Source file src/pkg/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, <script>alert('you have been pwned')</script>! 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 <i>you</i>? 90 <a title='{{.}}'> O'Reilly: How are you? 91 <a href="/{{.}}"> O'Reilly: How are %3ci%3eyou%3c/i%3e? 92 <a href="?q={{.}}"> O'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, <b>World<b>! 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