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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package template
import (
"fmt"
"reflect"
)
// Strings of content from a trusted source.
type (
// 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.
CSS 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.
//
// 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.
HTML string
// 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.
HTMLAttr 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.
//
// 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.
JS 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.
//
// 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.
JSStr 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.
//
// 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.
URL string
// Srcset encapsulates a known safe srcset attribute
// (see https://w3c.github.io/html/semantics-embedded-content.html#element-attrdef-img-srcset).
//
// 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.
Srcset string
)
type contentType uint8
const (
contentTypePlain contentType = iota
contentTypeCSS
contentTypeHTML
contentTypeHTMLAttr
contentTypeJS
contentTypeJSStr
contentTypeURL
contentTypeSrcset
// contentTypeUnsafe is used in attr.go for values that affect how
// embedded content and network messages are formed, vetted,
// or interpreted; or which credentials network messages carry.
contentTypeUnsafe
)
// indirect returns the value, after dereferencing as many times
// as necessary to reach the base type (or nil).
func indirect(a any) any {
if a == nil {
return nil
}
if t := reflect.TypeOf(a); t.Kind() != reflect.Pointer {
// Avoid creating a reflect.Value if it's not a pointer.
return a
}
v := reflect.ValueOf(a)
for v.Kind() == reflect.Pointer && !v.IsNil() {
v = v.Elem()
}
return v.Interface()
}
var (
errorType = reflect.TypeFor[error]()
fmtStringerType = reflect.TypeFor[fmt.Stringer]()
)
// indirectToStringerOrError returns the value, after dereferencing as many times
// as necessary to reach the base type (or nil) or an implementation of fmt.Stringer
// or error.
func indirectToStringerOrError(a any) any {
if a == nil {
return nil
}
v := reflect.ValueOf(a)
for !v.Type().Implements(fmtStringerType) && !v.Type().Implements(errorType) && v.Kind() == reflect.Pointer && !v.IsNil() {
v = v.Elem()
}
return v.Interface()
}
// stringify converts its arguments to a string and the type of the content.
// All pointers are dereferenced, as in the text/template package.
func stringify(args ...any) (string, contentType) {
if len(args) == 1 {
switch s := indirect(args[0]).(type) {
case string:
return s, contentTypePlain
case CSS:
return string(s), contentTypeCSS
case HTML:
return string(s), contentTypeHTML
case HTMLAttr:
return string(s), contentTypeHTMLAttr
case JS:
return string(s), contentTypeJS
case JSStr:
return string(s), contentTypeJSStr
case URL:
return string(s), contentTypeURL
case Srcset:
return string(s), contentTypeSrcset
}
}
i := 0
for _, arg := range args {
// We skip untyped nil arguments for backward compatibility.
// Without this they would be output as <nil>, escaped.
// See issue 25875.
if arg == nil {
continue
}
args[i] = indirectToStringerOrError(arg)
i++
}
return fmt.Sprint(args[:i]...), contentTypePlain
}
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