Plone CMS 3.x uses invariant data (a client username and a server secret) when calculating an HMAC-SHA1 value for an authentication cookie, which makes it easier for remote attackers to gain permanent access to an account by sniffing the network.
https://exchange.xforce.ibmcloud.com/vulnerabilities/41421
http://www.securityfocus.com/archive/1/489544/100/0/threaded