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Data Validation

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Untrusted data comes from many sources (users, third party sites, your own database!, ...) and all of it needs to be validated both on input and output.

Output Sanitization

The method of data sanitization depends on the type of data and the context in which it is used. Below are some common tasks in WordPress and how they should be sanitized.

Tip: It's best to do the output validation as late as possible, ideally as it's being outputted, as opposed to further up in your script. This way you can always be sure that your data is properly validated/escaped and you don't need to remember if the variable has been previously validated.

Integers

intval( $int ) or (int) $int
If it's supposed to be an integer, cast it as one.
absint( $int )
Ensures that the result is nonnegative.

HTML/XML

Note that many types of XML documents (as opposed to HTML documents) understand only a few named character references: apos, amp, gt, lt, quot. When outputting text to such an XML document, be sure to filter any text containing illegal named entities through WordPress's ent2ncr( $text ) function.

HTML/XML Fragments

wp_kses( (string) $fragment, (array) $allowed_html, (array) $protocols = null )
KSES Strips Evil Scripts. All untrusted HTML (post text, comment text, etc.) should be run through wp_kses().
To avoid having to pass an array of allowed HTML tags, you can use wp_kses_post( (string) $fragment ) for tags that are allowed in posts/pages or wp_kses_data( (string) $fragment ) for the small list of tags allowed in comments.
Note that the kses system can be resource-intensive, and should therefore not be run as an output sanitization filter directly, but as a filter to data after it has been input and processed, before it is saved in the database. WordPress runs kses on the pre_comment_content filter, for example, to filter the HTML before saving the comment.
wp_rel_nofollow( (string) $html )
Adds a "rel='nofollow'" attribute to any <a> link.
wp_kses_allowed_html( (string) $context )
Provides an array of allowed HTML tags for a given context. Allowed values are post | strip | data | entities or the name of a field filter such : as pre_user_description.

Text Nodes

esc_html( $text ) (since 2.8)
Encodes < > & " ' (less than, greater than, ampersand, double quote, single quote). Identical to esc_attr, except it applies the esc_html filter to the output.
esc_html__ (since 2.8)
Translates and encodes
esc_html_e (since 2.8)
Translates, encodes, and echoes
esc_textarea (since 3.1)
Encodes text for use inside a textarea element.
sanitize_text_field (since 2.9.0)
Sanitize a string from user input or from the db.

Attribute Nodes

esc_attr( $text ) (since 2.8)
Encodes < > & " ' (less than, greater than, ampersand, double quote, single quote). Identical to esc_html, except it applies the attribute_escape filter to the output.
esc_attr__()
Translates and encodes
esc_attr_e()
Translates, encodes, and echoes

JavaScript

esc_js( $text ) (since 2.8)

URLs

esc_url( $url, (array) $protocols = null ) (since 2.8)
Always use esc_url when sanitizing URLs (in text nodes, attribute nodes or anywhere else). Rejects URLs that do not have one of the provided whitelisted protocols (defaulting to http, https, ftp, ftps, mailto, news, irc, gopher, nntp, feed, and telnet), eliminates invalid characters, and removes dangerous characters. Replaces clean_url() which was deprecated in 3.0.
This function encodes characters as HTML entities: use it when generating an (X)HTML or XML document. Encodes ampersands (&) and single quotes (') as numeric entity references (&#038, &#039).
esc_url_raw( $url, (array) $protocols = null ) (since 2.8)
For inserting a URL in the database. This function does not encode characters as HTML entities: use it when storing a URL or in other cases where you need the non-encoded URL. This functionality can be replicated in the old clean_url function by setting $context to db.
urlencode( $scalar )
Encodes for use in URL (as a query parameter, for example)
urlencode_deep( $array )
urlencodes all array elements.

Database

$wpdb->insert( $table, (array) $data )
$data should be unescaped (the function will escape them for you). Keys are columns, Values are values.
$wpdb->update( $table, (array) $data, (array) $where )
$data should be unescaped. Keys are columns, Values are values. $where should be unescaped. Multiple WHERE conditions are ANDed together. 
$wpdb->update(
  'my_table',
  array( 'status' => $untrusted_status, 'title' => $untrusted_title ),
  array( 'id' => 123 )
);
$wpdb->prepare( $format, (scalar) $value1, (scalar) $value2, ... )
$format is a sprintf() like format string. It only understands %s, %d and %f, none of which need to be enclosed in quotation marks. 
$wpdb->get_var( $wpdb->prepare(
  "SELECT something FROM table WHERE foo = %s and status = %d",
  $name, // an unescaped string (function will do the sanitization for you)
  $status // an untrusted integer (function will do the sanitization for you)
) );
esc_sql( $sql )
Escapes a single string or string array for use in a SQL query. Glorified addslashes(). $wpdb->prepare is generally preferred because it corrects a few common formatting errors.
$wpdb->escape( $text )
Deprecated since 3.6. Use esc_sql() or $wpdb->prepare() instead.
$wpdb->escape_by_ref( &$text )
No return value. Since the parameter is passed by reference, the text is directly modified, so no need to assign any returned value.
$wpdb->esc_like( $text )
Sanitizes $text for use in a LIKE expression of a SQL query. Will still need to be SQL escaped (with one of the above functions).
like_escape( $string )
Deprecated since 4.0. Use $wpdb->esc_like() instead.

