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> Chapter 25 - Server Load Balancing > SSL/TLS load balancing > Disabling SSL/TLS re-negotiation

Disabling SSL/TLS re-negotiation

The vulnerability CVE-2009-3555 affects all SSL/TLS servers that support re-negotiation. FortiOS when configured for SSL/TLS offloading is operating as a SSL/TLS server. The IETF is working on a TLS protocol change that will fix the problem identified by CVE-2009-3555 while still supporting re-negotiation. Until that protocol change is available, you can use the ssl-client-renegotiation option to disable support for SSL/TLS re-negotiation. The default value of this option is allow, which allows an SSL client to renegotiate. You can change the setting to deny to abort any attempts by an SSL client to renegotiate. If you select deny as soon as a ClientHello message indicating a re-negotiation is received from the client FortiOS terminates the TCP connection.

Since SSL offloading does not support requesting client certificates the only circumstance in which a re-negotiation is required is when more than 2^32 bytes of data are exchanged over a single handshake. If you are sure that this volume of traffic will not occur then you can disable re-negotiation and avoid any possibility of the attack described in CVE-2009-3555.

The re-negotiation behavior can be tested using OpenSSL. The OpenSSL s_client application has the feature that the user can request that it do renegotiation by typing “R”. For example, the following shows a successful re-negotiation against a FortiGate unit configured with a VIP for 192.168.2.100:443:

$ openssl s_client -connect 192.168.2.100:443

CONNECTED(00000003)

depth=1 /C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

verify error:num=19:self signed certificate in certificate chain

verify return:0

---

Certificate chain

0

s:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Fortigate/CN=FW80CM3909604325/emailAddress=support@fortinet.com

i:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

1 s:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

i:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

---

Server certificate

-----BEGIN CERTIFICATE-----

 

---certificate not shown---

 

-----END CERTIFICATE-----

subject=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Fortigate/CN=FW80CM3909604325/emailAddress=support@fortinet.com

issuer=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

---

No client certificate CA names sent

---

SSL handshake has read 2370 bytes and written 316 bytes

---

New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA

Server public key is 1024 bit

Compression: NONE

Expansion: NONE

SSL-Session:

Protocol : TLSv1

Cipher : DHE-RSA-AES256-SHA

Session-ID:

02781E1E368DCCE97A95396FAA82E8F740F5BBA96CF022F6FEC3597B0CC88095

Session-ID-ctx:

Master-Key:

A6BBBD8477A2422D56E57C1792A4EA9C86F37D731E67D0A66E5CDB2B5C76650780C0E7F01CFF851EC4466186F4C48397

Key-Arg : None

Start Time: 1264453027

Timeout : 300 (sec)

Verify return code: 19 (self signed certificate in certificate

chain)

---

GET /main.c HTTP/1.0

R

RENEGOTIATING

depth=1 /C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

verify error:num=19:self signed certificate in certificate chain

verify return:0

HTTP/1.0 200 ok

Content-type: text/plain

 

/*

* Copyright (C) 2004-2007 Fortinet

*/

 

#include <stdio.h>

#include "vsd_ui.h"

 

int main(int argc, char **argv)

{

return vsd_ui_main(argc, argv);

}

closed

$

The following is the same test, but this time with the VIP configuration changed to ssl-client-renegotation deny:

 

$ openssl s_client -connect 192.168.2.100:443

CONNECTED(00000003)

depth=1 /C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

verify error:num=19:self signed certificate in certificate chain

verify return:0

---

Certificate chain

0

s:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Fortigate/CN=FW80CM3909604325/emailAddress=support@fortinet.com

i:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

1 s:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

i:/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

---

Server certificate

-----BEGIN CERTIFICATE-----

---certificate not shown---

-----END CERTIFICATE-----

subject=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Fortigate/CN=FW80CM3909604325/emailAddress=support@fortinet.com

issuer=/C=US/ST=California/L=Sunnyvale/O=Fortinet/OU=Certificate

Authority/CN=support/emailAddress=support@fortinet.com

---

No client certificate CA names sent

---

SSL handshake has read 2370 bytes and written 316 bytes

---

New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA

Server public key is 1024 bit

Compression: NONE

Expansion: NONE

SSL-Session:

Protocol : TLSv1

Cipher : DHE-RSA-AES256-SHA

Session-ID:

8253331D266DDE38E4D8A04AFCA9CBDED5B1134932CE1718EED6469C1FBC7474

Session-ID-ctx:

Master-Key:

ED05A3EF168AF2D06A486362FE91F1D6CAA55CEFC38A3C36FB8BD74236BF2657D4701B6C1456CEB5BB5EFAA7619EF12D

Key-Arg : None

Start Time: 1264452957

Timeout : 300 (sec)

Verify return code: 19 (self signed certificate in certificate

chain)

---

GET /main.c HTTP/1.0

R

RENEGOTIATING

19916:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake failure:s3_pkt.c:530:

Use the following command to check the SSL stats to see that the renegotiations blocked counter is now 1:

diagnose firewall vip virtual-server stats ssl

ssl

client

connections total 0 active 0 max 0

handshakes total 4 active 0 max 0 completed 4 abbreviated 0

session states total 4 active 4 max 4

cipher-suite failures 0

embryonics total 0 active 0 max 0 terminated 0

renegotiations blocked 1

server

connections total 0 active 0 max 0

handshakes total 3 active 0 max 0 completed 2 abbreviated 1

session states total 1 active 1 max 1

cipher-suite failures 0

internal error 0

bad handshake length 0

bad change cipher spec length 0

pubkey too big 0

persistence

find 0 found 0 clash 0 addr 0 error 0

If the virtual server debug log is examined (diagnose debug appl vs -1) then at the point the re-negotiation is blocked there is a log:

vs ssl 12 handshake recv ClientHello

vs ssl 12 handshake recv 1 (0100005403014b5e056c7f573a563bebe0258c3254bbaff7046a461164f34f94f4f3d019c41800002600390038003500160013000a00330032002f0005000400150012000900140011000800060003020100000400230000)

vs ssl 12 client renegotiation attempted rejected, abort

vs ssl 12 closing 0 up

vs src 12 close 0 in

vs src 12 error closing

vs dst 14 error closing

vs dst 14 closed

vs ssl 14 close

vs sock 14 free

vs src 12 closed

vs ssl 12 close

vs sock 12 free