libgo/go/html/template/doc.go

   1 // Copyright 2011 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 /*
   6 Package template (html/template) implements data-driven templates for
   7 generating HTML output safe against code injection. It provides the
   8 same interface as package text/template and should be used instead of
   9 text/template whenever the output is HTML.
  10
  11 The documentation here focuses on the security features of the package.
  12 For information about how to program the templates themselves, see the
  13 documentation for text/template.
  14
  15 Introduction
  16
  17 This package wraps package text/template so you can share its template API
  18 to parse and execute HTML templates safely.
  19
  20   tmpl, err := template.New("name").Parse(...)
  21   // Error checking elided
  22   err = tmpl.Execute(out, data)
  23
  24 If successful, tmpl will now be injection-safe. Otherwise, err is an error
  25 defined in the docs for ErrorCode.
  26
  27 HTML templates treat data values as plain text which should be encoded so they
  28 can be safely embedded in an HTML document. The escaping is contextual, so
  29 actions can appear within JavaScript, CSS, and URI contexts.
  30
  31 The security model used by this package assumes that template authors are
  32 trusted, while Execute's data parameter is not. More details are
  33 provided below.
  34
  35 Example
  36
  37   import "text/template"
  38   ...
  39   t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
  40   err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
  41
  42 produces
  43
  44   Hello, <script>alert('you have been pwned')</script>!
  45
  46 but the contextual autoescaping in html/template
  47
  48   import "html/template"
  49   ...
  50   t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
  51   err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
  52
  53 produces safe, escaped HTML output
  54
  55   Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!
  56
  57
  58 Contexts
  59
  60 This package understands HTML, CSS, JavaScript, and URIs. It adds sanitizing
  61 functions to each simple action pipeline, so given the excerpt
  62
  63   <a href="/search?q={{.}}">{{.}}</a>
  64
  65 At parse time each {{.}} is overwritten to add escaping functions as necessary.
  66 In this case it becomes
  67
  68   <a href="/search?q={{. | urlquery}}">{{. | html}}</a>
  69
  70
  71 Errors
  72
  73 See the documentation of ErrorCode for details.
  74
  75
  76 A fuller picture
  77
  78 The rest of this package comment may be skipped on first reading; it includes
  79 details necessary to understand escaping contexts and error messages. Most users
  80 will not need to understand these details.
  81
  82
  83 Contexts
  84
  85 Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows
  86 how {{.}} appears when used in the context to the left.
  87
  88   Context                          {{.}} After
  89   {{.}}                            O'Reilly: How are &lt;i&gt;you&lt;/i&gt;?
  90   <a title='{{.}}'>                O&#39;Reilly: How are you?
  91   <a href="/{{.}}">                O&#39;Reilly: How are %3ci%3eyou%3c/i%3e?
  92   <a href="?q={{.}}">              O&#39;Reilly%3a%20How%20are%3ci%3e...%3f
  93   <a onx='f("{{.}}")'>             O\x27Reilly: How are \x3ci\x3eyou...?
  94   <a onx='f({{.}})'>               "O\x27Reilly: How are \x3ci\x3eyou...?"
  95   <a onx='pattern = /{{.}}/;'>     O\x27Reilly: How are \x3ci\x3eyou...\x3f
  96
  97 If used in an unsafe context, then the value might be filtered out:
  98
  99   Context                          {{.}} After
 100   <a href="{{.}}">                 #ZgotmplZ
 101
 102 since "O'Reilly:" is not an allowed protocol like "http:".
 103
 104
 105 If {{.}} is the innocuous word, `left`, then it can appear more widely,
 106
 107   Context                              {{.}} After
 108   {{.}}                                left
 109   <a title='{{.}}'>                    left
 110   <a href='{{.}}'>                     left
 111   <a href='/{{.}}'>                    left
 112   <a href='?dir={{.}}'>                left
 113   <a style="border-{{.}}: 4px">        left
 114   <a style="align: {{.}}">             left
 115   <a style="background: '{{.}}'>       left
 116   <a style="background: url('{{.}}')>  left
 117   <style>p.{{.}} {color:red}</style>   left
 118
 119 Non-string values can be used in JavaScript contexts.
 120 If {{.}} is
 121
 122   struct{A,B string}{ "foo", "bar" }
 123
 124 in the escaped template
 125
 126   <script>var pair = {{.}};</script>
 127
 128 then the template output is
 129
 130   <script>var pair = {"A": "foo", "B": "bar"};</script>
 131
 132 See package json to understand how non-string content is marshalled for
 133 embedding in JavaScript contexts.
 134
 135
 136 Typed Strings
 137
 138 By default, this package assumes that all pipelines produce a plain text string.
 139 It adds escaping pipeline stages necessary to correctly and safely embed that
 140 plain text string in the appropriate context.
 141
 142 When a data value is not plain text, you can make sure it is not over-escaped
 143 by marking it with its type.
 144
 145 Types HTML, JS, URL, and others from content.go can carry safe content that is
 146 exempted from escaping.
 147
 148 The template
 149
 150   Hello, {{.}}!
 151
 152 can be invoked with
 153
 154   tmpl.Execute(out, HTML(`<b>World</b>`))
 155
 156 to produce
 157
 158   Hello, <b>World</b>!
 159
 160 instead of the
 161
 162   Hello, &lt;b&gt;World&lt;b&gt;!
 163
 164 that would have been produced if {{.}} was a regular string.
 165
 166
 167 Security Model
 168
 169 http://js-quasis-libraries-and-repl.googlecode.com/svn/trunk/safetemplate.html#problem_definition defines "safe" as used by this package.
 170
 171 This package assumes that template authors are trusted, that Execute's data
 172 parameter is not, and seeks to preserve the properties below in the face
 173 of untrusted data:
 174
 175 Structure Preservation Property:
 176 "... when a template author writes an HTML tag in a safe templating language,
 177 the browser will interpret the corresponding portion of the output as a tag
 178 regardless of the values of untrusted data, and similarly for other structures
 179 such as attribute boundaries and JS and CSS string boundaries."
 180
 181 Code Effect Property:
 182 "... only code specified by the template author should run as a result of
 183 injecting the template output into a page and all code specified by the
 184 template author should run as a result of the same."
 185
 186 Least Surprise Property:
 187 "A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who
 188 knows that contextual autoescaping happens should be able to look at a {{.}}
 189 and correctly infer what sanitization happens."
 190 */
 191 package template