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={{. | urlescaper | attrescaper}}">{{. | htmlescaper}}</a> 69 70 where urlescaper, attrescaper, and htmlescaper are aliases for internal escaping 71 functions. 72 73 Errors 74 75 See the documentation of ErrorCode for details. 76 77 78 A fuller picture 79 80 The rest of this package comment may be skipped on first reading; it includes 81 details necessary to understand escaping contexts and error messages. Most users 82 will not need to understand these details. 83 84 85 Contexts 86 87 Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows 88 how {{.}} appears when used in the context to the left. 89 90 Context {{.}} After 91 {{.}} O'Reilly: How are <i>you</i>? 92 <a title='{{.}}'> O'Reilly: How are you? 93 <a href="/{{.}}"> O'Reilly: How are %3ci%3eyou%3c/i%3e? 94 <a href="?q={{.}}"> O'Reilly%3a%20How%20are%3ci%3e...%3f 95 <a onx='f("{{.}}")'> O\x27Reilly: How are \x3ci\x3eyou...? 96 <a onx='f({{.}})'> "O\x27Reilly: How are \x3ci\x3eyou...?" 97 <a onx='pattern = /{{.}}/;'> O\x27Reilly: How are \x3ci\x3eyou...\x3f 98 99 If used in an unsafe context, then the value might be filtered out: 100 101 Context {{.}} After 102 <a href="{{.}}"> #ZgotmplZ 103 104 since "O'Reilly:" is not an allowed protocol like "http:". 105 106 107 If {{.}} is the innocuous word, `left`, then it can appear more widely, 108 109 Context {{.}} After 110 {{.}} left 111 <a title='{{.}}'> left 112 <a href='{{.}}'> left 113 <a href='/{{.}}'> left 114 <a href='?dir={{.}}'> left 115 <a style="border-{{.}}: 4px"> left 116 <a style="align: {{.}}"> left 117 <a style="background: '{{.}}'> left 118 <a style="background: url('{{.}}')> left 119 <style>p.{{.}} {color:red}</style> left 120 121 Non-string values can be used in JavaScript contexts. 122 If {{.}} is 123 124 struct{A,B string}{ "foo", "bar" } 125 126 in the escaped template 127 128 <script>var pair = {{.}};</script> 129 130 then the template output is 131 132 <script>var pair = {"A": "foo", "B": "bar"};</script> 133 134 See package json to understand how non-string content is marshaled for 135 embedding in JavaScript contexts. 136 137 138 Typed Strings 139 140 By default, this package assumes that all pipelines produce a plain text string. 141 It adds escaping pipeline stages necessary to correctly and safely embed that 142 plain text string in the appropriate context. 143 144 When a data value is not plain text, you can make sure it is not over-escaped 145 by marking it with its type. 146 147 Types HTML, JS, URL, and others from content.go can carry safe content that is 148 exempted from escaping. 149 150 The template 151 152 Hello, {{.}}! 153 154 can be invoked with 155 156 tmpl.Execute(out, template.HTML(`<b>World</b>`)) 157 158 to produce 159 160 Hello, <b>World</b>! 161 162 instead of the 163 164 Hello, <b>World<b>! 165 166 that would have been produced if {{.}} was a regular string. 167 168 169 Security Model 170 171 https://rawgit.com/mikesamuel/sanitized-jquery-templates/trunk/safetemplate.html#problem_definition defines "safe" as used by this package. 172 173 This package assumes that template authors are trusted, that Execute's data 174 parameter is not, and seeks to preserve the properties below in the face 175 of untrusted data: 176 177 Structure Preservation Property: 178 "... when a template author writes an HTML tag in a safe templating language, 179 the browser will interpret the corresponding portion of the output as a tag 180 regardless of the values of untrusted data, and similarly for other structures 181 such as attribute boundaries and JS and CSS string boundaries." 182 183 Code Effect Property: 184 "... only code specified by the template author should run as a result of 185 injecting the template output into a page and all code specified by the 186 template author should run as a result of the same." 187 188 Least Surprise Property: 189 "A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who 190 knows that contextual autoescaping happens should be able to look at a {{.}} 191 and correctly infer what sanitization happens." 192 */ 193 package template 194
