diff --git a/wp-includes/blocks.php b/wp-includes/blocks.php
index 829d916bfd..1ee3536f69 100644
--- a/wp-includes/blocks.php
+++ b/wp-includes/blocks.php
@@ -74,11 +74,11 @@ function has_blocks( $post = null ) {
  * @since 5.0.0
  * @see parse_blocks()
  *
- * @param string                  $block_type Full Block type to look for.
+ * @param string                  $block_name Full Block type to look for.
  * @param int|string|WP_Post|null $post Optional. Post content, post ID, or post object. Defaults to global $post.
  * @return bool Whether the post content contains the specified block.
  */
-function has_block( $block_type, $post = null ) {
+function has_block( $block_name, $post = null ) {
 	if ( ! has_blocks( $post ) ) {
 		return false;
 	}
@@ -90,7 +90,30 @@ function has_block( $block_type, $post = null ) {
 		}
 	}
 
-	return false !== strpos( $post, '<!-- wp:' . $block_type . ' ' );
+	/*
+	 * Normalize block name to include namespace, if provided as non-namespaced.
+	 * This matches behavior for WordPress 5.0.0 - 5.3.0 in matching blocks by
+	 * their serialized names.
+	 */
+	if ( false === strpos( $block_name, '/' ) ) {
+		$block_name = 'core/' . $block_name;
+	}
+
+	// Test for existence of block by its fully qualified name.
+	$has_block = false !== strpos( $post, '<!-- wp:' . $block_name . ' ' );
+
+	if ( ! $has_block ) {
+		/*
+		 * If the given block name would serialize to a different name, test for
+		 * existence by the serialized form.
+		 */
+		$serialized_block_name = strip_core_block_namespace( $block_name );
+		if ( $serialized_block_name !== $block_name ) {
+			$has_block = false !== strpos( $post, '<!-- wp:' . $serialized_block_name . ' ' );
+		}
+	}
+
+	return $has_block;
 }
 
 /**
@@ -113,6 +136,207 @@ function get_dynamic_block_names() {
 	return $dynamic_block_names;
 }
 
