Download the PHP package humanmade/whitelist-html without Composer

clean_html: Safe HTML for WordPress

Introduces an esc_*()-like function for when you need to allow some HTML.

Rationale

Background

Best practices when working with any sort of data is to escape your output, and do it as late as possible. Values can't usually know about where they're going to be used, so you need to escape based on whatever context you're outputting into. These are things like esc_attr() for HTML attribute values, esc_html() for text in HTML, and so on.

Even if values could know about their output context, it's still possible for users to craft malicious output if you're not escaping properly. For this reason, you need to do sanitization on input (to ensure your value is correct), as well as escaping on output (to ensure the value is output into the context correctly).

Right now across every WordPress site, there's a glaring hole in escaping, and hence in security.

When translating strings in WordPress, the most common functions to use are __() (translate and return) or _e() (translate and output). Where possible, these need to be escaped too, to ensure that translations don't accidentally break your output. For this reason, esc_html_e(), esc_attr_e(), etc are offered as convenience functions.

However, this falls down when you need to have HTML in the translation. Translation best practices say to include as much information as possible for translators when you translate a string. This means including HTML tags in the string so translators can understand how the sentence is formed.

It's possible to do "clever" hacks to get around this with placeholders, for example:

Note though that this is much harder for translators to understand, since they can't intuitively tell what's going on without checking the code. Even with translator comments, it's still harder to understand. There's also no guarantee that this is secure. You could swap the placeholders, or leave out pieces. Best practice states that we should instead have the following:

Right now, the policy is essentially to treat translated strings with HTML as trusted. Not only does this push the burden off to translation validators in GlotPress, but it means you're no longer in control of your output. This is an attack vector waiting to be exploited.

How do we solve this?

WordPress contains functions specifically designed to help with this problem. After all, people can submit comments or posts with HTML in them, but WP can handle this fine. WordPress handles this through a library called kses, which sanitizes HTML down to a small, safe subset of HTML. Posts can have more HTML tags than comments can, since they're usually semi-trusted users.

kses is great, but is not typically used outside of large HTML blocks like post or comment content. The reason for this is often stated as performance. It's well-known that kses is pretty slow, since it has to essentially disassemble the HTML, then reconstruct it with the allowed tags.

However, Zack Tollman wrote a fantastic post that calls into question this accepted knowledge of kses performance. Zack's findings show that while kses is worse with performance on longer pieces of content (like post content), it's actually closer to being on-par with other escaping for short strings. This is even more evident when reducing the allowed elements down from the default to just the elements you need.

clean_html

This library provides a nice, easy, performant way to perform sanitization on translated strings. Rather than requiring you to work with the internals of kses, it's much closer to functions like esc_html.

Security is only useful if it's also usable. For the most part, clean_html can be used in exactly the same way developers are used to using other escaping functions.

A quick example to demonstrate how easy it is:

Even if a malicious translator changed this to include a link to a spam site (or worse), this would be caught and stripped by clean_html.

Taking our original example from above, we can modify it to only allow a tags:

It's that easy. You can do this with multiple elements as well, using a comma-separated string or list of elements:

If you need custom attributes, you can use kses-style attribute specifiers. These can be mixed too:

Performance Test

In a quick test, the string 'hello with a <a href="wak://example.com">malicious extra link!<///q><o>b' was run through both clean_html (with only a) and esc_html with 10,000 iterations. While the two functions don't perform the same task, they're both escaping functions, so it's useful to compare performance to understand whether this approach can be used in production code.

In an unscientific trial, this gave figures of 0.96s for clean_html and 1.07s for esc_html for 10,000 trials each. This indicates that clean_html is at least on the order of other escaping functions.

Using this Library

Two steps to using this library:

1. Add this library in as a git submodule.
2. Load clean-html.php before you need to use it. We recommend in mu-plugins, but you can also load it in via wp-config.php if you want it earlier.

Done. Start using the function.