The Register explains:
Your antivirus and network protection efforts may actually be undermining network security, a new paper and subsequent US-CERT advisory have warned.
The issue comes with the use of HTTPS interception middleboxes and network monitoring products. They are extremely common and are used to check that nothing untoward is going on.
However, the very method by which these devices skirt the encryption on network traffic through protocols like SSL, and more recently TLS, is opening up the network to man-in-the-middle attacks.
In the paper [PDF], titled The Security Impact of HTTPS Interception, the researchers tested out a range of the most common TLS interception middleboxes and client-side interception software and found that the vast majority of them introduced security vulnerabilities.