Skip to content

Code review

VIP warnings

Issues reported as a VIP Warning should not be pushed to a production server unless a specific use case makes it acceptable. Some common issues reported as VIP warnings are listed below.

Uncached functions

WordPress provides a variety of functions that interact with the database, not all of which are cacheable. To ensure high performance and stability, please avoid using any of the functions listed on our Uncached Functions list.

Safelisting values for input/output validation

When working with user-submitted data, try where possible to accept data only from a finite list of known and trusted values. For example:

$possible_values = array( 'a', 1, 'good' );
if ( ! in_array( $untrusted, $possible_values, true ) )
die( "Don't do that!" );

Direct database queries

Thanks to WordPress’ API, you should almost never need to query database tables directly. Using WordPress APIs rather than rolling your own functions ensures compatibility with past and future versions of WordPress and PHP. It also makes code reviews go more smoothly because we know we can trust the APIs. Visit our page covering database queries for more information.

Additionally, direct database queries bypass internal caching. If absolutely necessary, you should evaluate the potential performance of these queries and add caching if needed.  Any queries that would modify database contents may also put the object cache out of sync with the data, causing problems.

Arbitrary JavaScript and CSS stored in options or meta

To limit attack vectors via malicious users or compromised accounts, arbitrary JavaScript cannot be stored in options or meta and then output as-is.

CSS in options or meta should also generally be avoided, but if absolutely necessary, it’s a good idea to properly sanitize it.

Encoding values used when creating a url or passed to add_query_arg()

Add_query_arg() is a really useful function, but it might not work as intended.
The values passed to it are not encoded meaning that passing

$m_yurl = 'admin.php?action=delete&post_id=321';
$my_url = add_query_arg( 'my_arg', 'somevalue&post_id=123', $my_url );

You would expect the url to be:

But in fact it becomes:

Using rawurlencode() on the values or variables passed to the query string prevents this and parameter hijacking.

Prefixing functions, constants, classes, and slugs

Best practice in WordPress is to prefix everything. This applies to obvious things such as names of function, constants, and classes, and also less obvious ones like post_type and taxonomy slugs, cron event names, etc.

Return values not checked

When defining a variable through a function call, you should always check the function’s return value before calling additional functions or methods using that variable.

function wpcom_vip_meta_desc() {
    $text = wpcom_vip_get_meta_desc();
    if ( ! empty( $text ) ) {
        echo "<meta name='description' content='" . esc_attr( $text ) . "' />";

Order By Rand

MySQL queries that use ORDER BY RAND() can be pretty challenging and slow on large datasets. An alternate option can be to retrieve 100 posts and pick one at random.

Skipping full page caching

Varnish is used for page caching at the edge, which improves performance by serving end-users a page that comes directly from the nearest datacenter. The GET parameters is always cached and done individually per GET parameter.  Varnish will only respect the Vary headers for X-Country-Code and Accept but not forCookie.

Ajax calls on every pageload

Making GET & POST requests to admin-ajax.php on every pageload, or on any pageload without user input, will cause performance issues and need to be rethought. If you have questions, we would be happy to help work through an alternate implementation.

Front-end database writes

Writing to the database on the frontend is highly discouraged. It typically will not work as expected because most pageviews are cached. It can also cause significant load on the database if a high percentage of requests are writing to the database. We generally recommend avoiding functions that write to the database on the frontend.

*_meta as a hit counters

Please don’t use meta (post_meta, comment_meta, etc.) to track counts of things (e.g. votes, pageviews, etc.). First of all, it won’t work properly because of caching and due to race conditions on high volume sites. It’s also just a recipe for disaster and an easy way to break your site. In general, you should not try to count/track user events within WordPress; consider using a JavaScript-based solution paired with a dedicated analytics service (such as Google Analytics) instead.

eval() and create_function()

Both these functions can execute arbitrary code that’s constructed at run time, which can be created through difficult-to-follow execution flows. These methods can make your site fragile because unforeseen conditions can cause syntax errors in the executed code, which becomes dynamic. A much better alternative is an Anonymous Function, which is hardcoded into the file and can never change during execution.

If there are no other options than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.

No LIMIT queries

Using posts_per_page (or numberposts) with the value set to -1 or an unreasonably high number or setting nopaging to true opens up the potential for scaling issues if the query ends up querying thousands of posts.

