How can I get LWP to validate SSL server certificates?

cjm_8355 0
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How can I get LWP to verify that the certificate of the server I'm connecting to is signed by a trusted authority and issued to the correct host? As far as I can tell, it doesn't even check that the certificate claims to be for the hostname I'm connecting to. That seems like a major security hole (especially with the recent DNS vulnerabilities).

Update: It turns out what I really wanted was HTTPS_CA_DIR, because I don't have a ca-bundle.crt. But HTTPS_CA_DIR=/usr/share/ca-certificates/ did the trick. I'm marking the answer as accepted anyway, because it was close enough.

Update 2: It turns out that HTTPS_CA_DIR and HTTPS_CA_FILE only apply if you're using Net::SSL as the underlying SSL library. But LWP also works with IO::Socket::SSL, which will ignore those environment variables and happily talk to any server, no matter what certificate it presents. Is there a more general solution?

Update 3: Unfortunately, the solution still isn't complete. Neither Net::SSL nor IO::Socket::SSL is checking the host name against the certificate. This means that someone can get a legitimate certificate for some domain, and then impersonate any other domain without LWP complaining.

Update 4: LWP 6.00 finally solves the problem. See my answer for details.

lwp https perl ssl
8 Answers

There are two means of doing this depending on which SSL module you have installed. The LWP docs recommend installing Crypt::SSLeay. If that's what you've done, setting the HTTPS_CA_FILE environment variable to point to your ca-bundle.crt should do the trick. (the Crypt::SSLeay docs mentions this but is a bit light on details). Also, depending on your setup, you may need to set the HTTPS_CA_DIR environment variable instead.

Example for Crypt::SSLeay:


use LWP::Simple qw(get);
$ENV{HTTPS_CA_FILE} = "/path/to/your/ca/file/ca-bundle";
$ENV{HTTPS_DEBUG} = 1;

print get("https://some-server-with-bad-certificate.com");

__END__
SSL_connect:before/connect initialization
SSL_connect:SSLv2/v3 write client hello A
SSL_connect:SSLv3 read server hello A
SSL3 alert write:fatal:unknown CA
SSL_connect:error in SSLv3 read server certificate B
SSL_connect:error in SSLv3 read server certificate B
SSL_connect:before/connect initialization
SSL_connect:SSLv3 write client hello A
SSL_connect:SSLv3 read server hello A
SSL3 alert write:fatal:bad certificate
SSL_connect:error in SSLv3 read server certificate B
SSL_connect:before/connect initialization
SSL_connect:SSLv2 write client hello A
SSL_connect:error in SSLv2 read server hello B

Note that get doesn't die, but it does return an undef.

Alternatively, you can use the IO::Socket::SSL module (also available from the CPAN). To make this verify the server certificate you need to modify the SSL context defaults:


use IO::Socket::SSL qw(debug3);
use Net::SSLeay;
BEGIN {
    IO::Socket::SSL::set_ctx_defaults(
        verify_mode => Net::SSLeay->VERIFY_PEER(),
        ca_file => "/path/to/ca-bundle.crt",
      # ca_path => "/alternate/path/to/cert/authority/directory"
    );
}
use LWP::Simple qw(get);

warn get("https:://some-server-with-bad-certificate.com");

This version also causes get() to return undef but prints a warning to STDERR when you execute it (as well as a bunch of debugging if you import the debug* symbols from IO::Socket::SSL):


% perl ssl_test.pl
DEBUG: .../IO/Socket/SSL.pm:1387: new ctx 139403496
DEBUG: .../IO/Socket/SSL.pm:269: socket not yet connected
DEBUG: .../IO/Socket/SSL.pm:271: socket connected
DEBUG: .../IO/Socket/SSL.pm:284: ssl handshake not started
DEBUG: .../IO/Socket/SSL.pm:327: Net::SSLeay::connect -> -1
DEBUG: .../IO/Socket/SSL.pm:1135: SSL connect attempt failed with unknown errorerror:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

DEBUG: .../IO/Socket/SSL.pm:333: fatal SSL error: SSL connect attempt failed with unknown errorerror:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
DEBUG: .../IO/Socket/SSL.pm:1422: free ctx 139403496 open=139403496
DEBUG: .../IO/Socket/SSL.pm:1425: OK free ctx 139403496
DEBUG: .../IO/Socket/SSL.pm:1135: IO::Socket::INET configuration failederror:00000000:lib(0):func(0):reason(0)
500 Can't connect to some-server-with-bad-certificate.com:443 (SSL connect attempt failed with unknown errorerror:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed)
Brian_Phillips_72300

If you use LWP::UserAgent directly (not via LWP::Simple) you can validate the hostname in the certificate by adding the "If-SSL-Cert-Subject" header to your HTTP::Request object. The value of the header is treated as a regular expression to be applied on the certificate subject, and if it does not match, the request fails. For example:

#!/usr/bin/perl 
use LWP::UserAgent;
my $ua = LWP::UserAgent->new();
my $req = HTTP::Request->new(GET => 'https://yourdomain.tld/whatever');
$req->header('If-SSL-Cert-Subject' => '/CN=make-it-fail.tld');

my $res = $ua->request( $req );

print "Status: " . $res->status_line . "\n"

will print

Status: 500 Bad SSL certificate subject: '/C=CA/ST=Ontario/L=Ottawa/O=Your Org/CN=yourdomain.tld' !~ //CN=make-it-fail.tld/
user_738860
user_27106080

You are right to be concerned about this. Unfortunately, I don't think it's possible to do it 100% securely under any of the low-level SSL/TLS bindings I looked at for Perl.

Essentially you need to pass in the hostname of the server you want to connect to the SSL library before the handshaking gets underway. Alternatively, you could arrange for a callback to occur at the right moment and abort the handshake from inside the callback if it doesn't check out. People writing Perl bindings to OpenSSL seemed to have troubles making the callback interface consistently.

The method to check the hostname against the server's cert is dependent on the protocol, too. So that would have to be a parameter to any perfect function.

You might want to see if there are any bindings to the Netscape/Mozilla NSS library. It seemed pretty good at doing this when I looked at it.

user_1162700
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