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:
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 (diag 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