src/html/template/content.go

// 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
}