bluemonday

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Published: Feb 7, 2022 License: BSD-3-Clause Imports: 11 Imported by: 1,135

README

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bluemonday is a HTML sanitizer implemented in Go. It is fast and highly configurable.

bluemonday takes untrusted user generated content as an input, and will return HTML that has been sanitised against an allowlist of approved HTML elements and attributes so that you can safely include the content in your web page.

If you accept user generated content, and your server uses Go, you need bluemonday.

The default policy for user generated content (bluemonday.UGCPolicy().Sanitize()) turns this:

Hello <STYLE>.XSS{background-image:url("javascript:alert('XSS')");}</STYLE><A CLASS=XSS></A>World

Into a harmless:

Hello World

And it turns this:

<a href="javascript:alert('XSS1')" onmouseover="alert('XSS2')">XSS<a>

Into this:

XSS

Whilst still allowing this:

<a href="http://www.google.com/">
  <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
</a>

To pass through mostly unaltered (it gained a rel="nofollow" which is a good thing for user generated content):

<a href="http://www.google.com/" rel="nofollow">
  <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
</a>

It protects sites from XSS attacks. There are many vectors for an XSS attack and the best way to mitigate the risk is to sanitize user input against a known safe list of HTML elements and attributes.

You should always run bluemonday after any other processing.

If you use blackfriday or Pandoc then bluemonday should be run after these steps. This ensures that no insecure HTML is introduced later in your process.

bluemonday is heavily inspired by both the OWASP Java HTML Sanitizer and the HTML Purifier.

Technical Summary

Allowlist based, you need to either build a policy describing the HTML elements and attributes to permit (and the regexp patterns of attributes), or use one of the supplied policies representing good defaults.

The policy containing the allowlist is applied using a fast non-validating, forward only, token-based parser implemented in the Go net/html library by the core Go team.

We expect to be supplied with well-formatted HTML (closing elements for every applicable open element, nested correctly) and so we do not focus on repairing badly nested or incomplete HTML. We focus on simply ensuring that whatever elements do exist are described in the policy allowlist and that attributes and links are safe for use on your web page. GIGO does apply and if you feed it bad HTML bluemonday is not tasked with figuring out how to make it good again.

Supported Go Versions

bluemonday is tested on all versions since Go 1.2 including tip.

We do not support Go 1.0 as we depend on golang.org/x/net/html which includes a reference to io.ErrNoProgress which did not exist in Go 1.0.

We support Go 1.1 but Travis no longer tests against it.

Is it production ready?

Yes

We are using bluemonday in production having migrated from the widely used and heavily field tested OWASP Java HTML Sanitizer.

We are passing our extensive test suite (including AntiSamy tests as well as tests for any issues raised). Check for any unresolved issues to see whether anything may be a blocker for you.

We invite pull requests and issues to help us ensure we are offering comprehensive protection against various attacks via user generated content.

Usage

Install in your ${GOPATH} using go get -u github.com/microcosm-cc/bluemonday

Then call it:

package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// Do this once for each unique policy, and use the policy for the life of the program
	// Policy creation/editing is not safe to use in multiple goroutines
	p := bluemonday.UGCPolicy()

	// The policy can then be used to sanitize lots of input and it is safe to use the policy in multiple goroutines
	html := p.Sanitize(
		`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`,
	)

	// Output:
	// <a href="http://www.google.com" rel="nofollow">Google</a>
	fmt.Println(html)
}

We offer three ways to call Sanitize:

p.Sanitize(string) string
p.SanitizeBytes([]byte) []byte
p.SanitizeReader(io.Reader) bytes.Buffer

If you are obsessed about performance, p.SanitizeReader(r).Bytes() will return a []byte without performing any unnecessary casting of the inputs or outputs. Though the difference is so negligible you should never need to care.

You can build your own policies:

package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	p := bluemonday.NewPolicy()

	// Require URLs to be parseable by net/url.Parse and either:
	//   mailto: http:// or https://
	p.AllowStandardURLs()

	// We only allow <p> and <a href="">
	p.AllowAttrs("href").OnElements("a")
	p.AllowElements("p")

	html := p.Sanitize(
		`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`,
	)

	// Output:
	// <a href="http://www.google.com">Google</a>
	fmt.Println(html)
}

We ship two default policies:

  1. bluemonday.StrictPolicy() which can be thought of as equivalent to stripping all HTML elements and their attributes as it has nothing on its allowlist. An example usage scenario would be blog post titles where HTML tags are not expected at all and if they are then the elements and the content of the elements should be stripped. This is a very strict policy.
  2. bluemonday.UGCPolicy() which allows a broad selection of HTML elements and attributes that are safe for user generated content. Note that this policy does not allow iframes, object, embed, styles, script, etc. An example usage scenario would be blog post bodies where a variety of formatting is expected along with the potential for TABLEs and IMGs.

Policy Building

The essence of building a policy is to determine which HTML elements and attributes are considered safe for your scenario. OWASP provide an XSS prevention cheat sheet to help explain the risks, but essentially:

  1. Avoid anything other than the standard HTML elements
  2. Avoid script, style, iframe, object, embed, base elements that allow code to be executed by the client or third party content to be included that can execute code
  3. Avoid anything other than plain HTML attributes with values matched to a regexp

Basically, you should be able to describe what HTML is fine for your scenario. If you do not have confidence that you can describe your policy please consider using one of the shipped policies such as bluemonday.UGCPolicy().