You should always fetch the lowest number possible that still gives you the number of results you find acceptable. Imagine that your site grows over time to include 10,000 posts. If you specify -1 for posts_per_page, you’ll query with no limit and fetch all 10,000 posts every time the query runs, which is going to destroy your site’s performance. If you know you’ll never have more than 15 posts, then set posts_per_page to 15. If you think you might have more than 15 that you’d want to display but doubt it’d hit more than 100 ever, set the limit to 100. If it gets much higher than that, you might need to rethink the page architecture a bit.

Cron schedules less than 15 minutes or expensive events

Overly frequent cron events (anything less than 15 minutes) can significantly impact the performance of the site, as can cron events that are expensive.

Flash (.swf) files

Flash (.swf) files are not advisable on the WordPress VIP Platform, as they often present a security threat (largely due to poor development practices or due to bugs in the Flash Player) and vulnerabilities that are difficult to detect and secure.

Ignore development only files

If it’s feasible within your development workflow, we ask that you .gitignore any files that are used exclusively in the local development of your theme, including but not limited to .svnignore, config.rb, sass-cache, grunt files, PHPUnit tests, etc.

Expensive 404 pages

The 404 page needs to be one of the fastest on the site, as it is only cached for 10 seconds. That means that a traffic spike from a broken link can cause performance and even availability problems if the 404 page is doing expensive db lookups.

Commented out code, debug code or output

VIP themes should not contain debug code and should not output debugging information. That includes the use of functions that provide backtrace information, such as wp_debug_backtrace_summary() or debug_backtrace(). If you’re encountering an issue that can’t be debugged in your development environment, we’ll be glad to help troubleshoot it with you. The use of commented out code should be avoided. Having code that is not ready for production on production is bad practice and could easily lead to mistakes while reviewing (since the commented out code might not have been reviewed and the removing on a comment might slip in accidentally).

Generating email

To prevent issues with spam, abuse or other unwanted communications, your code should not generate, or allow users to generate, email messages to site users or user-supplied email addresses. That includes mailing list functionality, invitations to view or share content, notifications of site activity, or other messages generated in bulk. Where needed, you can integrate third-party SMTP or ESP (Email Service Provider) services that allow sharing of content by email, as long as they don’t depend on the VIP infrastructure for message delivery. If you only need to send out a few emails to admins or to a specific email address not in bulk amounts, you can use the built-in wp_mail() functionality.

Custom wp_mail headers

Every time you want to create custom headers using user supplied data (e.g. “FROM” header), make sure you’re using filters provided by WordPress for you. See wp_mail_from() and wp_mail_from_name()

Serializing data

Unserialize has known vulnerability problems with Object Injection. JSON is generally a better approach for serializing data.

Including files with untrusted paths or filenames

locate_template(), get_template_part(), and sometimes include() or require() are typically used to include templates. If your template name, file name or path contains any non-static data or can be filtered, you must validate it against directory traversal using validate_file() or by detecting the string “..”

Relative file includes

Including files with relative paths may lead to unintended results. It’s recommended that all files are included with an absolute path.

Several functions are available to compose the file path, such as __DIR__ , dirname( __FILE__ ) , plugin_dir_path( __FILE__ ), or get_template_directory().

// Don't do this:
require_once 'file.php';
// Do this instead:
require_once __DIR__ . '/file.php';

Minified JavaScript files

JavaScript files that are minified should also be committed with changes to their unminified counterparts. Minified files cannot be read for review, and are much harder to work with when debugging issues.

reCaptcha for share by email

To protect against abuse of Jetpack’s share by email feature (aka Sharedaddy) it must be implemented along with reCaptcha. This helps protect against the risk of the network being seen as a source of email spam, which would adversely affect VIP sites. This blog post explains how to implement reCaptcha.

Removing the admin bar

Removing the WordPress admin tool bar is strongly discouraged, and it must not be removed for administrator or vip_support roles.

Remote calls

Remote calls such as fetching information from external APIs or resources should rely on the WordPress HTTP API (no cURL) and should be cached. Example of remote calls that should be cached are wp_remote_get(), wp_safe_remote_get() and wp_oembed_get(). For more information, review our page on retrieving remote data.

Using __FILE__ for page registration

When adding menus or registering your plugins, make sure that you use a unique handle or slug other than __FILE__ to ensure that you are not revealing system paths.

