# Zend\\Feed\\Reader and Security

As with any data coming from a source that is beyond the developer's control,
special attention needs to be given to securing, validating and filtering that
data. Similar to data input to our application by users, data coming from RSS
and Atom feeds should also be considered unsafe and potentially dangerous, as it
allows the delivery of HTML and [xHTML](http://tools.ietf.org/html/rfc4287#section-8.1).
Because data validation and filtration is out of `Zend\Feed`'s scope, this task
is left for implementation by the developer, by using libraries such as
zend-escaper for escaping and [HTMLPurifier](http://www.htmlpurifier.org/) for
validating and filtering feed data.

Escaping and filtering of potentially insecure data is highly recommended before
outputting it anywhere in our application or before storing that data in some
storage engine (be it a simple file or a database.).

## Filtering data using HTMLPurifier

Currently, the best available library for filtering and validating (x)HTML data
in PHP is [HTMLPurifier](http://www.htmlpurifier.org/), and, as such, is the
recommended tool for this task.  HTMLPurifier works by filtering out all (x)HTML
from the data, except for the tags and attributes specifically allowed in a
whitelist, and by checking and fixing nesting of tags, ensuring
standards-compliant output.

The following examples will show a basic usage of HTMLPurifier, but developers
are urged to go through and read [HTMLPurifier's documentation](http://www.htmlpurifier.org/docs).

```php
// Setting HTMLPurifier's options
$options = [
    // Allow only paragraph tags
    // and anchor tags wit the href attribute
    [
        'HTML.Allowed',
        'p,a[href]'
    ],
    // Format end output with Tidy
    [
        'Output.TidyFormat',
        true
    ],
    // Assume XHTML 1.0 Strict Doctype
    [
        'HTML.Doctype',
        'XHTML 1.0 Strict'
    ],
    // Disable cache, but see note after the example
    [
        'Cache.DefinitionImpl',
        null
    ]
];

// Configuring HTMLPurifier
$config = HTMLPurifier_Config::createDefault();
foreach ($options as $option) {
    $config->set($option[0], $option[1]);
}

// Creating a HTMLPurifier with it's config
$purifier = new HTMLPurifier($config);

// Fetch the RSS
try {
   $rss = Zend\Feed\Reader\Reader::import('http://www.planet-php.net/rss/');
} catch (Zend\Feed\Exception\Reader\RuntimeException $e) {
   // feed import failed
   echo "Exception caught importing feed: {$e->getMessage()}\n";
   exit;
}

// Initialize the channel data array
// See that we're cleaning the description with HTMLPurifier
$channel = [
   'title'       => $rss->getTitle(),
   'link'        => $rss->getLink(),
   'description' => $purifier->purify($rss->getDescription()),
   'items'       => [],
];

// Loop over each channel item and store relevant data
// See that we're cleaning the descriptions with HTMLPurifier
foreach ($rss as $item) {
   $channel['items'][] = [
       'title'       => $item->getTitle(),
       'link'        => $item->getLink(),
       'description' => $purifier->purify($item->getDescription()),
   ];
}
```

> ### Tidy is required
>
> HTMLPurifier is using the PHP [Tidy extension](http://php.net/tidy) to clean
> and repair the final output. If this extension is not available, it will
> silently fail, but its availability has no impact on the library's security.

> ### Caching
>
> For the sake of this example, the HTMLPurifier's cache is disabled, but it is
> recommended to configure caching and use its standalone include file as it can
> improve the performance of HTMLPurifier substantially.

## Escaping data using zend-escaper

To help prevent XSS attacks, Zend Framework provides the [zend-escaper component](https://github.com/zendframework/zend-escaper),
which complies to the current [OWASP recommendations](https://www.owasp.org/index.php/XSS_Prevention_Cheat_Sheet),
and as such, is the recommended tool for escaping HTML tags and attributes,
Javascript, CSS and URLs before outputting any potentially insecure data to the
users.

```php
try {
    $rss = Zend\Feed\Reader\Reader::import('http://www.planet-php.net/rss/');
} catch (Zend\Feed\Exception\Reader\RuntimeException $e) {
    // feed import failed
    echo "Exception caught importing feed: {$e->getMessage()}\n";
    exit;
}

// Validate all URIs
$linkValidator = new Zend\Validator\Uri;
$link = null;
if ($linkValidator->isValid($rss->getLink())) {
    $link = $rss->getLink();
}

// Escaper used for escaping data
$escaper = new Zend\Escaper\Escaper('utf-8');

// Initialize the channel data array
$channel = [
    'title'       => $escaper->escapeHtml($rss->getTitle()),
    'link'        => $escaper->escapeUrl($link),
    'description' => $escaper->escapeHtml($rss->getDescription()),
    'items'       => [],
];

// Loop over each channel item and store relevant data
foreach ($rss as $item) {
    $link = null;
    if ($linkValidator->isValid($rss->getLink())) {
        $link = $item->getLink();
    }
    $channel['items'][] = [
        'title'       => $escaper->escapeHtml($item->getTitle()),
        'link'        => $escaper->escapeUrl($link),
        'description' => $escaper->escapeHtml($item->getDescription()),
    ];
}
```

The feed data is now safe for output to HTML templates. You can, of course, skip
escaping when simply storing the data persistently, but remember to escape it on
output later!

Of course, these are just basic examples, and cannot cover all possible
scenarios that you, as a developer, can, and most likely will, encounter. Your
responsibility is to learn what libraries and tools are at your disposal, and
when and how to use them to secure your web applications.