To create a new policy:

p := bluemonday.NewPolicy()

To add elements to a policy either add just the elements:

p.AllowElements("b", "strong")

Or using a regex:

Note: if an element is added by name as shown above, any matching regex will be ignored

It is also recommended to ensure multiple patterns don't overlap as order of execution is not guaranteed and can result in some rules being missed.

p.AllowElementsMatching(regex.MustCompile(`^my-element-`))

Or add elements as a virtue of adding an attribute:

// Note the recommended pattern, see the recommendation on using .Matching() below
p.AllowAttrs("nowrap").OnElements("td", "th")

Again, this also supports a regex pattern match alternative:

p.AllowAttrs("nowrap").OnElementsMatching(regex.MustCompile(`^my-element-`))

Attributes can either be added to all elements:

p.AllowAttrs("dir").Matching(regexp.MustCompile("(?i)rtl|ltr")).Globally()

Or attributes can be added to specific elements:

// Not the recommended pattern, see the recommendation on using .Matching() below
p.AllowAttrs("value").OnElements("li")

It is always recommended that an attribute be made to match a pattern. XSS in HTML attributes is very easy otherwise:

// \p{L} matches unicode letters, \p{N} matches unicode numbers
p.AllowAttrs("title").Matching(regexp.MustCompile(`[\p{L}\p{N}\s\-_',:\[\]!\./\\\(\)&]*`)).Globally()

You can stop at any time and call .Sanitize():

// string htmlIn passed in from a HTTP POST
htmlOut := p.Sanitize(htmlIn)

And you can take any existing policy and extend it:

p := bluemonday.UGCPolicy()
p.AllowElements("fieldset", "select", "option")
Inline CSS

Although it's possible to handle inline CSS using AllowAttrs with a Matching rule, writing a single monolithic regular expression to safely process all inline CSS which you wish to allow is not a trivial task. Instead of attempting to do so, you can allow the style attribute on whichever element(s) you desire and use style policies to control and sanitize inline styles.

It is strongly recommended that you use Matching (with a suitable regular expression) MatchingEnum, or MatchingHandler to ensure each style matches your needs, but default handlers are supplied for most widely used styles.

Similar to attributes, you can allow specific CSS properties to be set inline:

p.AllowAttrs("style").OnElements("span", "p")
// Allow the 'color' property with valid RGB(A) hex values only (on any element allowed a 'style' attribute)
p.AllowStyles("color").Matching(regexp.MustCompile("(?i)^#([0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$")).Globally()

Additionally, you can allow a CSS property to be set only to an allowed value:

p.AllowAttrs("style").OnElements("span", "p")
// Allow the 'text-decoration' property to be set to 'underline', 'line-through' or 'none'
// on 'span' elements only
p.AllowStyles("text-decoration").MatchingEnum("underline", "line-through", "none").OnElements("span")

Or you can specify elements based on a regex pattern match:

p.AllowAttrs("style").OnElementsMatching(regex.MustCompile(`^my-element-`))
// Allow the 'text-decoration' property to be set to 'underline', 'line-through' or 'none'
// on 'span' elements only
p.AllowStyles("text-decoration").MatchingEnum("underline", "line-through", "none").OnElementsMatching(regex.MustCompile(`^my-element-`))

If you need more specific checking, you can create a handler that takes in a string and returns a bool to validate the values for a given property. The string parameter has been converted to lowercase and unicode code points have been converted.

myHandler := func(value string) bool{
	// Validate your input here
	return true
}
p.AllowAttrs("style").OnElements("span", "p")
// Allow the 'color' property with values validated by the handler (on any element allowed a 'style' attribute)
p.AllowStyles("color").MatchingHandler(myHandler).Globally()

Links are difficult beasts to sanitise safely and also one of the biggest attack vectors for malicious content.

It is possible to do this:

p.AllowAttrs("href").Matching(regexp.MustCompile(`(?i)mailto|https?`)).OnElements("a")

But that will not protect you as the regular expression is insufficient in this case to have prevented a malformed value doing something unexpected.

We provide some additional global options for safely working with links.

RequireParseableURLs will ensure that URLs are parseable by Go's net/url package:

p.RequireParseableURLs(true)

If you have enabled parseable URLs then the following option will AllowRelativeURLs. By default this is disabled (bluemonday is an allowlist tool... you need to explicitly tell us to permit things) and when disabled it will prevent all local and scheme relative URLs (i.e. href="localpage.html", href="../home.html" and even href="//www.google.com" are relative):

p.AllowRelativeURLs(true)

If you have enabled parseable URLs then you can allow the schemes (commonly called protocol when thinking of http and https) that are permitted. Bear in mind that allowing relative URLs in the above option will allow for a blank scheme:

p.AllowURLSchemes("mailto", "http", "https")

Regardless of whether you have enabled parseable URLs, you can force all URLs to have a rel="nofollow" attribute. This will be added if it does not exist, but only when the href is valid:

// This applies to "a" "area" "link" elements that have a "href" attribute
p.RequireNoFollowOnLinks(true)

Similarly, you can force all URLs to have "noreferrer" in their rel attribute.

