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388 lines
10 KiB
Go
388 lines
10 KiB
Go
// Package sanitize provides functions for sanitizing text.
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package sanitize
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import (
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"bytes"
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"html"
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"html/template"
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"io"
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"path"
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"regexp"
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"strings"
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parser "golang.org/x/net/html"
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)
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var (
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ignoreTags = []string{"title", "script", "style", "iframe", "frame", "frameset", "noframes", "noembed", "embed", "applet", "object", "base"}
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defaultTags = []string{"h1", "h2", "h3", "h4", "h5", "h6", "div", "span", "hr", "p", "br", "b", "i", "strong", "em", "ol", "ul", "li", "a", "img", "pre", "code", "blockquote", "article", "section"}
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defaultAttributes = []string{"id", "class", "src", "href", "title", "alt", "name", "rel"}
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)
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// HTMLAllowing sanitizes html, allowing some tags.
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// Arrays of allowed tags and allowed attributes may optionally be passed as the second and third arguments.
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func HTMLAllowing(s string, args ...[]string) (string, error) {
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allowedTags := defaultTags
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if len(args) > 0 {
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allowedTags = args[0]
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}
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allowedAttributes := defaultAttributes
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if len(args) > 1 {
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allowedAttributes = args[1]
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}
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// Parse the html
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tokenizer := parser.NewTokenizer(strings.NewReader(s))
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buffer := bytes.NewBufferString("")
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ignore := ""
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for {
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tokenType := tokenizer.Next()
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token := tokenizer.Token()
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switch tokenType {
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case parser.ErrorToken:
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err := tokenizer.Err()
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if err == io.EOF {
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return buffer.String(), nil
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}
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return "", err
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case parser.StartTagToken:
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if len(ignore) == 0 && includes(allowedTags, token.Data) {
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token.Attr = cleanAttributes(token.Attr, allowedAttributes)
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buffer.WriteString(token.String())
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} else if includes(ignoreTags, token.Data) {
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ignore = token.Data
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}
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case parser.SelfClosingTagToken:
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if len(ignore) == 0 && includes(allowedTags, token.Data) {
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token.Attr = cleanAttributes(token.Attr, allowedAttributes)
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buffer.WriteString(token.String())
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} else if token.Data == ignore {
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ignore = ""
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}
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case parser.EndTagToken:
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if len(ignore) == 0 && includes(allowedTags, token.Data) {
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token.Attr = []parser.Attribute{}
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buffer.WriteString(token.String())
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} else if token.Data == ignore {
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ignore = ""
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}
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case parser.TextToken:
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// We allow text content through, unless ignoring this entire tag and its contents (including other tags)
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if ignore == "" {
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buffer.WriteString(token.String())
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}
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case parser.CommentToken:
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// We ignore comments by default
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case parser.DoctypeToken:
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// We ignore doctypes by default - html5 does not require them and this is intended for sanitizing snippets of text
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default:
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// We ignore unknown token types by default
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}
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}
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}
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// HTML strips html tags, replace common entities, and escapes <>&;'" in the result.
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// Note the returned text may contain entities as it is escaped by HTMLEscapeString, and most entities are not translated.
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func HTML(s string) (output string) {
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// Shortcut strings with no tags in them
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if !strings.ContainsAny(s, "<>") {
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output = s
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} else {
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// First remove line breaks etc as these have no meaning outside html tags (except pre)
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// this means pre sections will lose formatting... but will result in less unintentional paras.
