passwordauth: prevent authentication bypass via multiple name parameters

Calling CGI::FormBuilder::field with a name argument in list context
returns zero or more user-specified values of the named field, even
if that field was not declared as supporting multiple values.
Passing the result of field as a function parameter counts as list
context. This is the same bad behaviour that is now discouraged
for CGI::param.

In this case we pass the multiple values to CGI::Session::param.
That accessor has six possible calling conventions, of which four are
documented. If an attacker passes (2*n + 1) values for the 'name'
field, for example name=a&name=b&name=c, we end up in one of the
undocumented calling conventions for param:

    # equivalent to: (name => 'a', b => 'c')
    $session->param('name', 'a', 'b', 'c')

and the 'b' session parameter is unexpectedly set to an
attacker-specified value.

In particular, if an attacker "bob" specifies
name=bob&name=name&name=alice, then authentication is carried out
for "bob" but the CGI::Session ends up containing {name => 'alice'},
an authentication bypass vulnerability.

This vulnerability is tracked as OVE-20170111-0001.

(cherry picked from commit e909eb93f4530a175d622360a8433e833ecf0254)

diff --git a/IkiWiki/Plugin/passwordauth.pm b/IkiWiki/Plugin/passwordauth.pm
index fb3fd9ee1620911558ec09c96b466405a3a0592a..0dde0386e7efce0e10624fb8b8b33e6e14f3e032 100644 (file)
--- a/IkiWiki/Plugin/passwordauth.pm
+++ b/IkiWiki/Plugin/passwordauth.pm
@@ -325,12 +325,13 @@ sub formbuilder (@) {
 
        if ($form->title eq "signin" || $form->title eq "register") {
                if (($form->submitted && $form->validate) || $do_register) {
+                       my $user_name = $form->field('name');
+
                        if ($form->submitted eq 'Login') {
-                               $session->param("name", $form->field("name"));
+                               $session->param("name", $user_name);
                                IkiWiki::cgi_postsignin($cgi, $session);
                        }
                        elsif ($form->submitted eq 'Create Account') {
-                               my $user_name=$form->field('name');
                                if (IkiWiki::userinfo_setall($user_name, {
                                        'email' => $form->field('email'),
                                        'regdate' => time})) {
@@ -344,7 +345,6 @@ sub formbuilder (@) {
                                }
                        }
                        elsif ($form->submitted eq 'Reset Password') {
-                               my $user_name=$form->field("name");
                                my $email=IkiWiki::userinfo_get($user_name, "email");
                                if (! length $email) {
                                        error(gettext("No email address, so cannot email password reset instructions."));