// This applies to "a" "area" "link" elements that have a "href" attribute
p.RequireNoReferrerOnLinks(true)

We provide a convenience method that applies all of the above, but you will still need to allow the linkable elements for the URL rules to be applied to:

p.AllowStandardURLs()
p.AllowAttrs("cite").OnElements("blockquote", "q")
p.AllowAttrs("href").OnElements("a", "area")
p.AllowAttrs("src").OnElements("img")

An additional complexity regarding links is the data URI as defined in RFC2397. The data URI allows for images to be served inline using this format:

<img src="data:image/webp;base64,UklGRh4AAABXRUJQVlA4TBEAAAAvAAAAAAfQ//73v/+BiOh/AAA=">

We have provided a helper to verify the mimetype followed by base64 content of data URIs links:

p.AllowDataURIImages()

That helper will enable GIF, JPEG, PNG and WEBP images.

It should be noted that there is a potential security risk with the use of data URI links. You should only enable data URI links if you already trust the content.

We also have some features to help deal with user generated content:

p.AddTargetBlankToFullyQualifiedLinks(true)

This will ensure that anchor <a href="" /> links that are fully qualified (the href destination includes a host name) will get target="_blank" added to them.

Additionally any link that has target="_blank" after the policy has been applied will also have the rel attribute adjusted to add noopener. This means a link may start like <a href="//host/path"/> and will end up as <a href="//host/path" rel="noopener" target="_blank">. It is important to note that the addition of noopener is a security feature and not an issue. There is an unfortunate feature to browsers that a browser window opened as a result of target="_blank" can still control the opener (your web page) and this protects against that. The background to this can be found here: https://dev.to/ben/the-targetblank-vulnerability-by-example

Policy Building Helpers

We also bundle some helpers to simplify policy building:


// Permits the "dir", "id", "lang", "title" attributes globally
p.AllowStandardAttributes()

// Permits the "img" element and its standard attributes
p.AllowImages()

// Permits ordered and unordered lists, and also definition lists
p.AllowLists()

// Permits HTML tables and all applicable elements and non-styling attributes
p.AllowTables()
Invalid Instructions

The following are invalid:

// This does not say where the attributes are allowed, you need to add
// .Globally() or .OnElements(...)
// This will be ignored without error.
p.AllowAttrs("value")

// This does not say where the attributes are allowed, you need to add
// .Globally() or .OnElements(...)
// This will be ignored without error.
p.AllowAttrs(
	"type",
).Matching(
	regexp.MustCompile("(?i)^(circle|disc|square|a|A|i|I|1)$"),
)

Both examples exhibit the same issue, they declare attributes but do not then specify whether they are allowed globally or only on specific elements (and which elements). Attributes belong to one or more elements, and the policy needs to declare this.

Limitations

We are not yet including any tools to help allow and sanitize CSS. Which means that unless you wish to do the heavy lifting in a single regular expression (inadvisable), you should not allow the "style" attribute anywhere.

In the same theme, both <script> and <style> are considered harmful. These elements (and their content) will not be rendered by default, and require you to explicitly set p.AllowUnsafe(true). You should be aware that allowing these elements defeats the purpose of using a HTML sanitizer as you would be explicitly allowing either JavaScript (and any plainly written XSS) and CSS (which can modify a DOM to insert JS), and additionally but limitations in this library mean it is not aware of whether HTML is validly structured and that can allow these elements to bypass some of the safety mechanisms built into the WhatWG HTML parser standard.

It is not the job of bluemonday to fix your bad HTML, it is merely the job of bluemonday to prevent malicious HTML getting through. If you have mismatched HTML elements, or non-conforming nesting of elements, those will remain. But if you have well-structured HTML bluemonday will not break it.

TODO

  • Investigate whether devs want to blacklist elements and attributes. This would allow devs to take an existing policy (such as the bluemonday.UGCPolicy() ) that encapsulates 90% of what they're looking for but does more than they need, and to remove the extra things they do not want to make it 100% what they want
  • Investigate whether devs want a validating HTML mode, in which the HTML elements are not just transformed into a balanced tree (every start tag has a closing tag at the correct depth) but also that elements and character data appear only in their allowed context (i.e. that a table element isn't a descendent of a caption, that colgroup, thead, tbody, tfoot and tr are permitted, and that character data is not permitted)

Development

If you have cloned this repo you will probably need the dependency:

go get golang.org/x/net/html

Gophers can use their familiar tools:

go build

go test

I personally use a Makefile as it spares typing the same args over and over whilst providing consistency for those of us who jump from language to language and enjoy just typing make in a project directory and watch magic happen.

make will build, vet, test and install the library.

make clean will remove the library from a single ${GOPATH}/pkg directory tree

make test will run the tests

make cover will run the tests and open a browser window with the coverage report

make lint will run golint (install via go get github.com/golang/lint/golint)

Long term goals

  1. Open the code to adversarial peer review similar to the Attack Review Ground Rules
  2. Raise funds and pay for an external security review

Documentation

Overview

Package bluemonday provides a way of describing an allowlist of HTML elements and attributes as a policy, and for that policy to be applied to untrusted strings from users that may contain markup. All elements and attributes not on the allowlist will be stripped.

The default bluemonday.UGCPolicy().Sanitize() turns this:

Hello <STYLE>.XSS{background-image:url("javascript:alert('XSS')");}</STYLE><A CLASS=XSS></A>World

Into the more harmless:

Hello World

And it turns this:

<a href="javascript:alert('XSS1')" onmouseover="alert('XSS2')">XSS<a>

Into this:

XSS

Whilst still allowing this:

<a href="http://www.google.com/">
  <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
</a>

To pass through mostly unaltered (it gained a rel="nofollow"):

<a href="http://www.google.com/" rel="nofollow">
  <img src="https://ssl.gstatic.com/accounts/ui/logo_2x.png"/>
</a>

The primary purpose of bluemonday is to take potentially unsafe user generated content (from things like Markdown, HTML WYSIWYG tools, etc) and make it safe for you to put on your website.

