XSS attribute filtering is inconsistent and strips valid attributes

smustgrave → credited longwave → .

Closed 🐛 Xss::filter() does not handle HTML tags inside attribute values Closed: duplicate as a duplicate.

The last patch is now a few lines off

Checking patch core/lib/Drupal/Component/Utility/Xss.php...
Checking patch core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php...
Hunk #1 succeeded at 403 (offset 8 lines).
Checking patch core/tests/Drupal/Tests/Component/Utility/XssTest.php...
Hunk #3 succeeded at 538 (offset 10 lines).
Applied patch core/lib/Drupal/Component/Utility/Xss.php cleanly.
Applied patch core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php cleanly.
Applied patch core/tests/Drupal/Tests/Component/Utility/XssTest.php cleanly.

Updated patch changes the line numbers and nothing else.

Just checked #146 and it applied fine to 10.1

Rerolled

error: patch failed: core/lib/Drupal/Component/Utility/Xss.php:267
error: core/lib/Drupal/Component/Utility/Xss.php: patch does not apply
error: patch failed: core/tests/Drupal/Tests/Component/Utility/XssTest.php:525
error: core/tests/Drupal/Tests/Component/Utility/XssTest.php: patch does not apply

1. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -29,6 +29,50 @@ class Xss {
   +   * The default list of safe attributes untouched by filter().
   

   nit - untouched => not sanitized ?

2. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -29,6 +29,50 @@ class Xss {
   +   * The default list of Unsafe attributes that must be touched by filter().
   

   ubernit - touched => sanitized ?

3. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -232,13 +277,13 @@ protected static function attributes($attributes) {
   +              substr($attribute_name, 0, 3) === 'ng-' ||
   

   Can we get a comment for this, I assume its something angular related

4. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -271,7 +316,15 @@ protected static function attributes($attributes) {
   +            if ($enforce_protocol_filtering) {
   +              $value = static::filterProtocol($attribute_name, $match[1]);
   +            }
   +            elseif ($skip_protocol_filtering) {
   +              $value = $match[1];
   +            }
   +            else {
   +              $value = static::filterProtocol($attribute_name, $match[1]);
   +            }
   

   I think we can simplify this to

   if ($enforce_protocol_filtering || !$skip_protocol_filtering) {
     // filter here
   }
   else {
     // skip here
   }
   
   

   applies in all three instances of this new code

5. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -340,6 +409,26 @@ protected static function needsRemoval($html_tags, $elem) {
   +  protected static function filterProtocol($name, $value) {
   

   We can typehint here, both for arguments and return type

6. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -340,6 +409,26 @@ protected static function needsRemoval($html_tags, $elem) {
   +    if (in_array($name, static::$safeAttributes)) {
   +      return preg_match('/^[a-zA-Z0-9]+\:[a-zA-Z0-9]+$/', $value) ? $value : UrlHelper::stripDangerousProtocols($value);
   

   can we get a comment here - its a safe attribute, but we're stripping something if a regex matches?

   could we document the regex a bit like we do in \Drupal\Component\Utility\Xss::filter where we have this

   // Strip any tags that are not in the list of allowed html tags.
       return preg_replace_callback('%
         (
         <(?=[^a-zA-Z!/])  # a lone <
         |                 # or
         <!--.*?-->        # a comment
         |                 # or
         <[^>]*(>|$)       # a string that starts with a <, up until the > or the end of the string
         |                 # or
         >                 # just a >
         )%x', $splitter, $string);
   

   Just as a courtesy to our future selves

7. +++ b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
   @@ -395,15 +395,15 @@ public function providerTestFilterXss() {
   +    $data[] = ['<META HTTP-EQUIV="refresh" CONTENT="0;url=javascript:alert(\'XSS\');">', '<META http-equiv="refresh" content="0;url=javascript:alert(\'XSS\');">'];
   

   Whilst content is not executed, for a http-equiv="refresh" it is

   I tested this in Chrome and it was blocked by the browser, but it was executed - so I think we may need to handle this differently

8. +++ b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
   @@ -395,15 +395,15 @@ public function providerTestFilterXss() {
   +    $data[] = ['<META HTTP-EQUIV="refresh" CONTENT="0;url=data:text/html base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K">', '<META http-equiv="refresh" content="0;url=data:text/html base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K">'];
   

   Same here, the browser evaluated it but blocked it in Firefox and Chrome

9. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -543,6 +546,104 @@ public function providerTestAttributes() {
   +        '<h2 property="javascript:alert(0);">The Title</h2>',
   

   In my testing this was safe, browsers ignored it

10. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
    @@ -543,6 +546,104 @@ public function providerTestAttributes() {
    +        '<section class="actions-menu-inner" ng-style="{ \'max-height\': maxPanelHeight }" ed-pretty-scrollbar ed-scroll-axis="y" ed-scroll-theme="light"></section>',
    

    shouldn't the ed- prefixed attributes be removed?

11. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
    @@ -543,6 +546,104 @@ public function providerTestAttributes() {
    +        '<img src="http://example.com/foo.jpg" typeof="foaf:Image">',
    +        '<img src="http://example.com/foo.jpg" typeof="foaf:Image">',
    +        'Image tag with RDFa with namespaced attribute',
    +        ['img'],
    +      ],
    +      [
    +        '<img src="http://example.com/foo.jpg" typeof="foaf:bad////value">',
    +        '<img src="http://example.com/foo.jpg" typeof="foaf:bad////value">',
    +        'Image tag with RDFa with bad with namespaced attribute',
    +        ['img'],
    +      ],
    +      [
    +        '<img src="http://example.com/foo.jpg" foo="bar:baz">',
    +        '<img src="http://example.com/foo.jpg" foo="baz">',
    +        'Image tag with non-RDFa attribute',
    +        ['img'],
    +      ],
    +      [
    +        '<h2 property="title">The Title</h2>',
    +        '<h2 property="title">The Title</h2>',
    +        'H2 tag with RDFa attribute without namespace',
    +        ['h2'],
    +      ],
    +      [
    +        '<h2 property="http://purl.org/dc/terms/title">The Title</h2>',
    +        '<h2 property="http://purl.org/dc/terms/title">The Title</h2>',
    +        'H2 tag with RDFa attribute with URL',
    +        ['h2'],
    +      ],
    +      [
    +        '<h2 property="javascript:alert(0);">The Title</h2>',
    +        '<h2 property="javascript:alert(0);">The Title</h2>',
    +        'H2 tag with RDFa attribute with XSS',
    +        ['h2'],
    

    these look to be duplicated in the test cases?

Will work on this tomorrow. If I don’t post anything in 3 days feel free to remove my name and work on.

#152

1 = Fixed
2 = Fixed
3 = Added comment
4 = Updated
5 = Fixed
6 = tried my best
7 = This was an existing test, only change I made was for javascript:alert(\'XSS\'); as content was marked a safe attribute
8 = This was an existing test, only change I made was for javascript:alert(\'XSS\'); as content was marked a safe attribute
9 = should it be removed?
10 = removed
11 = removed

Per a discussion with @larowlan removing "content" as one of the safe attributes and opened ✨ Investigate allowing "content" as allowed tag Active

Updated patch

The last submitted patch, 156: 2544110-156.patch, failed testing. View results →

1. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -29,6 +29,49 @@ class Xss {
   +    'rel',
   +    'rev',
   +    'about',
   +    'datatype',
   +    'datatype_callback',
   +    'datetime',
   +    'mailto',
   +    'media',
   +    'sizes',
   

   We don't have test coverage for all of these attributes - should we add it here? I realise there's some overlap with 🐛 Xss::filterAdmin() incorrectly filters datetime attribute Fixed

2. +++ b/core/lib/Drupal/Component/Utility/Xss.php
   @@ -229,16 +273,16 @@ protected static function attributes($attributes) {
   +              substr($attribute_name, 0, 3) === 'ng-' ||
   

   I think this was missed from my earlier review, can we get a comment regarding the significance of `ng-` - e.g. relates to Angular (I think?)

rel = is already included
rev = tests added
datetime = test added
mailto = test added
media = test added
sizes = test added

Removed some as I couldn't find examples
about = Removed
datatype = Removed
datatype_callback = Removed

#158.2 added to comment block in earlier patch.

Opened 📌 Considering making the XSS safe/unsafe attribute lists extendable Active as an additional follow-up, took the RDF list out of the issue summary since that's no long in the patch.

Needs re-roll for 🐛 Xss::filterAdmin() incorrectly filters datetime attribute Fixed .

Getting close, couple of questions and looks like at least one test case has the wrong label

1. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -507,6 +510,36 @@ public function providerTestAttributes() {
   +        'Link tag with rev attribute',
   

   I don't see a rev attribute here

2. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -507,6 +510,36 @@ public function providerTestAttributes() {
   +        'Link tag with mailto href',
   

   ah here is is, I think these titles are out of order

3. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -507,6 +510,36 @@ public function providerTestAttributes() {
   +        '<a media="print and (resolution:300dpi)">Drupal</a>',
   +        '<a media="print and (resolution:300dpi)">Drupal</a>',
   +        'Link tag with media attribute',
   +        ['a'],
   +      ],
   +      [
   +        '<a sizes="16x16">Drupal</a>',
   +        '<a sizes="16x16">Drupal</a>',
   

   are sizes and media valid attributes for an a tag? should we use img here or picture or something that reflects supported attributes?

4. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -507,6 +510,36 @@ public function providerTestAttributes() {
   +        '<time datetime="2017-02-14">',
   +        '<time datetime="2017-02-14">',
   +        'Time with datetime attribute',
   

   should we add a test-case with a : or did the related issue already resolve that? if so do we need this test case at all anymore?

5. +++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
   @@ -561,6 +594,68 @@ public function providerTestAttributes() {
   +        '<img src="http://example.com/foo.jpg" typeof="foaf:bad////value">',
   +        '<img src="http://example.com/foo.jpg" typeof="foaf:bad////value">',
   +        'Image tag with RDFa with bad with namespaced attribute',
   +        ['img'],
   +      ],
   +      [
   +        '<img src="http://example.com/foo.jpg" foo="bar:baz">',
   +        '<img src="http://example.com/foo.jpg" foo="baz">',
   +        'Image tag with non-RDFa attribute',
   

   I don't understand why one of these is stripped but the other isn't?

Not only attribute values are damaged, also attribute names.
Attribute aaa:bbb="X" becomes bbb="X".
Attribute aaa_bbb="X" becomes bbb="X".

E.g. try Xss::filter('<div aaa:bbb="AAA-BBB" bbb="BBB">&nbsp;</div>', Xss::getAdminTagList()).
This leads to a duplicate attribute `bbb=`.

You can argue that these are silly or invalid attribute names.
If this is the concern, the correct behavior would be to remove the attribute, _not_ to change the attribute name.

Interesting discussion: "What characters are allowed in an HTML attribute name?", https://stackoverflow.com/a/53563849/246724

Some things to consider.

• Custom html tags can have any custom attributes.
• Browsers typically accept custom attribute names even on basic html tags.
• Some legacy or poorly designed 3rd party systems might rely on poorly named attributes.
  In fact this is the case for me in a current project.

The culprit is definitely the attribute splitter function.
But to fix it, we need to define the desired behavior.

Review:

            if ($enforce_protocol_filtering || !$skip_protocol_filtering) {

These two variables always occur in this combination.
They can be combined into a single variable.

            $filter_protocols = in_array($attribute_name, static::$unSafeAttributes)
              || !(str_starts_with($attribute_name, 'data-')
                || str_starts_with($attribute_name, 'ng-')
                || in_array($attribute_name, static::$safeAttributes)
              );
[...]
            if ($filter_protocols) {

We also see that, with their default values, static::$unSafeAttributes has no effect whatsoever.
It would only have an effect if somebody adds a "data-" or "ng-" attribute to the unsafe list.

I wonder, do we support people modifying these class properties? Or could we use constants instead?

I wonder, do we support people modifying these class properties? Or could we use constants instead?

If we support them being modified, I would rather turn all the static methods to object methods, so that you can have different instances with different configuration.
For static methods, I would expect them to behave the same at all times.

Also,

  protected static function filterProtocol(string $name, string $value): string {
    // If the attribute is a safe attribute check that it doesn't contain a
    // protocol. If it does call UrlHelper::stripDangerousProtocols().
    if (in_array($name, static::$safeAttributes)) {

We now have two places where we compare the attribute name to a list:
One time in ::attributes() with case 0, another time in ::filterProtocol().
This can be collapsed to just one place.
Instead of passing the name to ::filterProtocol(), we can pass a filter mode.

But maybe the name parameter in ::filterProtocol() is meant for subclasses that want custom logic to determine the filter mode?
Do we care about inheritance?

despite several requests for comment, its still not clear why we need a special case for "ng-"

If we do need to special case it, hard coding it like this in its current location has a smell and suggests to me we haven't succeeded in making this "consistent" if such a special case for a framework is required.

While I haven't read every reply, I didn't see any results when I searched for ARIA in this issue, so adding my use case.

I got here because I'm trying to manipulate Views output to create accessible Read More links like described at https://www.w3.org/WAI/WCAG21/Techniques/aria/ARIA8 wherein I am adding an aria-label attribute with a token like <a href="/node/{{ nid }}" aria-label="Read more about {{ title }}">Read More</a> where the title in this case contains a colon. Everything in the ARIA Label up to and including the colon in the title is stripped.

Hide files

HI @ tommyk,

indeed. Got here by the same reason. The aria-* attributes may be 'whitelisted' as for the same reason 'data-' attributes are and perhaps a little more than 'ng-'.