Less than a week after it was introduced in Firefox 37, Opportunistic Encryption (OE) has been removed by Mozilla due to a flaw that was discovered in their HTTP Alternative Services implementation.
OE offered unauthenticated encryption over TLS, boosting the level of security for data that would've otherwise been transmitted via clear text. Thus the feature, wrote Patrick McManus, a network developer for Mozilla, created some level of confidentiality in the face of passive eavesdropping.
Security experts were pleased by OE, commenting that Firefox had taken a step in the right direction, removing almost all barriers to encrypting Web traffic.
However, in order for OE to work, website administrators needed to implement support for the Alternative Services specification, and that's where the problem came from.
"If an Alt-Svc header is specified in the HTTP/2 response, SSL certificate verification can be bypassed for the specified alternate server. As a result of this, warnings of invalid SSL certificates will not be displayed and an attacker could potentially impersonate another site through a man-in-the-middle (MTIM), replacing the original certificate with their own," a security advisory from Mozilla explained.
Given that HTTP/2 AltSvc breaks SSL certificate validation, Mozilla was really left with no other alternative than to remove OE and fix the issue offline.
They're encouraging all users to update to version 37.0.1 as quickly as possible in order to avoid potential problems.