It protects sites against XSS (http://en.wikipedia.org/wiki/Cross-site_scripting) and other malicious content that a user interface may deliver. There are many vectors for an XSS attack (https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet) and the safest thing to do is to sanitize user input against a known safe list of HTML elements and attributes.

Note: You should always run bluemonday after any other processing.

If you use blackfriday (https://github.com/russross/blackfriday) or Pandoc (http://johnmacfarlane.net/pandoc/) then bluemonday should be run after these steps. This ensures that no insecure HTML is introduced later in your process.

bluemonday is heavily inspired by both the OWASP Java HTML Sanitizer (https://code.google.com/p/owasp-java-html-sanitizer/) and the HTML Purifier (http://htmlpurifier.org/).

We ship two default policies, one is bluemonday.StrictPolicy() and can be thought of as equivalent to stripping all HTML elements and their attributes as it has nothing on its allowlist.

The other is bluemonday.UGCPolicy() and allows a broad selection of HTML elements and attributes that are safe for user generated content. Note that this policy does not allow iframes, object, embed, styles, script, etc.

The essence of building a policy is to determine which HTML elements and attributes are considered safe for your scenario. OWASP provide an XSS prevention cheat sheet ( https://www.google.com/search?q=xss+prevention+cheat+sheet ) to help explain the risks, but essentially:

1. Avoid allowing anything other than plain HTML elements
2. Avoid allowing `script`, `style`, `iframe`, `object`, `embed`, `base`
   elements
3. Avoid allowing anything other than plain HTML elements with simple
   values that you can match to a regexp
Example
package main

import (
	"fmt"
	"regexp"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// Create a new policy
	p := bluemonday.NewPolicy()

	// Add elements to a policy without attributes
	p.AllowElements("b", "strong")

	// Add elements as a virtue of adding an attribute
	p.AllowAttrs("nowrap").OnElements("td", "th")

	// Attributes can either be added to all elements
	p.AllowAttrs("dir").Globally()

	//Or attributes can be added to specific elements
	p.AllowAttrs("value").OnElements("li")

	// It is ALWAYS recommended that an attribute be made to match a pattern
	// XSS in HTML attributes is a very easy attack vector

	// \p{L} matches unicode letters, \p{N} matches unicode numbers
	p.AllowAttrs("title").Matching(regexp.MustCompile(`[\p{L}\p{N}\s\-_',:\[\]!\./\\\(\)&]*`)).Globally()

	// You can stop at any time and call .Sanitize()

	// Assumes that string htmlIn was passed in from a HTTP POST and contains
	// untrusted user generated content
	htmlIn := `untrusted user generated content <body onload="alert('XSS')">`
	fmt.Println(p.Sanitize(htmlIn))

	// And you can take any existing policy and extend it
	p = bluemonday.UGCPolicy()
	p.AllowElements("fieldset", "select", "option")

	// Links are complex beasts and one of the biggest attack vectors for
	// malicious content so we have included features specifically to help here.

	// This is not recommended:
	p = bluemonday.NewPolicy()
	p.AllowAttrs("href").Matching(regexp.MustCompile(`(?i)mailto|https?`)).OnElements("a")

	// The regexp is insufficient in this case to have prevented a malformed
	// value doing something unexpected.

	// This will ensure that URLs are not considered invalid by Go's net/url
	// package.
	p.RequireParseableURLs(true)

	// If you have enabled parseable URLs then the following option will allow
	// relative URLs. By default this is disabled and will prevent all local and
	// schema relative URLs (i.e. `href="//www.google.com"` is schema relative).
	p.AllowRelativeURLs(true)

	// If you have enabled parseable URLs then you can allow the schemas
	// that are permitted. Bear in mind that allowing relative URLs in the above
	// option allows for blank schemas.
	p.AllowURLSchemes("mailto", "http", "https")

	// Regardless of whether you have enabled parseable URLs, you can force all
	// URLs to have a rel="nofollow" attribute. This will be added if it does
	// not exist.

	// This applies to "a" "area" "link" elements that have a "href" attribute
	p.RequireNoFollowOnLinks(true)

	// We provide a convenience function that applies all of the above, but you
	// will still need to allow the linkable elements:
	p = bluemonday.NewPolicy()
	p.AllowStandardURLs()
	p.AllowAttrs("cite").OnElements("blockquote")
	p.AllowAttrs("href").OnElements("a", "area")
	p.AllowAttrs("src").OnElements("img")

	// Policy Building Helpers

	// If you've got this far and you're bored already, we also bundle some
	// other convenience functions
	p = bluemonday.NewPolicy()
	p.AllowStandardAttributes()
	p.AllowImages()
	p.AllowLists()
	p.AllowTables()
}
Output:

Index

Examples

Constants

This section is empty.

