL/TLS interception, how do you provide equivalent protection? Using interception may add some risk, but there are greater risks in not detecting and blocking malware, phishing attempts, intrusions, etc. Security is about minimizing the overall risk, and tradeoffs are required. SSL/TLS interception is a useful tool and when used well it adds minimal risk while providing a great security benefit. While the US-CERT Alert generalizes, the cited blog post raised specific technical concerns which have subsequently been raised by customers. These points are addressed in Appendix A. A second reference cited in the alert, a research paper, The Security Impact of HTTPS Interception, touches on several other potential issues. This include the protocols supported by the interception device (allowing a protocol downgrade), the ciphers allowed by the device, etc. On F5 products these are configurable in the associated profiles. The level of security, or vulnerability, is determined by the configuration used by the customer – and not F5 or the BIG-IP inherently. Customers who had additional concerns, or who desire assistance in configuring their BIG-IP to meet their specific needs, are encouraged