Date: Sun, 08 Nov 2009 21:42:38 +1300 From: Peter Gutmann <pgut001@...auckland.ac.nz> To: marsh@...endedsubset.com, rea-sec@...elabs.ru Cc: coley@...us.mitre.org, oss-security@...ts.openwall.com, tls@...f.org Subject: Re: [TLS] CVE-2009-3555 for TLS renegotiation MITM attacks Marsh Ray <marsh@...endedsubset.com> writes: >For one thing, browsers' behavior of allowing automatic certificate sending >is suspect and should be reconsidered. It's not actually safe to reconsider this because many servers (including some at very large sites) always request client auth, often without the site admins being aware of this or knowing how to disable it. I became aware of this when I changed my code to add a roadblock until the user explicitly responded to a client cert request, leading to many complaints about sites that formerly "worked" and now didn't (I've found all sorts of other broken behaviour around client-auth, for example servers that send a list of 150-odd CAs, every one they know of, as an indication of who they'll accept certs from, and other strangeness). Disabling the automatic sending of client certs would therefore obviously break, to the client, a number of (what I consider) broken servers. What my code currently does is: if( cert request received ) if( client cert present ) send cert; else send no-cert alert; This isn't totally safe though because there's no "tell user to make their cert available" option if they haven't pre-emptively done so, it "works" mostly because the extreme rarity of client cert use and stereotyped behaviour of existing clients hides all the glitches in implementations. Peter.
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