Filesystem

validate_file( (string) $filename, (array) $allowed_files = "" )
Used to prevent directory traversal attacks, or to test a filename against a whitelist. Returns 0 if $filename represents a valid relative path. After validating, you must treat $filename as a relative path (i.e. you must prepend it with an absolute path), since something like /etc/hosts will validate with this function. Returns an integer greater than zero if the given path contains .., ./, or :, or is not in the $allowed_files whitelist. Be careful making boolean interpretations of the result, since false (0) indicates the filename has passed validation, whereas true (> 0) indicates failure.

HTTP Headers

Header splitting attacks are annoying since they are dependent on the HTTP client. WordPress has little need to include user generated content in HTTP headers, but when it does, WordPress typically uses whitelisting for most of its HTTP headers.

WordPress does use user generated content in HTTP Location headers, and provides sanitization for those.

wp_redirect($location, $status = 302)
A safe way to redirect to any URL. Ensures the resulting HTTP Location header is legitimate.
wp_safe_redirect($location, $status = 302)
Even safer. Only allows redirects to whitelisted domains.

Input Validation

Many of the functions above in #Output_Sanitization are useful for input validation. In addition, WordPress uses the following functions.

Slugs

sanitize_title( $title )
Used in post slugs, for example
sanitize_user( $username, $strict = false )
Use $strict when creating a new user (though you should use the API for that).

HTML

balanceTags( $html ) or force_balance_tags( $html )
Tries to make sure HTML tags are balanced so that valid XML is output.
tag_escape( $html_tag_name )
Sanitizes an HTML tag name (does not escape anything, despite the name of the function).
sanitize_html_class( $class, $fallback )
Sanitizes a html classname to ensure it only contains valid characters. Strips the string down to A-Z,a-z,0-9,'-' if this results in an empty string then it will return the alternative value supplied.

Email

is_email( $email_address )
returns boolean false if invalid, or $email_address if valid

Arrays

array_map( 'absint', $array )
Ensures all elements are nonnegative integers. Replace callback 'absint' with whatever is appropriate for your data. array_map() is a core PHP function that runs array elements through an arbitrary callback function, in this example, absint().

Other

Some other functions that may be useful to sanitize data input:

Validation Philosophies

There are several different philosophies about how validation should be done. Each is appropriate for different scenarios.

Whitelist

Accept data only from a finite list of known and trusted values.

When comparing untrusted data against the whitelist, it's important to make sure that strict type checking is used. Otherwise an attacker could craft input in a way that will pass the whitelist but still have a malicious effect.

Comparison Operator

$untrusted_input = '1 malicious string';  // will evaluate to integer 1 during loose comparisons

if ( 1 === $untrusted_input ) {  // == would have evaluated to true, but === evaluates to false
	echo '<p>Valid data';
} else {
	wp_die( 'Invalid data' );
}

in_array()

$untrusted_input = '1 malicious string';  // will evaluate to integer 1 during loose comparisons
$safe_values     = array( 1, 5, 7 );

if ( in_array( $untrusted_input, $safe_values, true ) ) {  // `true` enables strict type checking
	echo '<p>Valid data';
} else {
	wp_die( 'Invalid data' );
}

switch()

$untrusted_input = '1 malicious string';  // will evaluate to integer 1 during loose comparisons

switch ( true ) {
	case 1 === $untrusted_input:  // do your own strict comparison instead of relying on switch()'s loose comparison
		echo '<p>Valid data';
		break;

	default:
		wp_die( 'Invalid data' );
}

Blacklist

Reject data from finite list of known untrusted values. This is very rarely a good idea.

Format Detection

Test to see if the data is of the correct format. Only accept it if it is. 

if ( ! ctype_alnum( $data ) ) {
  wp_die( "Invalid format" );
}

if ( preg_match( "/[^0-9.-]/", $data ) ) {
  wp_die( "Invalid format" );
}

Format Correction

Accept most any data, but remove or alter the dangerous pieces. 

$trusted_integer = (int) $untrusted_integer;
$trusted_alpha = preg_replace( '/[^a-z]/i', "", $untrusted_alpha );
$trusted_slug = sanitize_title( $untrusted_slug );

Changelog

External Resources

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Codex Resources