QT-1900 Add a CSRF token to the OIDC login flow.

This improves security by generating a CSRF token, passing it to the
OIDC IdP, and validating it afterwards.  The token is stored in
a cookie reverse-encrypted with MulkyID's private key.

4 files changed, 51 insertions, 13 deletions

diff --git a/www/authenticate.pl b/www/authenticate.pl
index 3c865ac..b2bd43f 100755
--- a/www/authenticate.pl
+++ b/www/authenticate.pl
@@ -13,6 +13,8 @@ use CGI::Fast;
 use OIDC::Lite;
 use OIDC::Lite::Client::WebServer;
 
+use Bytes::Random::Secure qw(random_bytes_base64);
+
 do "common.pl";
 
 while (my $cgi = new CGI::Fast) {
@@ -35,18 +37,20 @@ while (my $cgi = new CGI::Fast) {
         authorize_uri => 'https://accounts.google.com/o/oauth2/auth',
         access_token_uri => 'https://accounts.google.com/o/oauth2/token'
       );
-      # FIXME: Make `state` a unique, random session token!  (Maybe a
-      # signed, timestamped web token, so stateless?)
-      print $cgi->redirect($oidc_client->uri_to_redirect(
-        redirect_uri => reluri($cgi, 'login.pl'),
-        scope => 'openid email',
-        state => '',
-        extra => {
-          access_type => 'online',
-          login_hint => $claimed_email,
-          response_type => 'code'
-        }
-      ));
+      my $csrf_token = random_bytes_base64(32);  #256 bits
+      my $csrf_token_cookie = make_cookie('mulkyid_csrf_token', $csrf_token);
+      print $cgi->redirect(
+        -cookie => $csrf_token_cookie,
+        -url => $oidc_client->uri_to_redirect(
+          redirect_uri => reluri($cgi, 'login.pl'),
+          scope => 'openid email',
+          state => $csrf_token,
+          extra => {
+            access_type => 'online',
+            login_hint => $claimed_email,
+            response_type => 'code'
+          })
+      );
     }
     default {
       die "Invalid auth_type! " . $::MULKONF->{auth_type};
diff --git a/www/common.pl b/www/common.pl
index 63b8d0f..eb965f3 100644
--- a/www/common.pl
+++ b/www/common.pl
@@ -5,9 +5,12 @@ use common::sense;
 #use Modern::Perl 2011;
 use Modern::Perl;
 
+use File::Slurp;
 use Mail::ExpandAliases;
 use URI;
 use MIME::Base64 qw(encode_base64 decode_base64);
+use Crypt::OpenSSL::RSA;
+use CGI::Cookie;
 
 sub load_config() {
   $::MULKONF = { };
@@ -54,3 +57,27 @@ sub encode_base64_url($) {
   $s =~ s/\n//g;
   return $s;
 }
+
+sub acquire_private_key() {
+  my $key = Crypt::OpenSSL::RSA->new_private_key(scalar read_file($::MULKONF->{pemfile}));
+  $key->use_pkcs1_padding();
+  $key->use_sha256_hash();
+  return $key;
+}
+
+sub make_cookie($$) {
+  my ($name, $value) = @_;
+  my $key = acquire_private_key;
+  my $reverse_encrypted_value = $key->private_encrypt($value);
+  my $cookie = CGI::Cookie->new(-name => $name, -value =>encode_base64_url($reverse_encrypted_value));
+}
+
+sub read_cookie($$) {
+  my ($cgi, $name) = @_;
+  my $cookie = $cgi->cookie($name);
+  return unless ($cookie);
+  my $key = acquire_private_key;
+  my $value = $key->public_decrypt(decode_base64_url($cookie));
+  warn "cookie `$name` was forged!" unless $value;
+  return $value;
+}
diff --git a/www/logged_in_p.pl b/www/logged_in_p.pl
index b076618..5752959 100755
--- a/www/logged_in_p.pl
+++ b/www/logged_in_p.pl
@@ -52,4 +52,3 @@ while (my $cgi = new CGI::Fast) {
     say encode_json({logged_in_p => 0});
   }
 }
-
diff --git a/www/login.pl b/www/login.pl
index fe1729d..5d27cbe 100755
--- a/www/login.pl
+++ b/www/login.pl
@@ -79,6 +79,14 @@ while (my $cgi = new CGI::Fast) {
     }
     when ('google') {
       my $code = $cgi->param('code') or die "Authorization code is missing.";
+
+      # Validate CSRF token.
+      my $oauth_state = $cgi->param('state');
+      my $csrf_token = read_cookie($cgi, 'mulkyid_csrf_token');
+      unless ($csrf_token && $oauth_state && $csrf_token eq $oauth_state) {
+        die "CSRF token was forged!";
+      }
+
       my $oidc_client = OIDC::Lite::Client::WebServer->new(
         id => $::MULKONF->{'google_oauth2_client_id'},
         secret => $::MULKONF->{'google_oauth2_client_secret'},