Variables

View Source
var (
	// CellAlign handles the `align` attribute
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/td#attr-align
	CellAlign = regexp.MustCompile(`(?i)^(center|justify|left|right|char)$`)

	// CellVerticalAlign handles the `valign` attribute
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/td#attr-valign
	CellVerticalAlign = regexp.MustCompile(`(?i)^(baseline|bottom|middle|top)$`)

	// Direction handles the `dir` attribute
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/bdo#attr-dir
	Direction = regexp.MustCompile(`(?i)^(rtl|ltr)$`)

	// ImageAlign handles the `align` attribute on the `image` tag
	// http://www.w3.org/MarkUp/Test/Img/imgtest.html
	ImageAlign = regexp.MustCompile(
		`(?i)^(left|right|top|texttop|middle|absmiddle|baseline|bottom|absbottom)$`,
	)

	// Integer describes whole positive integers (including 0) used in places
	// like td.colspan
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/td#attr-colspan
	Integer = regexp.MustCompile(`^[0-9]+$`)

	// ISO8601 according to the W3 group is only a subset of the ISO8601
	// standard: http://www.w3.org/TR/NOTE-datetime
	//
	// Used in places like time.datetime
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/time#attr-datetime
	//
	// Matches patterns:
	//  Year:
	//     YYYY (eg 1997)
	//  Year and month:
	//     YYYY-MM (eg 1997-07)
	//  Complete date:
	//     YYYY-MM-DD (eg 1997-07-16)
	//  Complete date plus hours and minutes:
	//     YYYY-MM-DDThh:mmTZD (eg 1997-07-16T19:20+01:00)
	//  Complete date plus hours, minutes and seconds:
	//     YYYY-MM-DDThh:mm:ssTZD (eg 1997-07-16T19:20:30+01:00)
	//  Complete date plus hours, minutes, seconds and a decimal fraction of a
	//  second
	//      YYYY-MM-DDThh:mm:ss.sTZD (eg 1997-07-16T19:20:30.45+01:00)
	ISO8601 = regexp.MustCompile(
		`^[0-9]{4}(-[0-9]{2}(-[0-9]{2}([ T][0-9]{2}(:[0-9]{2}){1,2}(.[0-9]{1,6})` +
			`?Z?([\+-][0-9]{2}:[0-9]{2})?)?)?)?$`,
	)

	// ListType encapsulates the common value as well as the latest spec
	// values for lists
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/ol#attr-type
	ListType = regexp.MustCompile(`(?i)^(circle|disc|square|a|A|i|I|1)$`)

	// SpaceSeparatedTokens is used in places like `a.rel` and the common attribute
	// `class` which both contain space delimited lists of data tokens
	// http://www.w3.org/TR/html-markup/datatypes.html#common.data.tokens-def
	// Regexp: \p{L} matches unicode letters, \p{N} matches unicode numbers
	SpaceSeparatedTokens = regexp.MustCompile(`^([\s\p{L}\p{N}_-]+)$`)

	// Number is a double value used on HTML5 meter and progress elements
	// http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#the-meter-element
	Number = regexp.MustCompile(`^[-+]?[0-9]*\.?[0-9]+([eE][-+]?[0-9]+)?$`)

	// NumberOrPercent is used predominantly as units of measurement in width
	// and height attributes
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#attr-height
	NumberOrPercent = regexp.MustCompile(`^[0-9]+[%]?$`)

	// Paragraph of text in an attribute such as *.'title', img.alt, etc
	// https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes#attr-title
	// Note that we are not allowing chars that could close tags like '>'
	Paragraph = regexp.MustCompile(`^[\p{L}\p{N}\s\-_',\[\]!\./\\\(\)]*$`)
)

A selection of regular expressions that can be used as .Matching() rules on HTML attributes.

Functions

This section is empty.

Types

type Policy

type Policy struct {
	// contains filtered or unexported fields
}

Policy encapsulates the allowlist of HTML elements and attributes that will be applied to the sanitised HTML.

You should use bluemonday.NewPolicy() to create a blank policy as the unexported fields contain maps that need to be initialized.

func NewPolicy

func NewPolicy() *Policy

NewPolicy returns a blank policy with nothing allowed or permitted. This is the recommended way to start building a policy and you should now use AllowAttrs() and/or AllowElements() to construct the allowlist of HTML elements and attributes.

Example
package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// NewPolicy is a blank policy and we need to explicitly allow anything
	// that we wish to allow through
	p := bluemonday.NewPolicy()

	// We ensure any URLs are parseable and have rel="nofollow" where applicable
	p.AllowStandardURLs()

	// AllowStandardURLs already ensures that the href will be valid, and so we
	// can skip the .Matching()
	p.AllowAttrs("href").OnElements("a")

	// We allow paragraphs too
	p.AllowElements("p")

	html := p.Sanitize(
		`<p><a onblur="alert(secret)" href="http://www.google.com">Google</a></p>`,
	)

	fmt.Println(html)

}
Output:

<p><a href="http://www.google.com" rel="nofollow">Google</a></p>

func StrictPolicy

func StrictPolicy() *Policy

StrictPolicy returns an empty policy, which will effectively strip all HTML elements and their attributes from a document.

Example
package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// StrictPolicy is equivalent to NewPolicy and as nothing else is declared
	// we are stripping all elements (and their attributes)
	p := bluemonday.StrictPolicy()

	html := p.Sanitize(
		`Goodbye <a onblur="alert(secret)" href="http://en.wikipedia.org/wiki/Goodbye_Cruel_World_(Pink_Floyd_song)">Cruel</a> World`,
	)

	fmt.Println(html)

}
Output:

Goodbye Cruel World

func StripTagsPolicy

func StripTagsPolicy() *Policy

StripTagsPolicy is DEPRECATED. Use StrictPolicy instead.

func UGCPolicy

func UGCPolicy() *Policy

UGCPolicy returns a policy aimed at user generated content that is a result of HTML WYSIWYG tools and Markdown conversions.

