Feature Requests item #1767276, was opened at 2007-08-03 21:11
Message generated for change (Comment added) made by kb-ch
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Category: libcurl
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Scott Cantor (scantor)
Assigned to: Daniel Stenberg (bagder)
Summary: Request option to disable SSLv2

Initial Comment:
The current version selection option for SSL lets the caller turn on a specific SSL/TLS version, but not disable one. Normally SSLv3 and TLSv1 would both be acceptable, but SSLv2 is never acceptable because of its holes, so it would be good to have the option to allow anything but that version.

(We've tested that disabling the SSLv2 ciphers doesn't actually disable use of SSLv2 itself.)

I checked the openssl s_client options, and it supports turning off a specific version, so apparently it can be done.

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Comment By: Kaspar (kb-ch)
Date: 2008-02-17 16:27

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From http://curl.haxx.se/mail/lib-2008-02/0114.html:

> > I think starting disabling ssl2 by default is also a good thing
today.
>
> I agree. Related feature request:
>
http://sourceforge.net/tracker/index.php?func=detail&aid=1767276&group_id=976&atid=350976

While not exactly addressing the original request, here's a patch which
disables SSLv2 when CURL_SSLVERSION_DEFAULT is used (couldn't figure out
how to attach the file separately - maybe because I'm neither the reporter
nor the assignee?).

It includes the changes for all SSL toolkits currently supported by curl.
Note that for NSS, this will "automagically" turn on TLS extensions (server
name indication/SNI e.g.), if compiled against 3.11 or later. For GnuTLS,
no changes are needed - SSLv2 isn't implemented in that library on
purpose.

Index: lib/ssluse.c
===================================================================
RCS file: /cvsroot/curl/curl/lib/ssluse.c,v
retrieving revision 1.192
diff -u -p -r1.192 ssluse.c
--- lib/ssluse.c 7 Feb 2008 22:25:04 -0000 1.192
+++ lib/ssluse.c 17 Feb 2008 15:12:54 -0000
@@ -1324,6 +1331,10 @@ ossl_connect_step1(struct connectdata *c
   */
   SSL_CTX_set_options(connssl->ctx, SSL_OP_ALL);

+ /* disable SSLv2 in the default case (i.e. allow SSLv3 and TLSv1) */
+ if (data->set.ssl.version == CURL_SSLVERSION_DEFAULT)
+ SSL_CTX_set_options(connssl->ctx, SSL_OP_NO_SSLv2);
+
 #if 0
   /*
    * Not sure it's needed to tell SSL_connect() that socket is
Index: lib/nss.c
===================================================================
RCS file: /cvsroot/curl/curl/lib/nss.c,v
retrieving revision 1.15
diff -u -p -r1.15 nss.c
--- lib/nss.c 15 Jan 2008 23:19:02 -0000 1.15
+++ lib/nss.c 17 Feb 2008 15:12:54 -0000
@@ -873,7 +873,7 @@ CURLcode Curl_nss_connect(struct connect
   switch (data->set.ssl.version) {
   default:
   case CURL_SSLVERSION_DEFAULT:
- ssl2 = ssl3 = tlsv1 = PR_TRUE;
+ ssl3 = tlsv1 = PR_TRUE;
     break;
   case CURL_SSLVERSION_TLSv1:
     tlsv1 = PR_TRUE;
@@ -893,6 +893,9 @@ CURLcode Curl_nss_connect(struct connect
   if(SSL_OptionSet(model, SSL_ENABLE_TLS, tlsv1) != SECSuccess)
     goto error;

