Viewing Incoming/7506
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Major security issue: yes  no

Notes:

Notification:

Date: Mon, 28 Jan 2013 00:13:04 +0000
From: ben@bjencks.net
To: openldap-its@OpenLDAP.org
Subject: Diffie-Hellman parameters (DHParamFile) are improperly handled

Full_Name: Ben Jencks
Version: git master
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (2601:8:580:1d:5e6:2c43:96d0:4503)


Both tls_g.c for GnuTLS and tls_o.c for OpenSSL have major problems in their
handling of DHParamFile. Additionally, the documentation is not entirely
accurate.

For GnuTLS, DH parameters are entirely unimplemented. There appears to be an
attempt at generating them in tlsg_ctx_init, but they're never attached to
ctx->cred, so they're unused. Additionally, the size is hardcoded.

For OpenSSL, there's an attempt to support multiple sizes, but unfortunately
it's misusing the OpenSSL API and only 512 or 1024 bit parameters will ever be
used. The callback with specified size is only useful for export ciphers: the
size isn't negotiated, it's only specified as a maximum when the export flag is
set [1]. You want to specify exactly one size and parameter set.

The documentation refers to a non-existent TLSEphemeralDHParamFile option.

Patches are available below:
http://www.bjencks.net/openldap/0001-tls_g.c-Properly-support-DHParamFile.patch
http://www.bjencks.net/openldap/0002-tls_o.c-Fix-Diffie-Hellman-parameter-usage.patch
http://www.bjencks.net/openldap/0003-DHParamFile-Update-docs.patch

I'm a little uncomfortable with leaving a hardcoded parameter set in the OpenSSL
code, but I figured it would break compatibility to remove it. A better solution
(and the one recommended by OpenSSL) is to add parameter generation to the
install scripts. If you'd prefer that, I can modify the patch to get rid of the
hardcoded parameter and just not set a DH on the SSL_CTX without a file
specified.

The attached file is derived from OpenLDAP Software. All of the modifications to
OpenLDAP Software represented in the following patch(es) were developed by
Meddius. Meddius has not assigned rights and/or interest in this work to any
party. I, Ben Jencks am authorized by Meddius, my employer, to release this work
under the following terms.

Meddius hereby places the attached modifications to OpenLDAP Software (and only
these modifications) into the public domain. Hence, these modifications may be
freely used and/or redistributed for any purpose with or without attribution
and/or other notice.

[1] http://permalink.gmane.org/gmane.comp.encryption.openssl.user/45805

Followup 1

Date: Sun, 27 Jan 2013 19:28:17 -0500
From: Ben Jencks <ben@bjencks.net>
To: openldap-its@openldap.org
Subject: Re: (ITS#7506) Diffie-Hellman parameters (DHParamFile) are improperly
 handled

I suppose I should attach a test case:

cd $WORKDIR
openssl dhparam 1536 > dh1536.pem
openssl req -new -newkey rsa:2048 -keyout server.key \
  -out server.crt -sha256 -nodes -subj '/CN=localhost/' -x509

For GnuTLS, load the following config:
dn: cn=config
objectClass: olcGlobal
cn: config
olcTLSCertificateFile: $WORKDIR/server.crt
olcTLSCertificateKeyFile: $WORKDIR/server.key
olcTLSCipherSuite: NONE:+VERS-TLS1.1:+VERS-TLS1.0:+AES-128-CBC:
 +DHE-RSA:+SHA1:+COMP-NULL
olcTLSDHParamFile: $WORKDIR/dh1536.pem

dn: olcDatabase={0}config,cn=config
objectClass: olcDatabaseConfig
olcDatabase: {0}config

For Openssl, change olcTLSCipherSuite to:
olcTLSCipherSuite: aRSA+AES128+kEDH

Start up slapd with the config, listening on ldaps://localhost:6636.

openssl s_client -connect localhost:6636

With GnuTLS this will fail negotiation; with OpenSSL it will use 1024
bit parameters rather than the 1536 bits params we specified.

The only way I've found to find the actual parameter size is to tcpdump
the negotiation. First, verify that the negotiated cipher suite is
DHE-RSA-AES128-SHA according to s_client, then:

tcpdump -i lo -wldap.pcap port 6636
wireshark ldap.pcap
Apply the SSL decode to the traffic (right click on packet, Decode As,
Transport tab, SSL), find the Server Key Exchange packet, select the
Server Key Exchange portion of the handshake message, and look at the
hex dump.

It should start with 0c, followed by a three byte size, then the next
two bytes are the length of the DH parameters: 0080 for 1024 bits, 00c0
for 1536 bits.

On 01/27/2013 07:13 PM, openldap-its@OpenLDAP.org wrote:
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