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s = strings.Replace(s, "\n", "", -1)
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// Then replace line breaks with newlines, to preserve that formatting
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s = strings.Replace(s, "</p>", "\n", -1)
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s = strings.Replace(s, "<br>", "\n", -1)
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s = strings.Replace(s, "</br>", "\n", -1)
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s = strings.Replace(s, "<br/>", "\n", -1)
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s = strings.Replace(s, "<br />", "\n", -1)
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// Walk through the string removing all tags
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b := bytes.NewBufferString("")
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inTag := false
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for _, r := range s {
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switch r {
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case '<':
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inTag = true
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case '>':
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inTag = false
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default:
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if !inTag {
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b.WriteRune(r)
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}
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}
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}
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output = b.String()
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}
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// Remove a few common harmless entities, to arrive at something more like plain text
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output = strings.Replace(output, "‘", "'", -1)
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output = strings.Replace(output, "’", "'", -1)
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output = strings.Replace(output, "“", "\"", -1)
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output = strings.Replace(output, "”", "\"", -1)
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output = strings.Replace(output, " ", " ", -1)
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output = strings.Replace(output, """, "\"", -1)
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output = strings.Replace(output, "'", "'", -1)
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// Translate some entities into their plain text equivalent (for example accents, if encoded as entities)
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output = html.UnescapeString(output)
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// In case we have missed any tags above, escape the text - removes <, >, &, ' and ".
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output = template.HTMLEscapeString(output)
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// After processing, remove some harmless entities &, ' and " which are encoded by HTMLEscapeString
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output = strings.Replace(output, """, "\"", -1)
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output = strings.Replace(output, "'", "'", -1)
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output = strings.Replace(output, "& ", "& ", -1) // NB space after
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output = strings.Replace(output, "&amp; ", "& ", -1) // NB space after
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return output
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}
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// We are very restrictive as this is intended for ascii url slugs
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var illegalPath = regexp.MustCompile(`[^[:alnum:]\~\-\./]`)
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// Path makes a string safe to use as a URL path,
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// removing accents and replacing separators with -.
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// The path may still start at / and is not intended
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// for use as a file system path without prefix.
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func Path(s string) string {
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// Start with lowercase string
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filePath := strings.ToLower(s)
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filePath = strings.Replace(filePath, "..", "", -1)
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filePath = path.Clean(filePath)
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// Remove illegal characters for paths, flattening accents
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// and replacing some common separators with -
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filePath = cleanString(filePath, illegalPath)
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// NB this may be of length 0, caller must check
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return filePath
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}
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// Remove all other unrecognised characters apart from
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var illegalName = regexp.MustCompile(`[^[:alnum:]-.]`)
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// Name makes a string safe to use in a file name by first finding the path basename, then replacing non-ascii characters.
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func Name(s string) string {
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// Start with lowercase string
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fileName := strings.ToLower(s)
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fileName = path.Clean(path.Base(fileName))
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// Remove illegal characters for names, replacing some common separators with -
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fileName = cleanString(fileName, illegalName)
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// NB this may be of length 0, caller must check
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return fileName
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}
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// Replace these separators with -
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var baseNameSeparators = regexp.MustCompile(`[./]`)
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// BaseName makes a string safe to use in a file name, producing a sanitized basename replacing . or / with -.
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// No attempt is made to normalise a path or normalise case.
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func BaseName(s string) string {
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// Replace certain joining characters with a dash
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baseName := baseNameSeparators.ReplaceAllString(s, "-")
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// Remove illegal characters for names, replacing some common separators with -
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baseName = cleanString(baseName, illegalName)
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// NB this may be of length 0, caller must check
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return baseName
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}
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// A very limited list of transliterations to catch common european names translated to urls.
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// This set could be expanded with at least caps and many more characters.