+/**
+ * Given an array of attributes, returns a string in the serialized attributes
+ * format prepared for post content.
+ *
+ * The serialized result is a JSON-encoded string, with unicode escape sequence
+ * substitution for characters which might otherwise interfere with embedding
+ * the result in an HTML comment.
+ *
+ * @since 5.3.1
+ *
+ * @param array $attributes Attributes object.
+ * @return string Serialized attributes.
+ */
+function serialize_block_attributes( $block_attributes ) {
+	$encoded_attributes = json_encode( $block_attributes );
+	$encoded_attributes = preg_replace( '/--/', '\\u002d\\u002d', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/</', '\\u003c', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/>/', '\\u003e', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/&/', '\\u0026', $encoded_attributes );
+	// Regex: /\\"/
+	$encoded_attributes = preg_replace( '/\\\\"/', '\\u0022', $encoded_attributes );
+
+	return $encoded_attributes;
+}
+
+/**
+ * Returns the block name to use for serialization. This will remove the default
+ * "core/" namespace from a block name.
+ *
+ * @since 5.3.1
+ *
+ * @param string $block_name Original block name.
+ * @return string Block name to use for serialization.
+ */
+function strip_core_block_namespace( $block_name = null ) {
+	if ( is_string( $block_name ) && 0 === strpos( $block_name, 'core/' ) ) {
+		return substr( $block_name, 5 );
+	}
+
+	return $block_name;
+}
+
+/**
+ * Returns the content of a block, including comment delimiters.
+ *
+ * @since 5.3.1
+ *
+ * @param string $block_name Block name.
+ * @param array  $attributes Block attributes.
+ * @param string $content    Block save content.
+ * @return string Comment-delimited block content.
+ */
+function get_comment_delimited_block_content( $block_name = null, $block_attributes, $block_content ) {
+	if ( is_null( $block_name ) ) {
+		return $block_content;
+	}
+
+	$serialized_block_name = strip_core_block_namespace( $block_name );
+	$serialized_attributes = empty( $block_attributes ) ? '' : serialize_block_attributes( $block_attributes ) . ' ';
+
+	if ( empty( $block_content ) ) {
+		return sprintf( '<!-- wp:%s %s/-->', $serialized_block_name, $serialized_attributes );
+	}
+
+	return sprintf(
+		'<!-- wp:%s %s-->%s<!-- /wp:%s -->',
+		$serialized_block_name,
+		$serialized_attributes,
+		$block_content,
+		$serialized_block_name
+	);
+}
+
+/**
+ * Returns the content of a block, including comment delimiters, serializing all
+ * attributes from the given parsed block.
+ *
+ * This should be used when preparing a block to be saved to post content.
+ * Prefer `render_block` when preparing a block for display. Unlike
+ * `render_block`, this does not evaluate a block's `render_callback`, and will
+ * instead preserve the markup as parsed.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block $block A single parsed block object.
+ * @return string String of rendered HTML.
+ */
+function serialize_block( $block ) {
+	$block_content = '';
+
+	$index = 0;
+	foreach ( $block['innerContent'] as $chunk ) {
+		$block_content .= is_string( $chunk ) ? $chunk : serialize_block( $block['innerBlocks'][ $index++ ] );
+	}
+
+	if ( ! is_array( $block['attrs'] ) ) {
+		$block['attrs'] = array();
+	}
+
+	return get_comment_delimited_block_content(
+		$block['blockName'],
+		$block['attrs'],
+		$block_content
+	);
+}
+
+/**
+ * Returns a joined string of the aggregate serialization of the given parsed
+ * blocks.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block[] $blocks Parsed block objects.
+ * @return string String of rendered HTML.
+ */
+function serialize_blocks( $blocks ) {
+	return implode( '', array_map( 'serialize_block', $blocks ) );
+}
+
+/**
+ * Filters and sanitizes block content to remove non-allowable HTML from
+ * parsed block attribute values.
+ *
+ * @since 5.3.1
+ *
+ * @param string         $text              Text that may contain block content.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return string The filtered and sanitized content result.
+ */
+function filter_block_content( $text, $allowed_html = 'post', $allowed_protocols = array() ) {
+	$result = '';
+
+	$blocks = parse_blocks( $text );
+	foreach ( $blocks as $block ) {
+		$block   = filter_block_kses( $block, $allowed_html, $allowed_protocols );
+		$result .= serialize_block( $block );
+	}
+
+	return $result;
+}
+
+/**
+ * Filters and sanitizes a parsed block to remove non-allowable HTML from block
+ * attribute values.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block $block             The parsed block object.
+ * @param array[]|string        $allowed_html      An array of allowed HTML
+ *                                                 elements and attributes, or a
+ *                                                 context name such as 'post'.
+ * @param string[]              $allowed_protocols Allowed URL protocols.
+ * @return array The filtered and sanitized block object result.
+ */
+function filter_block_kses( $block, $allowed_html, $allowed_protocols = array() ) {
+	$block['attrs'] = filter_block_kses_value( $block['attrs'], $allowed_html, $allowed_protocols );
+
+	if ( is_array( $block['innerBlocks'] ) ) {
+		foreach ( $block['innerBlocks'] as $i => $inner_block ) {
+			$block['innerBlocks'][ $i ] = filter_block_kses( $inner_block, $allowed_html, $allowed_protocols );
+		}
+	}
+
+	return $block;
+}
+
+/**
+ * Filters and sanitizes a parsed block attribute value to remove non-allowable
+ * HTML.
+ *
+ * @since 5.3.1
+ *
+ * @param mixed          $value             The attribute value to filter.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return array The filtered and sanitized result.
+ */
+function filter_block_kses_value( $value, $allowed_html, $allowed_protocols = array() ) {
+	if ( is_array( $value ) ) {
+		foreach ( $value as $key => $inner_value ) {
+			$filtered_key   = filter_block_kses_value( $key, $allowed_html, $allowed_protocols );
+			$filtered_value = filter_block_kses_value( $inner_value, $allowed_html, $allowed_protocols );
+
+			if ( $filtered_key !== $key ) {
+				unset( $value[ $key ] );
+			}
+
+			$value[ $filtered_key ] = $filtered_value;
+		}
+	} elseif ( is_string( $value ) ) {
+		return wp_kses( $value, $allowed_html, $allowed_protocols );
+	}
+
+	return $value;
+}
+
 /**
  * Parses blocks out of a content string, and renders those appropriate for the excerpt.
  *
diff --git a/wp-includes/default-filters.php b/wp-includes/default-filters.php
index 1319826212..6379a55d24 100644
--- a/wp-includes/default-filters.php
+++ b/wp-includes/default-filters.php
@@ -245,6 +245,7 @@ add_filter( 'option_siteurl', '_config_wp_siteurl' );
 add_filter( 'tiny_mce_before_init', '_mce_set_direction' );
 add_filter( 'teeny_mce_before_init', '_mce_set_direction' );
 add_filter( 'pre_kses', 'wp_pre_kses_less_than' );
+add_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10, 3 );
 add_filter( 'sanitize_title', 'sanitize_title_with_dashes', 10, 3 );
 add_action( 'check_comment_flood', 'check_comment_flood_db', 10, 4 );
 add_filter( 'comment_flood_filter', 'wp_throttle_comment_flood', 10, 3 );
diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index d11700779b..a7ecf5b310 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -3041,8 +3041,26 @@ function wp_rel_nofollow_callback( $matches ) {
  */
 function wp_targeted_link_rel( $text ) {
 	// Don't run (more expensive) regex if no links with targets.
-	if ( stripos( $text, 'target' ) !== false && stripos( $text, '<a ' ) !== false ) {
-		$text = preg_replace_callback( '|<a\s([^>]*target\s*=[^>]*)>|i', 'wp_targeted_link_rel_callback', $text );
+	if ( stripos( $text, 'target' ) === false || stripos( $text, '<a ' ) === false || is_serialized( $text ) ) {
+		return $text;
+	}
+
+	$script_and_style_regex = '/<(script|style).*?<\/\\1>/si';
+
+	preg_match_all( $script_and_style_regex, $text, $matches );
+	$extra_parts = $matches[0];
+	$html_parts  = preg_split( $script_and_style_regex, $text );
+
+	foreach ( $html_parts as &$part ) {
+		$part = preg_replace_callback( '|<a\s([^>]*target\s*=[^>]*)>|i', 'wp_targeted_link_rel_callback', $part );
+	}
+
+	$text = '';
+	for ( $i = 0; $i < count( $html_parts ); $i++ ) {
+		$text .= $html_parts[ $i ];
+		if ( isset( $extra_parts[ $i ] ) ) {
+			$text .= $extra_parts[ $i ];
+		}
 	}
 
