Involved Source Filesattr.goattr_string.gocontent.gocontext.gocss.godelim_string.go Package template (html/template) implements data-driven templates for
generating HTML output safe against code injection. It provides the
same interface as [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 [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, <script>alert('you have been pwned')</script>!
# 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={{. | urlescaper | attrescaper}}">{{. | htmlescaper}}</a>
where urlescaper, attrescaper, and htmlescaper are aliases for internal escaping
functions.
For these internal escaping functions, if an action pipeline evaluates to
a nil interface value, it is treated as though it were an empty string.
# Namespaced and data- attributes
Attributes with a namespace are treated as if they had no namespace.
Given the excerpt
<a my:href="{{.}}"></a>
At parse time the attribute will be treated as if it were just "href".
So at parse time the template becomes:
<a my:href="{{. | urlescaper | attrescaper}}"></a>
Similarly to attributes with namespaces, attributes with a "data-" prefix are
treated as if they had no "data-" prefix. So given
<a data-href="{{.}}"></a>
At parse time this becomes
<a data-href="{{. | urlescaper | attrescaper}}"></a>
If an attribute has both a namespace and a "data-" prefix, only the namespace
will be removed when determining the context. For example
<a my:data-href="{{.}}"></a>
This is handled as if "my:data-href" was just "data-href" and not "href" as
it would be if the "data-" prefix were to be ignored too. Thus at parse
time this becomes just
<a my:data-href="{{. | attrescaper}}"></a>
As a special case, attributes with the namespace "xmlns" are always treated
as containing URLs. Given the excerpts
<a xmlns:title="{{.}}"></a>
<a xmlns:href="{{.}}"></a>
<a xmlns:onclick="{{.}}"></a>
At parse time they become:
<a xmlns:title="{{. | urlescaper | attrescaper}}"></a>
<a xmlns:href="{{. | urlescaper | attrescaper}}"></a>
<a xmlns:onclick="{{. | urlescaper | attrescaper}}"></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 <i>you</i>?
<a title='{{.}}'> O'Reilly: How are you?
<a href="/{{.}}"> O'Reilly: How are %3ci%3eyou%3c/i%3e?
<a href="?q={{.}}"> O'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 marshaled 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, template.HTML(`<b>World</b>`))
to produce
Hello, <b>World</b>!
instead of the
Hello, <b>World<b>!
that would have been produced if {{.}} was a regular string.
# Security Model
https://rawgit.com/mikesamuel/sanitized-jquery-templates/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."
As a consequence of the Least Surprise Property, template actions within an
ECMAScript 6 template literal are disabled by default.
Handling string interpolation within these literals is rather complex resulting
in no clear safe way to support it.
To re-enable template actions within ECMAScript 6 template literals, use the
GODEBUG=jstmpllitinterp=1 environment variable.element_string.goerror.goescape.gohtml.gojs.gojsctx_string.gostate_string.gotemplate.gotransition.gourl.gourlpart_string.go
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 https://www.w3.org/TR/css3-syntax/#parsing and
https://web.archive.org/web/20090211114933/http://w3.org/TR/css3-syntax#style
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
Error describes a problem encountered during template Escaping. Description is a human-readable description of the problem. ErrorCode describes the kind of error. Line is the line number of the error in the template source or 0. Name is the name of the template in which the error was encountered. Node is the node that caused the problem, if known.
If not nil, it overrides Name and Line.(*Error) Error() string
*Error : error
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.
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
HTMLAttr encapsulates an HTML attribute from a trusted source,
for example, ` dir="ltr"`.
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
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.
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
Using JS to include valid but untrusted JSON is not safe.
A safe alternative is to parse the JSON with json.Unmarshal and then
pass the resultant object into the template, where it will be
converted to sanitized JSON when presented in a JavaScript context.
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.
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
Template is a specialized Template from "text/template" that produces a safe
HTML document fragment. The underlying template's parse tree, updated to be HTML-safe. AddParseTree creates a new template with the name and parse tree
and associates it with t.
It returns an error if t or any associated template has already been executed. 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. DefinedTemplates returns a string listing the defined templates,
prefixed by the string "; defined templates are: ". If there are none,
it returns the empty string. Used to generate an error message. 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. 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, although if parallel
executions share a Writer the output may be interleaved. 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, although if parallel
executions share a Writer the output may be interleaved. Funcs adds the elements of the argument map to the template's function map.
It must be called before the template is parsed.
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. Lookup returns the template with the given name that is associated with t,
or nil if there is no such template. Name returns the name of the 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.
If a template with the given name already exists, the new HTML template
will replace it. The existing template will be reset and disassociated with
t. Option sets options for the template. Options are described by
strings, either a simple string or "key=value". There can be at
most one equals sign in an option string. If the option string
is unrecognized or otherwise invalid, Option panics.
Known options:
missingkey: Control the behavior during execution if a map is
indexed with a key that is not present in the map.
"missingkey=default" or "missingkey=invalid"
The default behavior: Do nothing and continue execution.
If printed, the result of the index operation is the string
"<no value>".
