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242 lines
8.1 KiB
Go
242 lines
8.1 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package template (html/template) implements data-driven templates for
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generating HTML output safe against code injection. It provides the
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same interface as [text/template] and should be used instead of
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[text/template] whenever the output is HTML.
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The documentation here focuses on the security features of the package.
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For information about how to program the templates themselves, see the
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documentation for [text/template].
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# Introduction
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This package wraps [text/template] so you can share its template API
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to parse and execute HTML templates safely.
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tmpl, err := template.New("name").Parse(...)
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// Error checking elided
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err = tmpl.Execute(out, data)
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If successful, tmpl will now be injection-safe. Otherwise, err is an error
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defined in the docs for ErrorCode.
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HTML templates treat data values as plain text which should be encoded so they
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can be safely embedded in an HTML document. The escaping is contextual, so
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actions can appear within JavaScript, CSS, and URI contexts.
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The security model used by this package assumes that template authors are
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trusted, while Execute's data parameter is not. More details are
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provided below.
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Example
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import template "github.com/gohugoio/hugo/tpl/internal/go_templates/texttemplate"
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...
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t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
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err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
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produces
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Hello, <script>alert('you have been pwned')</script>!
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but the contextual autoescaping in html/template
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import template "github.com/gohugoio/hugo/tpl/internal/go_templates/htmltemplate"
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...
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t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
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err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
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produces safe, escaped HTML output
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Hello, <script>alert('you have been pwned')</script>!
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# Contexts
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This package understands HTML, CSS, JavaScript, and URIs. It adds sanitizing
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functions to each simple action pipeline, so given the excerpt
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<a href="/search?q={{.}}">{{.}}</a>
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At parse time each {{.}} is overwritten to add escaping functions as necessary.
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In this case it becomes
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<a href="/search?q={{. | urlescaper | attrescaper}}">{{. | htmlescaper}}</a>
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where urlescaper, attrescaper, and htmlescaper are aliases for internal escaping
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functions.
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For these internal escaping functions, if an action pipeline evaluates to
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a nil interface value, it is treated as though it were an empty string.
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# Namespaced and data- attributes
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Attributes with a namespace are treated as if they had no namespace.
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Given the excerpt
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<a my:href="{{.}}"></a>
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At parse time the attribute will be treated as if it were just "href".
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So at parse time the template becomes:
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<a my:href="{{. | urlescaper | attrescaper}}"></a>
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Similarly to attributes with namespaces, attributes with a "data-" prefix are
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treated as if they had no "data-" prefix. So given
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<a data-href="{{.}}"></a>
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At parse time this becomes
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<a data-href="{{. | urlescaper | attrescaper}}"></a>
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If an attribute has both a namespace and a "data-" prefix, only the namespace
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will be removed when determining the context. For example
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<a my:data-href="{{.}}"></a>
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This is handled as if "my:data-href" was just "data-href" and not "href" as
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it would be if the "data-" prefix were to be ignored too. Thus at parse
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time this becomes just
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<a my:data-href="{{. | attrescaper}}"></a>
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As a special case, attributes with the namespace "xmlns" are always treated
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as containing URLs. Given the excerpts
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<a xmlns:title="{{.}}"></a>
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<a xmlns:href="{{.}}"></a>
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<a xmlns:onclick="{{.}}"></a>
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At parse time they become:
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<a xmlns:title="{{. | urlescaper | attrescaper}}"></a>
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<a xmlns:href="{{. | urlescaper | attrescaper}}"></a>
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<a xmlns:onclick="{{. | urlescaper | attrescaper}}"></a>
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# Errors
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See the documentation of ErrorCode for details.
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# A fuller picture
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The rest of this package comment may be skipped on first reading; it includes
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details necessary to understand escaping contexts and error messages. Most users
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will not need to understand these details.
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# Contexts
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Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows
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how {{.}} appears when used in the context to the left.
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Context {{.}} After
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{{.}} O'Reilly: How are <i>you</i>?
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<a title='{{.}}'> O'Reilly: How are you?
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<a href="/{{.}}"> O'Reilly: How are %3ci%3eyou%3c/i%3e?
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<a href="?q={{.}}"> O'Reilly%3a%20How%20are%3ci%3e...%3f
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<a onx='f("{{.}}")'> O\x27Reilly: How are \x3ci\x3eyou...?
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<a onx='f({{.}})'> "O\x27Reilly: How are \x3ci\x3eyou...?"
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<a onx='pattern = /{{.}}/;'> O\x27Reilly: How are \x3ci\x3eyou...\x3f
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If used in an unsafe context, then the value might be filtered out:
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Context {{.}} After
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<a href="{{.}}"> #ZgotmplZ
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since "O'Reilly:" is not an allowed protocol like "http:".
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If {{.}} is the innocuous word, `left`, then it can appear more widely,
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Context {{.}} After
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{{.}} left
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<a title='{{.}}'> left
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<a href='{{.}}'> left
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<a href='/{{.}}'> left
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<a href='?dir={{.}}'> left
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<a style="border-{{.}}: 4px"> left
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<a style="align: {{.}}"> left
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<a style="background: '{{.}}'> left
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<a style="background: url('{{.}}')> left
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<style>p.{{.}} {color:red}</style> left
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Non-string values can be used in JavaScript contexts.
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If {{.}} is
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struct{A,B string}{ "foo", "bar" }
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in the escaped template
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<script>var pair = {{.}};</script>
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then the template output is
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<script>var pair = {"A": "foo", "B": "bar"};</script>
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See package json to understand how non-string content is marshaled for
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embedding in JavaScript contexts.
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# Typed Strings
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By default, this package assumes that all pipelines produce a plain text string.
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It adds escaping pipeline stages necessary to correctly and safely embed that
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plain text string in the appropriate context.
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When a data value is not plain text, you can make sure it is not over-escaped
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by marking it with its type.
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Types HTML, JS, URL, and others from content.go can carry safe content that is
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exempted from escaping.
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The template
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Hello, {{.}}!
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can be invoked with
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tmpl.Execute(out, template.HTML(`<b>World</b>`))
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to produce
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Hello, <b>World</b>!
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instead of the
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Hello, <b>World<b>!
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that would have been produced if {{.}} was a regular string.
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# Security Model
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https://rawgit.com/mikesamuel/sanitized-jquery-templates/trunk/safetemplate.html#problem_definition defines "safe" as used by this package.
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This package assumes that template authors are trusted, that Execute's data
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parameter is not, and seeks to preserve the properties below in the face
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of untrusted data:
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Structure Preservation Property:
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"... when a template author writes an HTML tag in a safe templating language,
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the browser will interpret the corresponding portion of the output as a tag
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regardless of the values of untrusted data, and similarly for other structures
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such as attribute boundaries and JS and CSS string boundaries."
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Code Effect Property:
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"... only code specified by the template author should run as a result of
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injecting the template output into a page and all code specified by the
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template author should run as a result of the same."
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Least Surprise Property:
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"A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who
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knows that contextual autoescaping happens should be able to look at a {{.}}
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and correctly infer what sanitization happens."
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As a consequence of the Least Surprise Property, template actions within an
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ECMAScript 6 template literal are disabled by default.
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Handling string interpolation within these literals is rather complex resulting
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in no clear safe way to support it.
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To re-enable template actions within ECMAScript 6 template literals, use the
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GODEBUG=jstmpllitinterp=1 environment variable.
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*/
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package template
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