 	return $text;
@@ -3058,8 +3076,17 @@ function wp_targeted_link_rel( $text ) {
  * @return string HTML A Element with rel noreferrer noopener in addition to any existing values
  */
 function wp_targeted_link_rel_callback( $matches ) {
-	$link_html = $matches[1];
-	$rel_match = array();
+	$link_html          = $matches[1];
+	$original_link_html = $link_html;
+
+	// Consider the html escaped if there are no unescaped quotes
+	$is_escaped = ! preg_match( '/(^|[^\\\\])[\'"]/', $link_html );
+	if ( $is_escaped ) {
+		// Replace only the quotes so that they are parsable by wp_kses_hair, leave the rest as is
+		$link_html = preg_replace( '/\\\\([\'"])/', '$1', $link_html );
+	}
+
+	$atts = wp_kses_hair( $link_html, wp_allowed_protocols() );
 
 	/**
 	 * Filters the rel values that are added to links with `target` attribute.
@@ -3071,31 +3098,21 @@ function wp_targeted_link_rel_callback( $matches ) {
 	 */
 	$rel = apply_filters( 'wp_targeted_link_rel', 'noopener noreferrer', $link_html );
 
-	// Avoid additional regex if the filter removes rel values.
-	if ( ! $rel ) {
-		return "<a $link_html>";
+	// Return early if no rel values to be added or if no actual target attribute
+	if ( ! $rel || ! isset( $atts['target'] ) ) {
+		return "<a $original_link_html>";
 	}
 
-	// Value with delimiters, spaces around are optional.
-	$attr_regex = '|rel\s*=\s*?(\\\\{0,1}["\'])(.*?)\\1|i';
-	preg_match( $attr_regex, $link_html, $rel_match );
-
-	if ( empty( $rel_match[0] ) ) {
-		// No delimiters, try with a single value and spaces, because `rel =  va"lue` is totally fine...
-		$attr_regex = '|rel\s*=(\s*)([^\s]*)|i';
-		preg_match( $attr_regex, $link_html, $rel_match );
+	if ( isset( $atts['rel'] ) ) {
+		$all_parts = preg_split( '/\s/', "{$atts['rel']['value']} $rel", -1, PREG_SPLIT_NO_EMPTY );
+		$rel       = implode( ' ', array_unique( $all_parts ) );
 	}
 
