Security best practices guide

docs/guide/README.md

@@ -112,7 +112,7 @@ Security
 * [Authorization](security-authorization.md)
 * [Working with Passwords](security-passwords.md)
 * **TBD** [Auth Clients](security-auth-clients.md)
-* **TBD** [Best Practices](security-best-practices.md)
+* [Best Practices](security-best-practices.md)
 
 
 Caching

docs/guide/db-dao.md

@@ -194,7 +194,7 @@ $connection->createCommand()->update('user', ['status' => 1], 'age > 30')->execu
 $connection->createCommand()->delete('user', 'status = 0')->execute();
 ```
 
-Quoting Table and Column Names
+Quoting Table and Column Names <a name="quoting-table-and-column-names"></a>
 ------------------------------
 
 To make column and table names safe to use in queries, you can have Yii properly quote them for you:

docs/guide/security-best-practices.md

-Security best practice
-======================
+Security best practices
+=======================
 
-> Note: This section is under development.
->
-> It has no content yet.
+Below we'll review common security principles and describe how to avoid threats when developing applications using Yii. 
+
+Basic principles
+----------------
+
+There are two main principles when it comes to security no matter which application is being developed:
+
+1. Filter input.
+2. Escape output.
+
+
+### Filter input
+
+Filter input means that input should never be considered safe and you should always check if the value you've got is
+actually among allowed ones i.e. if we know that sorting could be done by three fields `title`, `created_at` and `status`
+and the field could be supplied via used input it's better to check the value we've got right where we're receiving it.
+In terms of basic PHP that would look like the following:
+
+```php
+$sortBy = $_GET['sort'];
+if (!in_array($sortBy, ['title', 'created_at', 'status'])) {
+	throw new Exception('Invalid sort value.');
+}
+```
+
+In Yii, most probably you'll use [form validation](input-validation.md) to do alike checks.
+
+
+### Escape output
+
+Escape output means that depending on context where we're using data it should be escaped i.e. in context of HTML you
+should escape `<`, `>` and alike special characters. In context of JavaScript or SQL it will be different set of characters.
+Since it's error-prone to escape everything automatically Yii provides various tools to perform escaping for different
+contexts.
+
+Avoiding SQL injections
+-----------------------
+
+SQL injection happens when query text is formed by concatenating unescaped strings such as the following:
+
+```php
+$username = $_GET['username'];
+$sql = "SELECT * FROM user WHERE username = '$username'";
+```
+
+Instead of supplying correct username attacker could give your applications something like `'; DROP TABLE user; --`.
+Resulting SQL will be the following:
+
+```sql
+SELECT * FROM user WHERE username = ''; DROP TABLE user; --'
+```
+
+This is valid query that will search for users with empty username and then will drop `user` table most probably
+resulting in broken website and data loss (you've set up regular backups, right?).
+
+In Yii most of database querying happens via [Active Record](db-active-record.md) which properly uses PDO perpared
+statements internally. In case of prepared statements it's not possible to manipulate query as was demonstrated above.
+
+Still, sometimes you need [raw queries](db-dao.md) or [query builder](db-query-builder.md). In this case you should use
+safe ways of passing data. If data is used for column values it's preferred to use prepared statements:
+
+```php
+// query builder
+$userIDs = (new Query())
+    ->select('id')
+    ->from('user')
+    ->where('status=:status', [':status' => $status])
+    ->all();
+    
+// DAO
+$userIDs = $connection
+    ->createCommand('SELECT id FROM user where status=:status')
+    ->bindValues([':status' => $status])
+    ->queryColumn();
+```
+
+If data is used to specify column names or table names it should be escaped. Yii has special syntax for such escaping
+which allows doing it the same way for all databases it supports:
+
+```php
+$sql = "SELECT COUNT([[$column]]) FROM {{table}}";
+$rowCount = $connection->createCommand($sql)->queryScalar();
+```
+
+You can get details about the syntax in [Quoting Table and Column Names](db-dao.md#quoting-table-and-column-names).
+
+
+Avoiding XSS
+------------
+
+XSS or cross-site scripting happens when output isn't escaped properly when outputting HTML to the browser. For example,
+if user can enter his name and instead of `Alexander` he enters `<script>alert('Hello!');</script>`, every page that
+outputs user name without escaping it will execute JavaScript `alert('Hello!');` resulting in alert box popping up
+in a browser. Depending on website instead of innocent alert such script could send messages using your name or even
+perform bank transactions.
+
+Avoiding XSS is quite easy in Yii. There are generally two cases:
+
+1. You want data to be outputted as plain text.
+2. You want data to be outputted as HTML.
+
+If all you need is plain text then escaping is as easy as the following:
+
+
+```php
+<?= \yii\helpers\Html::encode($username) ?>
+```
+
+If it should be HTML we could get some help from HtmlPurifier:
+
+```php
+<?= \yii\helpers\HtmlPurifier::process($description) ?>
+```
+
+Note that HtmlPurifier processing is quite heavy so consider adding caching.
+
+Avoiding CSRF
+-------------
+
+TBD
+
+
+Avoiding file exposure
+----------------------
+
+By default server webroot is meant to be pointed to `web` directory where `index.php` is. In case of shared hosting
+environments it could be impossible to achieve so we'll end up with all the code, configs and logs in server webroot.
+
+If it's the case don't forget to deny access to everything except `web`. If it can't be done consider hosting your
+application elsewhere.
+
+Avoiding debug info and tools at production
+-------------------------------------------
+
+In debug mode Yii shows quite verbose errors which are certainly helpful for development. The thing is that these
+verbose errors are handy for attacker as well since these could reveal database structure, configuration values and
+parts of your code. Never run production applications with `YII_DEBUG` set to `true` in your `index.php`.
+ 
+You should never enalble Gii at production. It could be used to get information about database structure, code and to
+simply rewrite code with what's generated by Gii.
+
+Debug toolbar should be avoided at production unless really necessary. It exposes all the application and config
+details possible. If you absolutely need it check twice that access is proerly restricted to your IP only.
\ No newline at end of file