US-CERT Warns That HTTPS Inspection Tools Weaken TLS

HTTPS inspection tools are, in essence, a security team's authorized man-in-the-middle attacker: they intercept encrypted SSL/TLS traffic, in order to, for example, search it for malware that uses HTTPS to connect to malicious servers. However, in an alert today, US-CERT warned that HTTPS interception weakens TLS security, advising that organizations "carefully consider the pros and cons of such products before implementing."

Normally, a Web browser will alert a user to weak ciphers, deprecated protocol versions, or other reasons that certificates should not be trusted and connections might be dangerous. Once an HTTPS interception tool is introduced, however, the user must put all its trust in the tool.

From the US-CERT alert:

"Because the HTTPS inspection product manages the protocols, ciphers, and certificate chain, the product must perform the necessary HTTPS validations. Failure to perform proper validation or adequately convey the validation status increases the probability that the client will fall victim to MiTM attacks by malicious third parties."

Unfortunately, researchers have found these products lacking when it comes to those validation practices. For example - as noted in works cited in the advisory, "The Risks of SSL Inspection" and "The Security Impact of HTTPS Interception" - some HTTPS inspection products do incomplete validation of upstream certificates, others conduct complete validation but fail to convey the results back to the client, and others will complete communication to the target server before issuing warnings to the user.   

HTTPS interception capabilities are built into a wide variety of security tools, including firewalls, secure web gateways, data loss prevention products, and other applications. A partial list of potentially affected applications is available here. 

US-CERT recommends that organizations use the testing resources at BadSSL.com to determine whether or not their HTTPS interception applications are properly validating certificates and preventing connections to sites using weak cryptography.

"At a minimum," states the alert, "if any of the tests in the Certificate section of badssl.com prevent a client with direct Internet access from connecting, those same clients should also refuse the connection when connected to the Internet by way of an HTTPS inspection product."