[development] Porting - Quick security reminders

Heine Deelstra hdeelstra at gmail.com
Tue Dec 19 14:30:03 UTC 2006

Dear all,

Porting a module is an excellent opportunity to keep an eye out for security
problems (evidence: <http://drupal.org/node/103943>).

Here's a quick security reminder regarding input (user-supplied data).

Code samples are only included to make a point, do not hold them against me.

Escape or filter text before display
If you do not escape or filter text before display, you enable a user to insert
arbitrary HTML and scripts into pages. This type of attack is know as cross site
scripting aka XSS.

For more information, see: <http://drupal.org/node/28984>

Escape / filter:
- check_plain or theme('placeholder') for plain text
- check_markup or filter_xss for markup containing text

Drupal 5 brought a (brilliant IMO) change to t():

Depending on the placeholder's sigil, it is passed through theme('placeholder')
(%) or check_plain (@) automatically.

t('I escape %user_data', '%user_data'); // I escape <em>user_data</em> (safe)
t('I escape @user_data', '@user_data'); // I escape user_data (safe)
t('I don't escape !user_data', '!user_data'); // XSS vulnerability

Sometimes the difficult part is to know whether a function requires escaped text
or does the escaping itself:

* Automatically escapes:

l() - Escapes text and attributes unless you pass TRUE for the $html parameter.
In that case only attributes are check_plained.

- menu items & breadcrumbs
- block descriptions (*not* titles)
- theme('username')

* Common pitfalls:
watchdog - you have to provide a safe $message
drupal_set_title - you have to provide a safe $title
Form elements - #description and #title of form_elements need to be safe
Form elements - #value of #type markup and item need to be safe. Note that the
default form element #type is markup!


drupal_set_title($node->title); // XSS vulnerability

watchdog(content', t("Deleted !title", array('!title' => $node->title)); // XSS
watchdog(content', t("Deleted %title", array('%title' => $node->title)); // or @

$form['unsafe'] = array('#value' => $user->name); //XSS
$form['safe'] = array('#value' => check_plain($user->name));
$form['safe'] = array('#value' => theme('username', $user));

$form['bad'] = array(
  '#type' => 'textfield';
  '#default_value' => check_plain($u_supplied),  // bad: escaped twice
  '#description' => t("Old data: !data", array('!data' => $u_supplied)), // XSS

$form['good'] = array(
  '#type' => 'textfield';
  '#default_value' => $u_supplied,
  '#description' => t("Old data: @data", array('@data' => $u_supplied)),

Database (ab)use

See also <http://drupal.org/node/101496>

Summary: Whatever you do, *never* use user supplied data directly in a query.
Use db_query as it was meant to be used: data goes in arguments. And make sure
to enclose %s in single quotes: '%s'.


1.db_query("SELECT t.s FROM {table} t WHERE t.field = $from_user");
2.db_query("SELECT t.s FROM {table} t WHERE t.field = %s", $from_user);

db_query("SELECT t.s FROM {table} t WHERE t.field = '%s'", $from_user);

BAD ($from_user is an array of numbers):

db_query("SELECT t.s FROM {table} t WHERE t.field IN (%s)", $from_user);


$placeholders = implode(',', array_fill(0, count($from_user), "%d"));

db_query("SELECT t.s FROM {table} t WHERE t.field IN ($placeholders)", $from_user);

Anything else: bad. Why is this bad? Suppose you check the data that you stuff
in to a query (e.g. 'all numeric'). Or, you know a user name is validated to
contain only certain 'safe' characters.

The problem is that you open the possibility that a code path develops where
unfiltered content ends up at your query. In that case you're toast.

This code path can arise because you 1) accidentally remove validation in future
development, 2) a user adds a module that is more lenient in the data it
accepts or 3) you add a bunch of code that skips validation (recent xTracker

Writing secure code
If you want to read more about security, I suggest that you start at "Input the
root of all evil" <http://drupal.org/node/101495> in the "Writing secure code"
<http://drupal.org/node/62304> pages.

I hope to expand and improve those pages in the future, but some of them are
already useful.

You found a vulnerability?
What to do when you find a vulnerability in your module? Simple; write us at
security at drupal.org.

Thank you for listening,

Heine Deelstra

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