Functions that use JOINS, taxonomy relation queries, -cat, -tax queries, subselects or API calls

Close evaluation of the queries is recommended as these can be expensive and lead to performance issues. Queries with known problems when working with large datasets:

  • category__and, tag__and, tax_query with AND
  • category__not_in, tag__not_in, and tax_query with NOT IN
  • tax_query with multiple taxonomies
  • meta_query with a large result set (e.g. looking for only posts with a thumbnail_id meta on a large site, looking for posts with a specific meta value on a key)

Taxonomy queries that do not specify 'include_children' => false

Almost all taxonomy queries include 'include_children' => true by default.  This can have a very significant performance impact on code, and in some cases queries will time out.  We recommend 'include_children' => false to be added to all taxonomy queries when possible.

In many instances where all posts in either a parent or child term are wanted, this can be replaced by only querying for the parent term and using a save_post() hook to determine a child term is added, and if so enforce that its parent term is also added. A one-time WP-CLI command might be needed to ensure previous data integrity.

Custom roles

For best compatibility between environments and for added security, custom user roles and capabilities need to be managed via our helper functions.

Using extract()

extract() should never be used because it is too opaque and difficult to understand how it will behave under a variety of inputs. It makes it too easy to unknowingly introduce new variables into a function’s scope, potentially leading to unintended and difficult to debug conflicts.


$_REQUEST should never be used because it is hard to track where the data is coming from (i.e., was it POST, or GET, or a cookie), which makes reviewing the code more difficult. Additionally, it makes it easy to introduce sneaky and hard to find bugs, as any of the aforementioned locations can supply the data, which is hard to predict.  A much better practice is to be explicit and use either $_POST or $_GET instead.

Not using the settings API

Instead of handling the output of settings pages and storage yourself, use the WordPress Settings API as it handles a lot of the heavy lifting for you including added security.

Make sure to also validate and sanitize submitted values from users using the sanitize callback in the register_setting call.

Using page templates instead of rewrites

A common “hack” in the WordPress community when requiring a custom feature to live at a vanity URL (e.g. /lifestream/) is to use a Page + Page Template. This isn’t ideal for numerous reasons:

  • It requires WordPress to do multiple queries to handle the lookup for the page and any additional loops you manually run through.
  • It impedes development workflow as it requires the page to be manually created in each environment and new developer machines as well.

Use wp_parse_url() instead of parse_url()

In PHP versions lower than 5.4.7 schemeless and relative urls would not be parsed correctly by parse_url(). We therefore recommend that you use wp_parse_url() for backward compatibility.

Use wp_safe_redirect() instead of wp_redirect()

Using wp_safe_redirect(), along with the allowed_redirect_hosts filter, can help avoid any chances of malicious redirects within code.  It’s also important to remember to call exit() after a redirect so that no other unwanted code is executed.

Mobile Detection

When targeting mobile visitors, use jetpack_is_mobile() instead of wp_is_mobile(). It is more robust and works better with full-page caching.

function jetpack_is_mobile( $kind = 'any', $return_matched_agent = false )

Where kind can be:

  • smart
  • dumb
  • any

You can also use:

  • Jetpack_User_Agent_Info::is_ipad()
  • Jetpack_User_Agent_Info::is_tablet()

Views from mobile devices are cached and to make sure things work as expected, please let us know before including any additional server-side logic (PHP) in your code.

These are loaded automatically.

Custom API endpoints without permissions callback

For custom API routes that perform writes or read private data, registering a valid permissions callback is recommended to ensure that no data is exposed or manipulated. 

Using bloginfo() without escaping

get_bloginfo() should be used instead of bloginfo() so that data can be properly late escaped on output. get_bloginfo() can also return multiple types of data, be used in multiple places, and may need to be escaped with many different functions depending on the context. For example:

echo '<a href="' . esc_url( get_bloginfo( 'url' ) ) . '">' . esc_html( get_bloginfo( 'name' ) ) . '</a>';
echo '<meta property="og:description" content="' . esc_attr( get_bloginfo( 'description' ) ) . '">';

Plugin registration hooks

register_activation_hook() and register_deactivation_hook() are not supported because of the way plugins are loaded on WordPress VIP using wpcom_vip_load_plugin().

Last updated: March 15, 2022