Re: [TLS] SSL Renegotiation DOS

Nikos Mavrogiannopoulos <> Tue, 15 March 2011 11:45 UTC

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Date: Tue, 15 Mar 2011 12:46:14 +0100
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From: Nikos Mavrogiannopoulos <>
To: "Jorge A. Orchilles" <>
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Subject: Re: [TLS] SSL Renegotiation DOS
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On Tue, Mar 15, 2011 at 12:31 PM, Jorge A. Orchilles
<> wrote:
> Hello all,
> Marsh Ray has invited me to present my research and report on SSL/TLS
> Renegotiation Denial of Service on this mailing list. I have posted this on
> my site and will paste here for your feedback:
> SSL/TLS Renegotiation Denial of Service
> An SSL/TLS handshake requires at least 10 times more processing power on the
> server than on the client. The handshake is only performed at the beginning
> of a secure connection to establish it. When SSL/TLS Renegotiation is
> enabled on the server, a user is allowed to send a renegotiation request
> which initiates a new handshake. Since it takes much less resources for a
> client to perform a handshake, requesting multiple handshakes per second
> could cause a denial of service on the server side SSL/TLS interface.
> Therefore, if a malicious user on one host requests multiple renegotiation
> requests it will exhaust the server’s resources and not allow any other user
> to establish a connection.

 I'm curious, what is the effect of that in typical HTTPS servers? Do
servers allow
for renegotiation initiated by the client? (apache with mod_gnutls doesn't)