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Package template

import "html/template"
Overview
Index

Overview ▾

Package template (html/template) implements data-driven templates for generating HTML output safe against code injection. It provides the same interface as package text/template and should be used instead of text/template whenever the output is HTML.

The documentation here focuses on the security features of the package. For information about how to program the templates themselves, see the documentation for text/template.

Introduction

This package wraps package text/template so you can share its template API to parse and execute HTML templates safely.

tmpl, err := template.New("name").Parse(...)
// Error checking elided
err = tmpl.Execute(out, data)

If successful, tmpl will now be injection-safe. Otherwise, err is an error defined in the docs for ErrorCode.

HTML templates treat data values as plain text which should be encoded so they can be safely embedded in an HTML document. The escaping is contextual, so actions can appear within JavaScript, CSS, and URI contexts.

The security model used by this package assumes that template authors are trusted, while Execute's data parameter is not. More details are provided below.

Example

import "text/template"
...
t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")

produces

Hello, <script>alert('you have been pwned')</script>!

but the contextual autoescaping in html/template

import "html/template"
...
t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")

produces safe, escaped HTML output

Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!

Contexts

This package understands HTML, CSS, JavaScript, and URIs. It adds sanitizing functions to each simple action pipeline, so given the excerpt

<a href="/search?q={{.}}">{{.}}</a>

At parse time each {{.}} is overwritten to add escaping functions as necessary. In this case it becomes

<a href="/search?q={{. | urlquery}}">{{. | html}}</a>

Errors

See the documentation of ErrorCode for details.

A fuller picture

The rest of this package comment may be skipped on first reading; it includes details necessary to understand escaping contexts and error messages. Most users will not need to understand these details.

Contexts

Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows how {{.}} appears when used in the context to the left.

Context                          {{.}} After
{{.}}                            O'Reilly: How are &lt;i&gt;you&lt;/i&gt;?
<a title='{{.}}'>                O&#39;Reilly: How are you?
<a href="/{{.}}">                O&#39;Reilly: How are %3ci%3eyou%3c/i%3e?
<a href="?q={{.}}">              O&#39;Reilly%3a%20How%20are%3ci%3e...%3f
<a onx='f("{{.}}")'>             O\x27Reilly: How are \x3ci\x3eyou...?
<a onx='f({{.}})'>               "O\x27Reilly: How are \x3ci\x3eyou...?"
<a onx='pattern = /{{.}}/;'>     O\x27Reilly: How are \x3ci\x3eyou...\x3f

If used in an unsafe context, then the value might be filtered out:

Context                          {{.}} After
<a href="{{.}}">                 #ZgotmplZ

since "O'Reilly:" is not an allowed protocol like "http:".

If {{.}} is the innocuous word, `left`, then it can appear more widely,

Context                              {{.}} After
{{.}}                                left
<a title='{{.}}'>                    left
<a href='{{.}}'>                     left
<a href='/{{.}}'>                    left
<a href='?dir={{.}}'>                left
<a style="border-{{.}}: 4px">        left
<a style="align: {{.}}">             left
<a style="background: '{{.}}'>       left
<a style="background: url('{{.}}')>  left
<style>p.{{.}} {color:red}</style>   left

Non-string values can be used in JavaScript contexts. If {{.}} is

struct{A,B string}{ "foo", "bar" }

in the escaped template

<script>var pair = {{.}};</script>

then the template output is

<script>var pair = {"A": "foo", "B": "bar"};</script>

See package json to understand how non-string content is marshalled for embedding in JavaScript contexts.

Typed Strings

By default, this package assumes that all pipelines produce a plain text string. It adds escaping pipeline stages necessary to correctly and safely embed that plain text string in the appropriate context.

When a data value is not plain text, you can make sure it is not over-escaped by marking it with its type.

Types HTML, JS, URL, and others from content.go can carry safe content that is exempted from escaping.

The template

Hello, {{.}}!

can be invoked with

tmpl.Execute(out, HTML(`<b>World</b>`))

to produce

Hello, <b>World</b>!

instead of the

Hello, &lt;b&gt;World&lt;b&gt;!

that would have been produced if {{.}} was a regular string.

Security Model

http://js-quasis-libraries-and-repl.googlecode.com/svn/trunk/safetemplate.html#problem_definition defines "safe" as used by this package.

This package assumes that template authors are trusted, that Execute's data parameter is not, and seeks to preserve the properties below in the face of untrusted data:

Structure Preservation Property: "... when a template author writes an HTML tag in a safe templating language, the browser will interpret the corresponding portion of the output as a tag regardless of the values of untrusted data, and similarly for other structures such as attribute boundaries and JS and CSS string boundaries."