This is expected to be a fairly rich document where as much markup as possible should be retained. Markdown permits raw HTML so we are basically providing a policy to sanitise HTML5 documents safely but with the least intrusion on the formatting expectations of the user.

Example
package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// UGCPolicy is a convenience policy for user generated content.
	p := bluemonday.UGCPolicy()

	html := p.Sanitize(
		`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`,
	)

	fmt.Println(html)

}
Output:

<a href="http://www.google.com" rel="nofollow">Google</a>

func (*Policy) AddSpaceWhenStrippingTag

func (p *Policy) AddSpaceWhenStrippingTag(allow bool) *Policy

AddSpaceWhenStrippingTag states whether to add a single space " " when removing tags that are not allowed by the policy.

This is useful if you expect to strip tags in dense markup and may lose the value of whitespace.

For example: "<p>Hello</p><p>World</p>"" would be sanitized to "HelloWorld" with the default value of false, but you may wish to sanitize this to " Hello World " by setting AddSpaceWhenStrippingTag to true as this would retain the intent of the text.

func (p *Policy) AddTargetBlankToFullyQualifiedLinks(require bool) *Policy

AddTargetBlankToFullyQualifiedLinks will result in all a, area and link tags that point to a non-local destination (i.e. starts with a protocol and has a host) having a target="_blank" added to them if one does not already exist

Note: This requires p.RequireParseableURLs(true) and will enable it.

func (*Policy) AllowAttrs

func (p *Policy) AllowAttrs(attrNames ...string) *attrPolicyBuilder

AllowAttrs takes a range of HTML attribute names and returns an attribute policy builder that allows you to specify the pattern and scope of the allowed attribute.

The attribute policy is only added to the core policy when either Globally() or OnElements(...) are called.

Example
package main

import (
	"github.com/microcosm-cc/bluemonday"
)

func main() {
	p := bluemonday.NewPolicy()

	// Allow the 'title' attribute on every HTML element that has been
	// allowed
	p.AllowAttrs("title").Matching(bluemonday.Paragraph).Globally()

	// Allow the 'abbr' attribute on only the 'td' and 'th' elements.
	p.AllowAttrs("abbr").Matching(bluemonday.Paragraph).OnElements("td", "th")

	// Allow the 'colspan' and 'rowspan' attributes, matching a positive integer
	// pattern, on only the 'td' and 'th' elements.
	p.AllowAttrs("colspan", "rowspan").Matching(
		bluemonday.Integer,
	).OnElements("td", "th")
}
Output:

func (*Policy) AllowComments added in v1.0.10

func (p *Policy) AllowComments()

AllowComments allows comments.

Please note that only one type of comment will be allowed by this, this is the the standard HTML comment <!-- --> which includes the use of that to permit conditionals as per https://docs.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/compatibility/ms537512(v=vs.85)?redirectedfrom=MSDN

What is not permitted are CDATA XML comments, as the x/net/html package we depend on does not handle this fully and we are not choosing to take on that work: https://pkg.go.dev/golang.org/x/net/html#Tokenizer.AllowCDATA . If the x/net/html package changes this then these will be considered, otherwise if you AllowComments but provide a CDATA comment, then as per the documentation in x/net/html this will be treated as a plain HTML comment.

func (*Policy) AllowDataAttributes

func (p *Policy) AllowDataAttributes()

AllowDataAttributes permits all data attributes. We can't specify the name of each attribute exactly as they are customized.

NOTE: These values are not sanitized and applications that evaluate or process them without checking and verification of the input may be at risk if this option is enabled. This is a 'caveat emptor' option and the person enabling this option needs to fully understand the potential impact with regards to whatever application will be consuming the sanitized HTML afterwards, i.e. if you know you put a link in a data attribute and use that to automatically load some new window then you're giving the author of a HTML fragment the means to open a malicious destination automatically. Use with care!

func (*Policy) AllowDataURIImages

func (p *Policy) AllowDataURIImages()

AllowDataURIImages permits the use of inline images defined in RFC2397 http://tools.ietf.org/html/rfc2397 http://en.wikipedia.org/wiki/Data_URI_scheme

Images must have a mimetype matching:

image/gif
image/jpeg
image/png
image/webp

NOTE: There is a potential security risk to allowing data URIs and you should only permit them on content you already trust. http://palizine.plynt.com/issues/2010Oct/bypass-xss-filters/ https://capec.mitre.org/data/definitions/244.html

func (*Policy) AllowElements

func (p *Policy) AllowElements(names ...string) *Policy

AllowElements will append HTML elements to the allowlist without applying an attribute policy to those elements (the elements are permitted sans-attributes)

Example
package main

import (
	"github.com/microcosm-cc/bluemonday"
)

func main() {
	p := bluemonday.NewPolicy()

	// Allow styling elements without attributes
	p.AllowElements("br", "div", "hr", "p", "span")
}
Output:

func (*Policy) AllowElementsContent

func (p *Policy) AllowElementsContent(names ...string) *Policy

AllowElementsContent marks the HTML elements whose content should be retained after removing the tag.

func (*Policy) AllowElementsMatching added in v1.0.3

func (p *Policy) AllowElementsMatching(regex *regexp.Regexp) *Policy

AllowElementsMatching will append HTML elements to the allowlist if they match a regexp.

func (*Policy) AllowIFrames added in v1.0.17

func (p *Policy) AllowIFrames(vals ...SandboxValue)

func (*Policy) AllowImages

func (p *Policy) AllowImages()

AllowImages enables the img element and some popular attributes. It will also ensure that URL values are parseable. This helper does not enable data URI images, for that you should also use the AllowDataURIImages() helper.

func (*Policy) AllowLists

func (p *Policy) AllowLists()

AllowLists will enabled ordered and unordered lists, as well as definition lists

func (*Policy) AllowNoAttrs

func (p *Policy) AllowNoAttrs() *attrPolicyBuilder

AllowNoAttrs says that attributes on element are optional.