+ if(SSL_OptionSet(model, SSL_V2_COMPATIBLE_HELLO, ssl2) != SECSuccess)
+ goto error;
+
   if(data->set.ssl.cipher_list) {
     if(set_ciphers(data, model, data->set.ssl.cipher_list) != SECSuccess)
{
       curlerr = CURLE_SSL_CIPHER;
Index: lib/qssl.c
===================================================================
RCS file: /cvsroot/curl/curl/lib/qssl.c,v
retrieving revision 1.9
diff -u -p -r1.9 qssl.c
--- lib/qssl.c 11 Feb 2008 22:03:31 -0000 1.9
+++ lib/qssl.c 17 Feb 2008 15:12:54 -0000
@@ -90,7 +90,7 @@ static CURLcode Curl_qsossl_init_session
   memset((char *) &initappstr, 0, sizeof initappstr);
   initappstr.applicationID = certname;
   initappstr.applicationIDLen = strlen(certname);
- initappstr.protocol = SSL_VERSION_CURRENT;
+ initappstr.protocol = TLSV1_SSLV3;
   initappstr.sessionType = SSL_REGISTERED_AS_CLIENT;
   rc = SSL_Init_Application(&initappstr);

@@ -190,7 +190,7 @@ static CURLcode Curl_qsossl_handshake(st

   default:
   case CURL_SSLVERSION_DEFAULT:
- h->protocol = SSL_VERSION_CURRENT;
+ h->protocol = TLSV1_SSLV3;
     break;

   case CURL_SSLVERSION_TLSv1:
Index: docs/libcurl/curl_easy_setopt.3
===================================================================
RCS file: /cvsroot/curl/curl/docs/libcurl/curl_easy_setopt.3,v
retrieving revision 1.211
diff -u -p -r1.211 curl_easy_setopt.3
--- docs/libcurl/curl_easy_setopt.3 11 Jan 2008 14:20:41 -0000
1.211
+++ docs/libcurl/curl_easy_setopt.3 17 Feb 2008 15:12:54 -0000
@@ -1379,10 +1379,9 @@ Pass a long as parameter to control what
 The available options are:
 .RS
 .IP CURL_SSLVERSION_DEFAULT
-The default action. When libcurl built with OpenSSL or NSS, this will
attempt
-to figure out the remote SSL protocol version. Unfortunately there are a
lot of
-ancient and broken servers in use which cannot handle this technique and
will
-fail to connect. When libcurl is built with GnuTLS, this will mean
SSLv3.
+The default action. This will attempt to figure out the remote SSL
protocol
+version, i.e. either SSLv3 or TLSv1 (but not SSLv2, which became
disabled
+by default with 7.18.1).
 .IP CURL_SSLVERSION_TLSv1
 Force TLSv1
 .IP CURL_SSLVERSION_SSLv2

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Comment By: Scott Cantor (scantor)
Date: 2007-08-03 23:38

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Quick update...it uses SSL_CTX_set_options using some bit flags that turn
off specific protocols, you'll see it in s_client.c. It looks like it only
works with "non-buggy" servers, but that's not too surprising.

Don't know how far back the option exists, I'm looking at 0.9.8

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Comment By: Daniel Stenberg (bagder)
Date: 2007-08-03 23:37

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Thanks, that lead me exactly to the right place.

It seems SSL_CTX_set_options() (that libcurl already uses) has bits to
disable specific protocols:

       SSL_OP_NO_SSLv2
           Do not use the SSLv2 protocol.

       SSL_OP_NO_SSLv3
           Do not use the SSLv3 protocol.

       SSL_OP_NO_TLSv1
           Do not use the TLSv1 protocol.

So yes, this should be fairly easy to support for OpenSSL at least...

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Comment By: Scott Cantor (scantor)
Date: 2007-08-03 23:32

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I don't know how it does it internally (haven't looked yet), but the flag
is here:

$ openssl s_client help
...
-no_tls1/-no_ssl3/-no_ssl2 - turn off that protocol
...

I wouldn't have asked if I hadn't seen the option, I was expecting it had
the same limitation libcurl did but then I saw it listed, so at least
there's code we could copy from the sample tool.

It's probably not worth the hassle if it takes anything exotic that could
create regressions, but if there's a simple API it would be worth it.

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Comment By: Daniel Stenberg (bagder)
Date: 2007-08-03 23:25

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How does the s_client support this? (I mean option/command etc)

I don't find any easy API for this in the OpenSSL docs, so possibly the
lib would have to do a version check after a connect has been performed and
then close down again if using the wrong version...

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Received on 2008-02-17