Code Effect Property: "... only code specified by the template author should run as a result of injecting the template output into a page and all code specified by the template author should run as a result of the same."

Least Surprise Property: "A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who knows that contextual autoescaping happens should be able to look at a {{.}} and correctly infer what sanitization happens."

Index ▾

Package files

attr.go content.go context.go css.go doc.go error.go escape.go html.go js.go template.go transition.go url.go

func HTMLEscape

func HTMLEscape(w io.Writer, b []byte)

HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.

func HTMLEscapeString

func HTMLEscapeString(s string) string

HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.

func HTMLEscaper

func HTMLEscaper(args ...interface{}) string

HTMLEscaper returns the escaped HTML equivalent of the textual representation of its arguments.

func JSEscape

func JSEscape(w io.Writer, b []byte)

JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.

func JSEscapeString

func JSEscapeString(s string) string

JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.

func JSEscaper

func JSEscaper(args ...interface{}) string

JSEscaper returns the escaped JavaScript equivalent of the textual representation of its arguments.

func URLQueryEscaper

func URLQueryEscaper(args ...interface{}) string

URLQueryEscaper returns the escaped value of the textual representation of its arguments in a form suitable for embedding in a URL query.

type CSS

type CSS string

CSS encapsulates known safe content that matches any of:

1. The CSS3 stylesheet production, such as `p { color: purple }`.
2. The CSS3 rule production, such as `a[href=~"https:"].foo#bar`.
3. CSS3 declaration productions, such as `color: red; margin: 2px`.
4. The CSS3 value production, such as `rgba(0, 0, 255, 127)`.

See http://www.w3.org/TR/css3-syntax/#parsing and https://web.archive.org/web/20090211114933/http://w3.org/TR/css3-syntax#style

type Error

type Error struct {
        // ErrorCode describes the kind of error.
        ErrorCode ErrorCode
        // Name is the name of the template in which the error was encountered.
        Name string
        // Line is the line number of the error in the template source or 0.
        Line int
        // Description is a human-readable description of the problem.
        Description string
}

Error describes a problem encountered during template Escaping.

func (*Error) Error

func (e *Error) Error() string

type ErrorCode

type ErrorCode int

ErrorCode is a code for a kind of error.

const (
        // OK indicates the lack of an error.
        OK ErrorCode = iota

        // ErrAmbigContext: "... appears in an ambiguous URL context"
        // Example:
        //   <a href="
        //      {{if .C}}
        //        /path/
        //      {{else}}
        //        /search?q=
        //      {{end}}
        //      {{.X}}
        //   ">
        // Discussion:
        //   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
        //  it may be either a URL suffix or a query parameter.
        //   Moving {{.X}} into the condition removes the ambiguity:
        //   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
        ErrAmbigContext

        // ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
        //   "... in unquoted attr", "... in attribute name"
        // Example:
        //   <a href = /search?q=foo>
        //   <href=foo>
        //   <form na<e=...>
        //   <option selected<
        // Discussion:
        //   This is often due to a typo in an HTML element, but some runes
        //   are banned in tag names, attribute names, and unquoted attribute
        //   values because they can tickle parser ambiguities.
        //   Quoting all attributes is the best policy.
        ErrBadHTML

        // ErrBranchEnd: "{{if}} branches end in different contexts"
        // Example:
        //   {{if .C}}<a href="{{end}}{{.X}}
        // Discussion:
        //   Package html/template statically examines each path through an
        //   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
        //   The example is ambiguous since {{.X}} might be an HTML text node,
        //   or a URL prefix in an HTML attribute. The context of {{.X}} is
        //   used to figure out how to escape it, but that context depends on
        //   the run-time value of {{.C}} which is not statically known.
        //
        //   The problem is usually something like missing quotes or angle
        //   brackets, or can be avoided by refactoring to put the two contexts
        //   into different branches of an if, range or with. If the problem
        //   is in a {{range}} over a collection that should never be empty,
        //   adding a dummy {{else}} can help.
        ErrBranchEnd

        // ErrEndContext: "... ends in a non-text context: ..."
        // Examples:
        //   <div
        //   <div title="no close quote>
        //   <script>f()
        // Discussion:
        //   Executed templates should produce a DocumentFragment of HTML.
        //   Templates that end without closing tags will trigger this error.
        //   Templates that should not be used in an HTML context or that
        //   produce incomplete Fragments should not be executed directly.
        //
        //   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
        //   {{define "helper"}} document.write(' <div title=" ') {{end}}
        //
        //   "helper" does not produce a valid document fragment, so should
        //   not be Executed directly.
        ErrEndContext

