Web servers that permit users to inject new negotiations into ongoing secure sessions may allow interlopers to inject themselves into encrypted Internet transmissions.
A correctly-implemented server opens each new session with the new negotiation and then closes the old one. Outdated server software that permits renegotiation within a single session is problematic.
The negotiation or "handshake" consists of the servers opening and acknowledging communications, and then exchanging information about at least one digital certificate. The digital certificate(s) contains a long number. The servers plug the number (called a "public key") into an algorithm to encrypt the communications, so that only the servers that know the number can decrypt the information.
Some outdated server software allows a third party to interject a new negotiation or handshake into the middle of the data stream. Because the servers do not reset the communication session, the confidential communication may be compromised. In an attack called Man-in-the-Middle, the third party may interject itself into the communication and hijack data. "Practical attacks against…Microsoft IIS and Apache httpd on a variety of platforms," have been noted, write Ray and Dispensa.
The fault lies not with SSL or TLS per se, but is rather a flaw to which many current implementations of SSL-capable WebServer software are vulnerable, advises Robin Alden, Chief Technical Officer at Comodo CA. "Web server administrators should take great care to install the latest software updates to patch vulnerabilities in general and, in this case, to avoid compromised communications," Alden said.