"missingkey=zero"
The operation returns the zero value for the map type's element.
"missingkey=error"
Execution stops immediately with an error. Parse parses text as a template body for t.
Named template definitions ({{define ...}} or {{block ...}} statements) in text
define additional templates associated with t and are removed from the
definition of t itself.
Templates can be redefined in successive calls to Parse,
before the first use of Execute on t or any associated template.
A template definition with a body containing only white space and comments
is considered empty and will not replace an existing template's body.
This allows using Parse to add new named template definitions without
overwriting the main template body. ParseFS is like ParseFiles or ParseGlob but reads from the file system fs
instead of the host operating system's file system.
It accepts a list of glob patterns.
(Note that most file names serve as glob patterns matching only themselves.) 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.
When parsing multiple files with the same name in different directories,
the last one mentioned will be the one that results.
ParseFiles returns an error if t or any associated template has already been executed. ParseGlob parses the template definitions in the files identified by the
pattern and associates the resulting templates with t. The files are matched
according to the semantics of filepath.Match, and the pattern must match at
least one file. ParseGlob is equivalent to calling t.ParseFiles with the
list of files matched by the pattern.
When parsing multiple files with the same name in different directories,
the last one mentioned will be the one that results.
ParseGlob returns an error if t or any associated template has already been executed. Templates returns a slice of the templates associated with t, including t
itself.
func Must(t *Template, err error) *Template
func New(name string) *Template
func ParseFiles(filenames ...string) (*Template, error)
func ParseFS(fs fs.FS, patterns ...string) (*Template, error)
func ParseGlob(pattern string) (*Template, error)
func (*Template).AddParseTree(name string, tree *parse.Tree) (*Template, error)
func (*Template).Clone() (*Template, error)
func (*Template).Delims(left, right string) *Template
func (*Template).Funcs(funcMap FuncMap) *Template
func (*Template).Lookup(name string) *Template
func (*Template).New(name string) *Template
func (*Template).Option(opt ...string) *Template
func (*Template).Parse(text string) (*Template, error)
func (*Template).ParseFiles(filenames ...string) (*Template, error)
func (*Template).ParseFS(fs fs.FS, patterns ...string) (*Template, error)
func (*Template).ParseGlob(pattern string) (*Template, error)
func (*Template).Templates() []*Template
func Must(t *Template, err error) *Template
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.
Use of this type presents a security risk:
the encapsulated content should come from a trusted source,
as it will be included verbatim in the template output.
Package-Level Functions (total 13)
HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
HTMLEscaper returns the escaped HTML equivalent of the textual
representation of its arguments.
HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
and whether the value has a meaningful truth value. This is the definition of
truth used by if and other such actions.
JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
JSEscaper returns the escaped JavaScript equivalent of the textual
representation of its arguments.
JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
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"))
New allocates a new HTML template with the given name.
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.
When parsing multiple files with the same name in different directories,
the last one mentioned will be the one that results.
For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template
named "foo", while "a/foo" is unavailable.
ParseFS is like ParseFiles or ParseGlob but reads from the file system fs
instead of the host operating system's file system.
It accepts a list of glob patterns.
(Note that most file names serve as glob patterns matching only themselves.)
ParseGlob creates a new Template and parses the template definitions from
the files identified by the pattern. The files are matched according to the
semantics of filepath.Match, and the pattern 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.
When parsing multiple files with the same name in different directories,
the last one mentioned will be the one that results.
URLQueryEscaper returns the escaped value of the textual representation of
its arguments in a form suitable for embedding in a URL query.
Package-Level Constants (total 13)
ErrAmbigContext: "... appears in an ambiguous context within a URL"
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}}">
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.
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.
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.
ErrJSTemplate: "... appears in a JS template literal"
Example:
<script>var tmpl = `{{.Interp}}`</script>
Discussion:
Package html/template does not support actions inside of JS template
literals.
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.
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.
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.
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`)
ErrPredefinedEscaper: "predefined escaper ... disallowed in template"
Example:
<div class={{. | html}}>Hello<div>
Discussion:
Package html/template already contextually escapes all pipelines to
produce HTML output safe against code injection. Manually escaping
pipeline output using the predefined escapers "html" or "urlquery" is
unnecessary, and may affect the correctness or safety of the escaped
pipeline output in Go 1.8 and earlier.
In most cases, such as the given example, this error can be resolved by
simply removing the predefined escaper from the pipeline and letting the
contextual autoescaper handle the escaping of the pipeline. In other
instances, where the predefined escaper occurs in the middle of a
pipeline where subsequent commands expect escaped input, e.g.
{{.X | html | makeALink}}
where makeALink does
return `<a href="`+input+`">link</a>`
consider refactoring the surrounding template to make use of the
contextual autoescaper, i.e.
<a href="{{.X}}">link</a>
To ease migration to Go 1.9 and beyond, "html" and "urlquery" will
continue to be allowed as the last command in a pipeline. However, if the
pipeline occurs in an unquoted attribute value context, "html" is
disallowed. Avoid using "html" and "urlquery" entirely in new templates.
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>
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.
OK indicates the lack of an error.
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