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Another Protocol Bites The Dust

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For the last 6 weeks or so, a bunch of us have been working on a really serious issue in SSL. In short, a man-in-the-middle can use SSL renegotiation to inject an arbitrary prefix into any SSL session, undetected by either end.

To make matters even worse, through a piece of (in retrospect) incredibly bad design, HTTP servers will, under some circumstances, replay that arbitrary prefix in a new authentication context. For example, this is what happens if you configure Apache to require client certificates for one directory but not another. Once it emerges that your request is for a protected directory, a renegotiation will occur to obtain the appropriate client certificate, and then the original request (i.e. the stuff from the bad guy) gets replayed as if it had been authenticated by the client certificate. But it hasn’t.

Not that the picture is all rosy even when client certificates are not involved.

Vulnerability in SSL/TLS protocol According to reports, vulnerabilities in the SSL/TLS protocol can be exploited by attackers to insert content into secure connections. If this is correct, it would affect HTTPS and all other protocols which use TLS for security, including IMAP. The precise effects of the problem are not discussed in the reports. It would, however, appear to be possible to manipulate HTML content from websites during data transfer and, for example, inject malicious code.

The crux of the problem is, rather than a flawed implementation, a design flaw in the TLS protocol when renegotiating parameters for an existing TLS connection. This occurs when, for example, a client wants to access a secure area on a web server which requires the requesting client certificates. When the server establishes that is the case, it begins a renegotiation to obtain the appropriate client certificate. The original request gets replayed during this renegotiation as if it had been authenticated by the client certificate, but it has not. The discoverer of the problem describes this as an "authentication gap".

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