Apache HTTP Server Version 2.2
The solution to this problem is trivial and is left as an exercise for the reader.
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How to solve particular security problems for an SSL-aware webserver is not always obvious because of the interactions between SSL, HTTP and Apache's way of processing requests. This chapter gives instructions on how to solve some typical situations. Treat it as a first step to find out the final solution, but always try to understand the stuff before you use it. Nothing is worse than using a security solution without knowing its restrictions and how it interacts with other systems.
The following creates an SSL server which speaks only the SSLv2 protocol and its ciphers.
SSLProtocol -all +SSLv2
SSLCipherSuite SSLv2:+HIGH:+MEDIUM:+LOW:+EXP
The following enables only the seven strongest ciphers:
SSLProtocol all
SSLCipherSuite HIGH:MEDIUM
This facility is called Server Gated Cryptography (SGC) and requires a Global ID server certificate, signed by a special CA certificate from Verisign. This enables strong encryption in 'export' versions of browsers, which traditionally could not support it (because of US export restrictions).
When a browser connects with an export cipher, the server sends its Global ID certificate. The browser verifies this, and can then upgrade its cipher suite before any HTTP communication takes place. The problem lies in allowing browsers to upgrade in this fashion, but still requiring strong encryption. In other words, we want browsers to either start a connection with strong encryption, or to start with export ciphers but upgrade to strong encryption before beginning HTTP communication.
This can be done as follows:
# allow all ciphers for the initial handshake,
# so export browsers can upgrade via SGC facility
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
<Directory /usr/local/apache2/htdocs>
# but finally deny all browsers which haven't upgraded
SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128
</Directory>
Obviously, a server-wide SSLCipherSuite
which restricts
ciphers to the strong variants, isn't the answer here. However,
mod_ssl
can be reconfigured within Location
blocks, to give a per-directory solution, and can automatically force
a renegotiation of the SSL parameters to meet the new configuration.
This can be done as follows:
# be liberal in general
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
<Location /strong/area>
# but https://hostname/strong/area/ and below
# requires strong ciphers
SSLCipherSuite HIGH:MEDIUM
</Location>
When you know all of your users (eg, as is often the case on a corporate
Intranet), you can require plain certificate authentication. All you
need to do is to create client certificates signed by your own CA
certificate (ca.crt
) and then verify the clients against this
certificate.
# require a client certificate which has to be directly
# signed by our CA certificate in ca.crt
SSLVerifyClient require
SSLVerifyDepth 1
SSLCACertificateFile conf/ssl.crt/ca.crt
To force clients to authenticate using certificates for a particular URL,
you can use the per-directory reconfiguration features of mod_ssl
:
SSLVerifyClient none
SSLCACertificateFile conf/ssl.crt/ca.crt
<Location /secure/area>
SSLVerifyClient require
SSLVerifyDepth 1
</Location>
The key to doing this is checking that part of the client certificate
matches what you expect. Usually this means checking all or part of the
Distinguished Name (DN), to see if it contains some known string.
There are two ways to do this, using either mod_auth_basic
or
SSLRequire
.
The mod_auth_basic
method is generally required when
the certificates are completely arbitrary, or when their DNs have
no common fields (usually the organisation, etc.). In this case,
you should establish a password database containing all
clients allowed, as follows:
SSLVerifyClient none <Directory /usr/local/apache2/htdocs/secure/area> SSLVerifyClient require SSLVerifyDepth 5 SSLCACertificateFile conf/ssl.crt/ca.crt SSLCACertificatePath conf/ssl.crt SSLOptions +FakeBasicAuth SSLRequireSSL AuthName "Snake Oil Authentication" AuthType Basic AuthBasicProvider file AuthUserFile /usr/local/apache2/conf/httpd.passwd Require valid-user </Directory>
The password used in this example is the DES encrypted string "password".
See the SSLOptions
docs for more
information.
/C=DE/L=Munich/O=Snake Oil, Ltd./OU=Staff/CN=Foo:xxj31ZMTZzkVA /C=US/L=S.F./O=Snake Oil, Ltd./OU=CA/CN=Bar:xxj31ZMTZzkVA /C=US/L=L.A./O=Snake Oil, Ltd./OU=Dev/CN=Quux:xxj31ZMTZzkVA
When your clients are all part of a common hierarchy, which is encoded
into the DN, you can match them more easily using SSLRequire
, as follows:
SSLVerifyClient none <Directory /usr/local/apache2/htdocs/secure/area> SSLVerifyClient require SSLVerifyDepth 5 SSLCACertificateFile conf/ssl.crt/ca.crt SSLCACertificatePath conf/ssl.crt SSLOptions +FakeBasicAuth SSLRequireSSL SSLRequire %{SSL_CLIENT_S_DN_O} eq "Snake Oil, Ltd." \ and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"} </Directory>
These examples presume that clients on the Intranet have IPs in the range
192.168.1.0/24, and that the part of the Intranet website you want to allow
internet access to is /usr/local/apache2/htdocs/subarea
.
This configuration should remain outside of your HTTPS virtual host, so
that it applies to both HTTPS and HTTP.
SSLCACertificateFile conf/ssl.crt/company-ca.crt <Directory /usr/local/apache2/htdocs> # Outside the subarea only Intranet access is granted Order deny,allow Deny from all Allow from 192.168.1.0/24 </Directory> <Directory /usr/local/apache2/htdocs/subarea> # Inside the subarea any Intranet access is allowed # but from the Internet only HTTPS + Strong-Cipher + Password # or the alternative HTTPS + Strong-Cipher + Client-Certificate # If HTTPS is used, make sure a strong cipher is used. # Additionally allow client certs as alternative to basic auth. SSLVerifyClient optional SSLVerifyDepth 1 SSLOptions +FakeBasicAuth +StrictRequire SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128 # Force clients from the Internet to use HTTPS RewriteEngine on RewriteCond %{REMOTE_ADDR} !^192\.168\.1\.[0-9]+$ RewriteCond %{HTTPS} !=on RewriteRule .* - [F] # Allow Network Access and/or Basic Auth Satisfy any # Network Access Control Order deny,allow Deny from all Allow 192.168.1.0/24 # HTTP Basic Authentication AuthType basic AuthName "Protected Intranet Area" AuthBasicProvider file AuthUserFile conf/protected.passwd Require valid-user </Directory>