-	if ( ! empty( $rel_match[0] ) ) {
-		$parts     = preg_split( '|\s+|', strtolower( $rel_match[2] ) );
-		$parts     = array_map( 'esc_attr', $parts );
-		$needed    = explode( ' ', $rel );
-		$parts     = array_unique( array_merge( $parts, $needed ) );
-		$delimiter = trim( $rel_match[1] ) ? $rel_match[1] : '"';
-		$rel       = 'rel=' . $delimiter . trim( implode( ' ', $parts ) ) . $delimiter;
-		$link_html = str_replace( $rel_match[0], $rel, $link_html );
-	} else {
-		$link_html .= " rel=\"$rel\"";
+	$atts['rel']['whole'] = 'rel="' . esc_attr( $rel ) . '"';
+	$link_html            = join( ' ', array_column( $atts, 'whole' ) );
+
+	if ( $is_escaped ) {
+		$link_html = preg_replace( '/[\'"]/', '\\\\$0', $link_html );
 	}
 
 	return "<a $link_html>";
@@ -4796,6 +4813,31 @@ function wp_pre_kses_less_than_callback( $matches ) {
 	return $matches[0];
 }
 
+/**
+ * Remove non-allowable HTML from parsed block attribute values when filtering
+ * in the post context.
+ *
+ * @since 5.3.1
+ *
+ * @param string         $string            Content to be run through KSES.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return string Filtered text to run through KSES.
+ */
+function wp_pre_kses_block_attributes( $string, $allowed_html, $allowed_protocols ) {
+	/*
+	 * `filter_block_content` is expected to call `wp_kses`. Temporarily remove
+	 * the filter to avoid recursion.
+	 */
+	remove_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10 );
+	$string = filter_block_content( $string, $allowed_html, $allowed_protocols );
+	add_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10, 3 );
+
+	return $string;
+}
+
 /**
  * WordPress implementation of PHP sprintf() with filters.
  *
diff --git a/wp-includes/kses.php b/wp-includes/kses.php
index 4bbc5aeed5..78d55ae6b1 100644
--- a/wp-includes/kses.php
+++ b/wp-includes/kses.php
@@ -1658,7 +1658,7 @@ function wp_kses_html_error( $string ) {
  */
 function wp_kses_bad_protocol_once( $string, $allowed_protocols, $count = 1 ) {
 	$string  = preg_replace( '/(&#0*58(?![;0-9])|&#x0*3a(?![;a-f0-9]))/i', '$1;', $string );
-	$string2 = preg_split( '/:|&#0*58;|&#x0*3a;/i', $string, 2 );
+	$string2 = preg_split( '/:|&#0*58;|&#x0*3a;|&colon;/i', $string, 2 );
 	if ( isset( $string2[1] ) && ! preg_match( '%/\?%', $string2[0] ) ) {
 		$string   = trim( $string2[1] );
 		$protocol = wp_kses_bad_protocol_once2( $string2[0], $allowed_protocols );
diff --git a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
index bc0a179f7f..5e756231a4 100644
--- a/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
+++ b/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php
@@ -499,7 +499,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			return new WP_Error( 'rest_cannot_edit_others', __( 'Sorry, you are not allowed to create posts as this user.' ), array( 'status' => rest_authorization_required_code() ) );
 		}
 
-		if ( ! empty( $request['sticky'] ) && ! current_user_can( $post_type->cap->edit_others_posts ) ) {
+		if ( ! empty( $request['sticky'] ) && ! current_user_can( $post_type->cap->edit_others_posts ) && ! current_user_can( $post_type->cap->publish_posts ) ) {
 			return new WP_Error( 'rest_cannot_assign_sticky', __( 'Sorry, you are not allowed to make posts sticky.' ), array( 'status' => rest_authorization_required_code() ) );
 		}
 
@@ -654,7 +654,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			return new WP_Error( 'rest_cannot_edit_others', __( 'Sorry, you are not allowed to update posts as this user.' ), array( 'status' => rest_authorization_required_code() ) );
 		}
 
-		if ( ! empty( $request['sticky'] ) && ! current_user_can( $post_type->cap->edit_others_posts ) ) {
+		if ( ! empty( $request['sticky'] ) && ! current_user_can( $post_type->cap->edit_others_posts ) && ! current_user_can( $post_type->cap->publish_posts ) ) {
 			return new WP_Error( 'rest_cannot_assign_sticky', __( 'Sorry, you are not allowed to make posts sticky.' ), array( 'status' => rest_authorization_required_code() ) );
 		}
 
@@ -956,7 +956,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 	 * @return stdClass|WP_Error Post object or WP_Error.
 	 */
 	protected function prepare_item_for_database( $request ) {
-		$prepared_post = new stdClass;
+		$prepared_post = new stdClass();
 
 		// Post ID.
 		if ( isset( $request['id'] ) ) {