Goals of Bleach ¶

This document lists the goals and non-goals of Bleach. My hope is that by focusing on these goals and explicitly listing the non-goals, the project will evolve in a stronger direction.

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Goals of Bleach

Goals ¶

Always take a allowed-list-based approach ¶

Bleach should always take a allowed-list-based approach to markup filtering. Specifying disallowed lists is error-prone and not future proof.

For example, you should have to opt-in to allowing the onclick attribute, not opt-out of all the other on* attributes. Future versions of HTML may add new event handlers, like ontouch, that old disallow would not prevent.

Main goal is to sanitize input of malicious content ¶

The primary goal of Bleach is to sanitize user input that is allowed to contain some HTML as markup and is to be included in the content of a larger page in an HTML context.

Examples of such content might include:

User comments on a blog.
“Bio” sections of a user profile.
Descriptions of a product or application.

These examples, and others, are traditionally prone to security issues like XSS or other script injection, or annoying issues like unclosed tags and invalid markup. Bleach will take a proactive, allowed-list-only approach to allowing HTML content, and will use the HTML5 parsing algorithm to handle invalid markup.

See the chapter on clean() for more info.

Safely create links ¶

The secondary goal of Bleach is to provide a mechanism for finding or altering links (<a> tags with href attributes, or things that look like URLs or email addresses) in text.

While Bleach itself will always operate on a allowed-list-based security model, the linkify() method is flexible enough to allow the creation, alteration, and removal of links based on an extremely wide range of use cases.

Bleach does not try to verify the validity or safety of the domains linked to beyond being well-formed (see Linkifying text fragments for details).

Non-Goals ¶

Bleach is designed to work with fragments of HTML by untrusted users. Some non-goal use cases include:

Sanitize complete HTML documents ¶

Bleach’s clean is not for sanitizing entire HTML documents. Once you’re creating whole documents, you have to allow so many tags that a disallow-list approach (e.g. forbidding <script> or <object>) may be more appropriate.

Sanitize for use in HTML attributes, CSS, JSON, xhtml, SVG, or other contexts ¶

Bleach’s clean is used for sanitizing content to be used in an HTML context–not for HTML attributes, CSS, JSON, xhtml, SVG, or other contexts.

For example, this is a safe use of clean output in an HTML context:

<p>
  {{ bleach.clean(user_bio) }}
</p>

This is a not safe use of clean output in an HTML attribute:

<body data-bio="{{ bleach.clean(user_bio) }}">

If you need to use the output of bleach.clean() in an HTML attribute, you need to pass it through your template library’s escape function. For example, Jinja2’s escape or django.utils.html.escape or something like that.

If you need to use the output of bleach.clean() in any other context, you need to pass it through an appropriate sanitizer/escaper for that context.

Remove all HTML or transforming content for some non-web-page purpose ¶

There are much faster tools available if you want to remove or escape all HTML from a document.

Clean up after trusted users ¶

Bleach is powerful but it is not fast. If you trust your users, trust them and don’t rely on Bleach to clean up their mess.

Make malicious content look pretty or sane ¶

Malicious content is designed to be malicious. Making it safe is a design goal of Bleach. Making it pretty or sane-looking is not.

If you want your malicious content to look pretty, you should pass it through Bleach to make it safe and then do your own transform afterwards.

Allow arbitrary styling ¶

There are a number of interesting CSS properties that can do dangerous things, like Opera’s -o-link. Painful as it is, if you want your users to be able to change nearly anything in a style attribute, you should have to opt into this.

Usage with Javascript frameworks and template languages ¶

A number of Javascript frameworks and template languages allow XSS via Javascript Gadgets. While Bleach usually produces output safe for these contexts, it is not tested against them nor guaranteed to produce safe output. Check that bleach properly strips or escapes language-specific syntax like data-bind attributes for Knockout.js or ng-* attributes from Angular templates before using bleach-sanitized output with your framework or template language.

Protect against CSS-based XSS attacks in legacy browsers ¶

Bleach will not protect against CSS-based XSS vectors that only worked in legacy IE, Opera, or Netscape/Mozilla/Firefox browsers. For example, it will not remove expression or url functions in CSS component values in style elements or attributes and other vectors.

Protect against privacy, cross site, or HTTP leaks ¶

Bleach does not prevent output from fingerprinting users or leaking information about users via requests to external sites. For example, it will not remove CSS Media Queries or tracking pixels.

Bleach vs html5lib ¶

Bleach is built upon html5lib, and html5lib has a built-in sanitizer filter, so why use Bleach?

Bleach’s API is simpler.
Bleach’s sanitizer allows a map to be provided for ALLOWED_ATTRIBUTES, giving you a lot more control over sanitizing attributes: you can sanitize attributes for specific tags, you can sanitize based on value, etc.
Bleach’s sanitizer always alphabetizes attributes, but uses an alphabetizer that works with namespaces — the html5lib one is broken in that regard.
Bleach’s sanitizer always quotes attribute values because that’s the safe thing to do. The html5lib one makes that configurable. In this case, Bleach doesn’t make something configurable that isn’t safe.
Bleach’s sanitizer has a very restricted set of ALLOWED_PROTOCOLS by default. html5lib has a much more expansive one that Bleach’s authors claim is less safe.
html5lib.filters.sanitizer.Filter’s sanitize_css is broken and doesn’t work.