The attribute policy is only added to the core policy when OnElements(...) are called.

func (*Policy) AllowRelativeURLs

func (p *Policy) AllowRelativeURLs(require bool) *Policy

AllowRelativeURLs enables RequireParseableURLs and then permits URLs that are parseable, have no schema information and url.IsAbs() returns false This permits local URLs

func (*Policy) AllowStandardAttributes

func (p *Policy) AllowStandardAttributes()

AllowStandardAttributes will enable "id", "title" and the language specific attributes "dir" and "lang" on all elements that are allowed

func (*Policy) AllowStandardURLs

func (p *Policy) AllowStandardURLs()

AllowStandardURLs is a convenience function that will enable rel="nofollow" on "a", "area" and "link" (if you have allowed those elements) and will ensure that the URL values are parseable and either relative or belong to the "mailto", "http", or "https" schemes

func (*Policy) AllowStyles added in v1.0.3

func (p *Policy) AllowStyles(propertyNames ...string) *stylePolicyBuilder

AllowStyles takes a range of CSS property names and returns a style policy builder that allows you to specify the pattern and scope of the allowed property.

The style policy is only added to the core policy when either Globally() or OnElements(...) are called.

Example
package main

import (
	"fmt"
	"regexp"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	p := bluemonday.NewPolicy()

	// Allow only 'span' and 'p' elements
	p.AllowElements("span", "p", "strong")

	// Only allow 'style' attributes on 'span' and 'p' elements
	p.AllowAttrs("style").OnElements("span", "p")

	// Allow the 'text-decoration' property to be set to 'underline', 'line-through' or 'none'
	// on 'span' elements only
	p.AllowStyles("text-decoration").MatchingEnum("underline", "line-through", "none").OnElements("span")

	// Allow the 'color' property with valid RGB(A) hex values only
	// on every HTML element that has been allowed
	p.AllowStyles("color").Matching(regexp.MustCompile("(?i)^#([0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$")).Globally()

	// Default handler
	p.AllowStyles("background-origin").Globally()

	// The span has an invalid 'color' which will be stripped along with other disallowed properties
	html := p.Sanitize(
		`<p style="color:#f00;">
	<span style="text-decoration: underline; background-image: url(javascript:alert('XSS')); color: #f00ba; background-origin: invalidValue">
		Red underlined <strong style="text-decoration:none;">text</strong>
	</span>
</p>`,
	)

	fmt.Println(html)

}
Output:

<p style="color: #f00">
	<span style="text-decoration: underline">
		Red underlined <strong>text</strong>
	</span>
</p>

func (*Policy) AllowStyling

func (p *Policy) AllowStyling()

AllowStyling presently enables the class attribute globally.

Note: When bluemonday ships a CSS parser and we can safely sanitise that, this will also allow sanitized styling of elements via the style attribute.

func (*Policy) AllowTables

func (p *Policy) AllowTables()

AllowTables will enable a rich set of elements and attributes to describe HTML tables

func (*Policy) AllowURLSchemeWithCustomPolicy

func (p *Policy) AllowURLSchemeWithCustomPolicy(
	scheme string,
	urlPolicy func(url *url.URL) (allowUrl bool),
) *Policy

AllowURLSchemeWithCustomPolicy will append URL schemes with a custom URL policy to the allowlist. Only the URLs with matching schema and urlPolicy(url) returning true will be allowed.

func (*Policy) AllowURLSchemes

func (p *Policy) AllowURLSchemes(schemes ...string) *Policy

AllowURLSchemes will append URL schemes to the allowlist Example: p.AllowURLSchemes("mailto", "http", "https")

func (*Policy) AllowUnsafe added in v1.0.16

func (p *Policy) AllowUnsafe(allowUnsafe bool) *Policy

AllowUnsafe permits fundamentally unsafe elements.

If false (default) then elements such as `style` and `script` will not be permitted even if declared in a policy. These elements when combined with untrusted input cannot be safely handled by bluemonday at this point in time.