        // ErrNoSuchTemplate: "no such template ..."
        // Examples:
        //   {{define "main"}}<div {{template "attrs"}}>{{end}}
        //   {{define "attrs"}}href="{{.URL}}"{{end}}
        // Discussion:
        //   Package html/template looks through template calls to compute the
        //   context.
        //   Here the {{.URL}} in "attrs" must be treated as a URL when called
        //   from "main", but you will get this error if "attrs" is not defined
        //   when "main" is parsed.
        ErrNoSuchTemplate

        // ErrOutputContext: "cannot compute output context for template ..."
        // Examples:
        //   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
        // Discussion:
        //   A recursive template does not end in the same context in which it
        //   starts, and a reliable output context cannot be computed.
        //   Look for typos in the named template.
        //   If the template should not be called in the named start context,
        //   look for calls to that template in unexpected contexts.
        //   Maybe refactor recursive templates to not be recursive.
        ErrOutputContext

        // ErrPartialCharset: "unfinished JS regexp charset in ..."
        // Example:
        //     <script>var pattern = /foo[{{.Chars}}]/</script>
        // Discussion:
        //   Package html/template does not support interpolation into regular
        //   expression literal character sets.
        ErrPartialCharset

        // ErrPartialEscape: "unfinished escape sequence in ..."
        // Example:
        //   <script>alert("\{{.X}}")</script>
        // Discussion:
        //   Package html/template does not support actions following a
        //   backslash.
        //   This is usually an error and there are better solutions; for
        //   example
        //     <script>alert("{{.X}}")</script>
        //   should work, and if {{.X}} is a partial escape sequence such as
        //   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
        ErrPartialEscape

        // ErrRangeLoopReentry: "on range loop re-entry: ..."
        // Example:
        //   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
        // Discussion:
        //   If an iteration through a range would cause it to end in a
        //   different context than an earlier pass, there is no single context.
        //   In the example, there is missing a quote, so it is not clear
        //   whether {{.}} is meant to be inside a JS string or in a JS value
        //   context.  The second iteration would produce something like
        //
        //     <script>var x = ['firstValue,'secondValue]</script>
        ErrRangeLoopReentry

        // ErrSlashAmbig: '/' could start a division or regexp.
        // Example:
        //   <script>
        //     {{if .C}}var x = 1{{end}}
        //     /-{{.N}}/i.test(x) ? doThis : doThat();
        //   </script>
        // Discussion:
        //   The example above could produce `var x = 1/-2/i.test(s)...`
        //   in which the first '/' is a mathematical division operator or it
        //   could produce `/-2/i.test(s)` in which the first '/' starts a
        //   regexp literal.
        //   Look for missing semicolons inside branches, and maybe add
        //   parentheses to make it clear which interpretation you intend.
        ErrSlashAmbig
)

We define codes for each error that manifests while escaping templates, but escaped templates may also fail at runtime.

Output: "ZgotmplZ" Example:

<img src="{{.X}}">
where {{.X}} evaluates to `javascript:...`

Discussion:

"ZgotmplZ" is a special value that indicates that unsafe content reached a
CSS or URL context at runtime. The output of the example will be
  <img src="#ZgotmplZ">
If the data comes from a trusted source, use content types to exempt it
from filtering: URL(`javascript:...`).

type FuncMap

type FuncMap map[string]interface{}

FuncMap is the type of the map defining the mapping from names to functions. Each function must have either a single return value, or two return values of which the second has type error. In that case, if the second (error) argument evaluates to non-nil during execution, execution terminates and Execute returns that error. FuncMap has the same base type as FuncMap in "text/template", copied here so clients need not import "text/template".

type HTML

type HTML string

HTML encapsulates a known safe HTML document fragment. It should not be used for HTML from a third-party, or HTML with unclosed tags or comments. The outputs of a sound HTML sanitizer and a template escaped by this package are fine for use with HTML.

type HTMLAttr

type HTMLAttr string

HTMLAttr encapsulates an HTML attribute from a trusted source, for example, ` dir="ltr"`.

type JS

type JS string

JS encapsulates a known safe EcmaScript5 Expression, for example, `(x + y * z())`. Template authors are responsible for ensuring that typed expressions do not break the intended precedence and that there is no statement/expression ambiguity as when passing an expression like "{ foo: bar() }\n['foo']()", which is both a valid Expression and a valid Program with a very different meaning.

type JSStr

type JSStr string

JSStr encapsulates a sequence of characters meant to be embedded between quotes in a JavaScript expression. The string must match a series of StringCharacters:

StringCharacter :: SourceCharacter but not `\` or LineTerminator
                 | EscapeSequence

