[HtmlSanitizer] Introduce HtmlSanitizer component

Author: tgalopin

.gitattributes

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+/Tests export-ignore
+/phpunit.xml.dist export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore

.gitignore

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+composer.lock
+phpunit.xml
+vendor/

CHANGELOG.md

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+CHANGELOG
+=========
+
+6.1
+---
+
+ * Add the component as experimental

HtmlSanitizer.php

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+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\HtmlSanitizer;
+
+use Symfony\Component\HtmlSanitizer\Parser\MastermindsParser;
+use Symfony\Component\HtmlSanitizer\Parser\ParserInterface;
+use Symfony\Component\HtmlSanitizer\Reference\W3CReference;
+use Symfony\Component\HtmlSanitizer\TextSanitizer\StringSanitizer;
+use Symfony\Component\HtmlSanitizer\Visitor\DomVisitor;
+
+/**
+ * @author Titouan Galopin <[email protected]>
+ *
+ * @experimental
+ */
+final class HtmlSanitizer implements HtmlSanitizerInterface
+{
+    private HtmlSanitizerConfig $config;
+    private int $maxInputLength;
+    private ParserInterface $parser;
+
+    /**
+     * @var array<string, DomVisitor>
+     */
+    private array $domVisitors = [];
+
+    public function __construct(HtmlSanitizerConfig $config, int $maxInputLength = 20000, ParserInterface $parser = null)
+    {
+        $this->config = $config;
+        $this->maxInputLength = $maxInputLength;
+        $this->parser = $parser ?? new MastermindsParser();
+    }
+
+    public function sanitize(string $input): string
+    {
+        return $this->sanitizeWithContext(W3CReference::CONTEXT_BODY, $input);
+    }
+
+    public function sanitizeFor(string $element, string $input): string
+    {
+        return $this->sanitizeWithContext(
+            W3CReference::CONTEXTS_MAP[StringSanitizer::htmlLower($element)] ?? W3CReference::CONTEXT_BODY,
+            $input
+        );
+    }
+
+    private function sanitizeWithContext(string $context, string $input): string
+    {
+        // Text context: early return with HTML encoding
+        if (W3CReference::CONTEXT_TEXT === $context) {
+            return StringSanitizer::encodeHtmlEntities($input);
+        }
+
+        // Other context: build a DOM visitor
+        $this->domVisitors[$context] ??= $this->createDomVisitorForContext($context);
+
+        // Prevent DOS attack induced by extremely long HTML strings
+        if (\strlen($input) > $this->maxInputLength) {
+            $input = substr($input, 0, $this->maxInputLength);
+        }
+
+        // Only operate on valid UTF-8 strings. This is necessary to prevent cross
+        // site scripting issues on Internet Explorer 6. Idea from Drupal (filter_xss).
+        if (!$this->isValidUtf8($input)) {
+            return '';
+        }
+
+        // Remove NULL character
+        $input = str_replace(\chr(0), '', $input);
+
+        // Parse as HTML
+        if (!$parsed = $this->parser->parse($input)) {
+            return '';
+        }
+
+        // Visit the DOM tree and render the sanitized nodes
+        return $this->domVisitors[$context]->visit($parsed)?->render() ?? '';
+    }
+
+    private function isValidUtf8(string $html): bool
+    {
+        // preg_match() fails silently on strings containing invalid UTF-8.
+        return '' === $html || preg_match('//u', $html);
+    }
+
+    private function createDomVisitorForContext(string $context): DomVisitor
+    {
+        $elementsConfig = [];
+
+        // Head: only a few elements are allowed
+        if (W3CReference::CONTEXT_HEAD === $context) {
+            foreach ($this->config->getAllowedElements() as $allowedElement => $allowedAttributes) {
+                if (\array_key_exists($allowedElement, W3CReference::HEAD_ELEMENTS)) {
+                    $elementsConfig[$allowedElement] = $allowedAttributes;
+                }
+            }
+
+            foreach ($this->config->getBlockedElements() as $blockedElement => $v) {
+                if (\array_key_exists($blockedElement, W3CReference::HEAD_ELEMENTS)) {
+                    $elementsConfig[$blockedElement] = false;
+                }
+            }
+
+            return new DomVisitor($this->config, $elementsConfig);
+        }
+
+        // Body: allow any configured element that isn't in <head>
+        foreach ($this->config->getAllowedElements() as $allowedElement => $allowedAttributes) {
+            if (!\array_key_exists($allowedElement, W3CReference::HEAD_ELEMENTS)) {
+                $elementsConfig[$allowedElement] = $allowedAttributes;
+            }
+        }
+
+        foreach ($this->config->getBlockedElements() as $blockedElement => $v) {
+            if (!\array_key_exists($blockedElement, W3CReference::HEAD_ELEMENTS)) {
+                $elementsConfig[$blockedElement] = false;
+            }
+        }
+
+        return new DomVisitor($this->config, $elementsConfig);
+    }
+}