If true then `style` and `script` would be permitted by bluemonday if a policy declares them. However this is not recommended under any circumstance and can lead to XSS being rendered thus defeating the purpose of using a HTML sanitizer.

func (*Policy) RequireCrossOriginAnonymous added in v1.0.6

func (p *Policy) RequireCrossOriginAnonymous(require bool) *Policy

RequireCrossOriginAnonymous will result in all audio, img, link, script, and video tags having a crossorigin="anonymous" added to them if one does not already exist

func (p *Policy) RequireNoFollowOnFullyQualifiedLinks(require bool) *Policy

RequireNoFollowOnFullyQualifiedLinks will result in all a, area, and link tags that point to a non-local destination (i.e. starts with a protocol and has a host) having a rel="nofollow" added to them if one does not already exist

Note: This requires p.RequireParseableURLs(true) and will enable it.

func (p *Policy) RequireNoFollowOnLinks(require bool) *Policy

RequireNoFollowOnLinks will result in all a, area, link tags having a rel="nofollow"added to them if one does not already exist

Note: This requires p.RequireParseableURLs(true) and will enable it.

func (p *Policy) RequireNoReferrerOnFullyQualifiedLinks(require bool) *Policy

RequireNoReferrerOnFullyQualifiedLinks will result in all a, area, and link tags that point to a non-local destination (i.e. starts with a protocol and has a host) having a rel="noreferrer" added to them if one does not already exist

Note: This requires p.RequireParseableURLs(true) and will enable it.

func (p *Policy) RequireNoReferrerOnLinks(require bool) *Policy

RequireNoReferrerOnLinks will result in all a, area, and link tags having a rel="noreferrrer" added to them if one does not already exist

Note: This requires p.RequireParseableURLs(true) and will enable it.

func (*Policy) RequireParseableURLs

func (p *Policy) RequireParseableURLs(require bool) *Policy

RequireParseableURLs will result in all URLs requiring that they be parseable by "net/url" url.Parse() This applies to: - a.href - area.href - blockquote.cite - img.src - link.href - script.src

func (*Policy) RequireSandboxOnIFrame added in v1.0.17

func (p *Policy) RequireSandboxOnIFrame(vals ...SandboxValue)

RequireSandboxOnIFrame will result in all iframe tags having a sandbox="" tag Any sandbox values not specified here will be filtered from the generated HTML

func (*Policy) Sanitize

func (p *Policy) Sanitize(s string) string

Sanitize takes a string that contains a HTML fragment or document and applies the given policy allowlist.

It returns a HTML string that has been sanitized by the policy or an empty string if an error has occurred (most likely as a consequence of extremely malformed input)

Example
package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// UGCPolicy is a convenience policy for user generated content.
	p := bluemonday.UGCPolicy()

	// string in, string out
	html := p.Sanitize(`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`)

	fmt.Println(html)

}
Output:

<a href="http://www.google.com" rel="nofollow">Google</a>

func (*Policy) SanitizeBytes

func (p *Policy) SanitizeBytes(b []byte) []byte

SanitizeBytes takes a []byte that contains a HTML fragment or document and applies the given policy allowlist.

It returns a []byte containing the HTML that has been sanitized by the policy or an empty []byte if an error has occurred (most likely as a consequence of extremely malformed input)

Example
package main

import (
	"fmt"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// UGCPolicy is a convenience policy for user generated content.
	p := bluemonday.UGCPolicy()

	// []byte in, []byte out
	b := []byte(`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`)
	b = p.SanitizeBytes(b)

	fmt.Println(string(b))

}
Output:

<a href="http://www.google.com" rel="nofollow">Google</a>

func (*Policy) SanitizeReader

func (p *Policy) SanitizeReader(r io.Reader) *bytes.Buffer

SanitizeReader takes an io.Reader that contains a HTML fragment or document and applies the given policy allowlist.

It returns a bytes.Buffer containing the HTML that has been sanitized by the policy. Errors during sanitization will merely return an empty result.

Example
package main

import (
	"fmt"
	"strings"

	"github.com/microcosm-cc/bluemonday"
)

func main() {
	// UGCPolicy is a convenience policy for user generated content.
	p := bluemonday.UGCPolicy()

	// io.Reader in, bytes.Buffer out
	r := strings.NewReader(`<a onblur="alert(secret)" href="http://www.google.com">Google</a>`)
	buf := p.SanitizeReader(r)

	fmt.Println(buf.String())

}
Output:

<a href="http://www.google.com" rel="nofollow">Google</a>

func (*Policy) SanitizeReaderToWriter added in v1.0.14

func (p *Policy) SanitizeReaderToWriter(r io.Reader, w io.Writer) error

SanitizeReaderToWriter takes an io.Reader that contains a HTML fragment or document and applies the given policy allowlist and writes to the provided writer returning an error if there is one.

func (*Policy) SkipElementsContent

func (p *Policy) SkipElementsContent(names ...string) *Policy

SkipElementsContent adds the HTML elements whose tags is needed to be removed with its content.

type Query added in v1.0.6

type Query struct {
	Key      string
	Value    string
	HasValue bool
}

Query represents a single part of the query string, a query param

type SandboxValue added in v1.0.17

type SandboxValue int64
const (
	SandboxAllowDownloads SandboxValue = iota
	SandboxAllowDownloadsWithoutUserActivation
	SandboxAllowForms
	SandboxAllowModals
	SandboxAllowOrientationLock
	SandboxAllowPointerLock
	SandboxAllowPopups
	SandboxAllowPopupsToEscapeSandbox
	SandboxAllowPresentation
	SandboxAllowSameOrigin
	SandboxAllowScripts
	SandboxAllowStorageAccessByUserActivation
	SandboxAllowTopNavigation
	SandboxAllowTopNavigationByUserActivation
)

Directories

Path Synopsis
cmd
sanitise_html_email
Package main demonstrates a HTML email cleaner.
Package main demonstrates a HTML email cleaner.
sanitise_ugc
Package main demonstrates a simple user generated content sanitizer.
Package main demonstrates a simple user generated content sanitizer.

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