Note that LineContinuations are not allowed. JSStr("foo\\nbar") is fine, but JSStr("foo\\\nbar") is not.

type Template

type Template struct {

        // The underlying template's parse tree, updated to be HTML-safe.
        Tree *parse.Tree
        // contains filtered or unexported fields
}

Template is a specialized Template from "text/template" that produces a safe HTML document fragment.

func Must

func Must(t *Template, err error) *Template

Must is a helper that wraps a call to a function returning (*Template, error) and panics if the error is non-nil. It is intended for use in variable initializations such as

var t = template.Must(template.New("name").Parse("html"))

func New

func New(name string) *Template

New allocates a new HTML template with the given name.

func ParseFiles

func ParseFiles(filenames ...string) (*Template, error)

ParseFiles creates a new Template and parses the template definitions from the named files. The returned template's name will have the (base) name and (parsed) contents of the first file. There must be at least one file. If an error occurs, parsing stops and the returned *Template is nil.

func ParseGlob

func ParseGlob(pattern string) (*Template, error)

ParseGlob creates a new Template and parses the template definitions from the files identified by the pattern, which must match at least one file. The returned template will have the (base) name and (parsed) contents of the first file matched by the pattern. ParseGlob is equivalent to calling ParseFiles with the list of files matched by the pattern.

func (*Template) AddParseTree

func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error)

AddParseTree creates a new template with the name and parse tree and associates it with t.

It returns an error if t has already been executed.

func (*Template) Clone

func (t *Template) Clone() (*Template, error)

Clone returns a duplicate of the template, including all associated templates. The actual representation is not copied, but the name space of associated templates is, so further calls to Parse in the copy will add templates to the copy but not to the original. Clone can be used to prepare common templates and use them with variant definitions for other templates by adding the variants after the clone is made.

It returns an error if t has already been executed.

func (*Template) Delims

func (t *Template) Delims(left, right string) *Template

Delims sets the action delimiters to the specified strings, to be used in subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template definitions will inherit the settings. An empty delimiter stands for the corresponding default: {{ or }}. The return value is the template, so calls can be chained.

func (*Template) Execute

func (t *Template) Execute(wr io.Writer, data interface{}) error

Execute applies a parsed template to the specified data object, writing the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel.

func (*Template) ExecuteTemplate

func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error

ExecuteTemplate applies the template associated with t that has the given name to the specified data object and writes the output to wr. If an error occurs executing the template or writing its output, execution stops, but partial results may already have been written to the output writer. A template may be executed safely in parallel.

func (*Template) Funcs

func (t *Template) Funcs(funcMap FuncMap) *Template

Funcs adds the elements of the argument map to the template's function map. It panics if a value in the map is not a function with appropriate return type. However, it is legal to overwrite elements of the map. The return value is the template, so calls can be chained.

func (*Template) Lookup

func (t *Template) Lookup(name string) *Template

Lookup returns the template with the given name that is associated with t, or nil if there is no such template.

func (*Template) Name

func (t *Template) Name() string

Name returns the name of the template.

func (*Template) New

func (t *Template) New(name string) *Template

New allocates a new HTML template associated with the given one and with the same delimiters. The association, which is transitive, allows one template to invoke another with a {{template}} action.

func (*Template) Parse

func (t *Template) Parse(src string) (*Template, error)

Parse parses a string into a template. Nested template definitions will be associated with the top-level template t. Parse may be called multiple times to parse definitions of templates to associate with t. It is an error if a resulting template is non-empty (contains content other than template definitions) and would replace a non-empty template with the same name. (In multiple calls to Parse with the same receiver template, only one call can contain text other than space, comments, and template definitions.)

func (*Template) ParseFiles

func (t *Template) ParseFiles(filenames ...string) (*Template, error)

ParseFiles parses the named files and associates the resulting templates with t. If an error occurs, parsing stops and the returned template is nil; otherwise it is t. There must be at least one file.

func (*Template) ParseGlob

func (t *Template) ParseGlob(pattern string) (*Template, error)

ParseGlob parses the template definitions in the files identified by the pattern and associates the resulting templates with t. The pattern is processed by filepath.Glob and must match at least one file. ParseGlob is equivalent to calling t.ParseFiles with the list of files matched by the pattern.

func (*Template) Templates

func (t *Template) Templates() []*Template

Templates returns a slice of the templates associated with t, including t itself.

type URL

type URL string

URL encapsulates a known safe URL or URL substring (see RFC 3986). A URL like `javascript:checkThatFormNotEditedBeforeLeavingPage()` from a trusted source should go in the page, but by default dynamic `javascript:` URLs are filtered out since they are a frequently exploited injection vector.