Re: Disabling Client-Initiated TLS renegotiation

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I suppose one could abuse SSL_set_msg_callback() to create a filter that rewrites the initial re-handshake message into something innocuous.  Though I doubt that would work, once the client starts a handshake, it expects a response from the server, and may well time out and close if it does not get one.  The way TLS works, it is always the client starting a renegotiation.  The server can send a message asking the client to renegotiate, but the client can ignore that. I don’t believe the opposite is true.

 

From: openssl-users [mailto:openssl-users-bounces@xxxxxxxxxxx] On Behalf Of Sashank Mullapudi (samullap)
Sent: Monday, November 28, 2016 10:56 PM
To: openssl-users@xxxxxxxxxxx
Cc: Ram Mohan R (rmohanr) <rmohanr@xxxxxxxxx>; Nikhil Mittal (nimittal) <nimittal@xxxxxxxxx>; Anil Kumar (anilkum) <anilkum@xxxxxxxxx>
Subject: Re: Disabling Client-Initiated TLS renegotiation

 

Resending this hoping for a response from someone who has information on disabling TLS renegotiation from the Client side.

 

Thanks,

Sashank

 

From: samullap <samullap@xxxxxxxxx>
Date: Tuesday, 22 November 2016 at 12:21 PM
To: "openssl-users@xxxxxxxxxxx" <openssl-users@xxxxxxxxxxx>
Cc: "Ram Mohan R (rmohanr)" <rmohanr@xxxxxxxxx>, "Anil Kumar (anilkum)" <anilkum@xxxxxxxxx>, "Nikhil Mittal (nimittal)" <nimittal@xxxxxxxxx>
Subject: Disabling Client-Initiated TLS renegotiation

 

Hi,

 

As part of securing our web interfaces, we wanted to disable client-initiated TLS renegotiation. 

 

The reasoning for this requirement is as follows- Generally, renegotiation of TLS sessions is much more resource-intensive for the server than the client, and should therefore not be performed at will to avoid degrading performance. Disabling client from renegotiating secures the server from undergoing a DoS attack due to continuous renegotiation requests.

 

I see that there is an option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION, but that is to secure the renegotiation, not disable it.

 

I wanted to check if there is a patch or flag available to disable any negotiation initiated from the client side.

 

Thanks and Regards,

Sashank

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