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var transliterations = map[rune]string{
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'À': "A",
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'Á': "A",
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'Â': "A",
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'Ã': "A",
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'Ä': "A",
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'Å': "AA",
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'Æ': "AE",
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'Ç': "C",
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'È': "E",
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'É': "E",
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'Ê': "E",
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'Ë': "E",
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'Ì': "I",
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'Í': "I",
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'Î': "I",
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'Ï': "I",
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'Ð': "D",
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'Ł': "L",
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'Ñ': "N",
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'Ò': "O",
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'Ó': "O",
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'Ô': "O",
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'Õ': "O",
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'Ö': "OE",
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'Ø': "OE",
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'Œ': "OE",
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'Ù': "U",
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'Ú': "U",
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'Ü': "UE",
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'Û': "U",
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'Ý': "Y",
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'Þ': "TH",
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'ẞ': "SS",
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'à': "a",
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'á': "a",
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'â': "a",
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'ã': "a",
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'ä': "ae",
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'å': "aa",
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'æ': "ae",
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'ç': "c",
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'è': "e",
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'é': "e",
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'ê': "e",
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'ë': "e",
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'ì': "i",
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'í': "i",
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'î': "i",
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'ï': "i",
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'ð': "d",
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'ł': "l",
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'ñ': "n",
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'ń': "n",
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'ò': "o",
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'ó': "o",
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'ô': "o",
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'õ': "o",
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'ō': "o",
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'ö': "oe",
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'ø': "oe",
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'œ': "oe",
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'ś': "s",
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'ù': "u",
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'ú': "u",
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'û': "u",
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'ū': "u",
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'ü': "ue",
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'ý': "y",
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'ÿ': "y",
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'ż': "z",
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'þ': "th",
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'ß': "ss",
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}
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// Accents replaces a set of accented characters with ascii equivalents.
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func Accents(s string) string {
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// Replace some common accent characters
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b := bytes.NewBufferString("")
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for _, c := range s {
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// Check transliterations first
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if val, ok := transliterations[c]; ok {
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b.WriteString(val)
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} else {
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b.WriteRune(c)
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}
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}
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return b.String()
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}
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var (
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// If the attribute contains data: or javascript: anywhere, ignore it
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// we don't allow this in attributes as it is so frequently used for xss
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// NB we allow spaces in the value, and lowercase.
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illegalAttr = regexp.MustCompile(`(d\s*a\s*t\s*a|j\s*a\s*v\s*a\s*s\s*c\s*r\s*i\s*p\s*t\s*)\s*:`)
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// We are far more restrictive with href attributes.
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legalHrefAttr = regexp.MustCompile(`\A[/#][^/\\]?|mailto:|http://|https://`)
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)
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// cleanAttributes returns an array of attributes after removing malicious ones.
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func cleanAttributes(a []parser.Attribute, allowed []string) []parser.Attribute {
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if len(a) == 0 {
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return a
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}
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var cleaned []parser.Attribute
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for _, attr := range a {
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if includes(allowed, attr.Key) {
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val := strings.ToLower(attr.Val)
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// Check for illegal attribute values
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if illegalAttr.FindString(val) != "" {
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attr.Val = ""
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}
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// Check for legal href values - / mailto:// http:// or https://
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if attr.Key == "href" {
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if legalHrefAttr.FindString(val) == "" {
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attr.Val = ""
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}
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}
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// If we still have an attribute, append it to the array
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if attr.Val != "" {
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cleaned = append(cleaned, attr)
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}
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}
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}
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return cleaned
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}
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// A list of characters we consider separators in normal strings and replace with our canonical separator - rather than removing.
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var (
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separators = regexp.MustCompile(`[ &_=+:]`)
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dashes = regexp.MustCompile(`[\-]+`)
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)
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// cleanString replaces separators with - and removes characters listed in the regexp provided from string.
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// Accents, spaces, and all characters not in A-Za-z0-9 are replaced.
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func cleanString(s string, r *regexp.Regexp) string {
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// Remove any trailing space to avoid ending on -
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s = strings.Trim(s, " ")
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// Flatten accents first so that if we remove non-ascii we still get a legible name
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s = Accents(s)
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// Replace certain joining characters with a dash
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s = separators.ReplaceAllString(s, "-")
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// Remove all other unrecognised characters - NB we do allow any printable characters
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s = r.ReplaceAllString(s, "")
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// Remove any multiple dashes caused by replacements above
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s = dashes.ReplaceAllString(s, "-")
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return s
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}
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// includes checks for inclusion of a string in a []string.
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func includes(a []string, s string) bool {
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for _, as := range a {
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if as == s {
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return true
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}
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}
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return false
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}
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