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Why do HTTPS interception tools weaken TLS security?

HTTPS interception tools help protect websites, but they can also hurt TLS security. Expert Judith Myerson explains how this works and what enterprises can do about it.

Many companies use HTTPS to better protect their websites, but I just found out HTTPS interception can weaken TLS security. How does this happen? What measures are available to prevent attackers from taking advantage of this weakness?

All HTTPS interception products intercept HTTPS network traffic and perform a man-in-the-middle attack on the encrypted connection to make sure the attack doesn't succeed.

To get the HTTPS interception products to work, the administrators must install trusted certificates, including the server-side Transport Layer Security (TLS) certificate. TLS is a protocol that encrypts communications between the client and server. Browsers and other client applications use the certificate to validate encrypted connections created by the HTTPS interception product.

To secure a website, a trusted third party needs to install the certificate on a legitimate server. If you enter https:// without the certificate installed, the browser will alert you that it can't connect securely to the page it's trying to reach. If the connection failure message comes up after the certificate is installed, you should check with the third party to find out if the failure is due to improper TLS security settings. If the settings are proper, you should ask to rebound the certificate to your account.

The problem is some organizations aren't making sure their HTTPS interception products are performing correct TLS certificate validation. The client systems have no way of independently validating the HTTPS connection. The failure of some interception products to send warnings or error messages to the user weakens the protections HTTPS aims to provide.

To prevent attackers from taking advantage of this weakness, organizations should:

  • Consider the pros and cons of HTTPS interception products before implementing them.
  • Verify the product properly validates certificate chains and passes any warnings or errors to the client.
  • Read "The Risks of SSL Inspection" for a list of potentially affected software.
  • Take other steps to secure end-to-end communications, as presented in US-CERT Alert TA15-120A.

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