diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index 4ab8d11e24..067249a510 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -3754,6 +3754,11 @@ function _deep_replace( $search, $subject ) {
  * Sometimes, spot-escaping is required or useful. One example
  * is preparing an array for use in an IN clause.
  *
+ * NOTE: Since 4.8.3, '%' characters will be replaced with a placeholder string,
+ * this prevents certain SQLi attacks from taking place. This change in behaviour
+ * may cause issues for code that expects the return value of esc_sql() to be useable
+ * for other purposes.
+ *
  * @since 2.8.0
  *
  * @global wpdb $wpdb WordPress database abstraction object.
diff --git a/wp-includes/meta.php b/wp-includes/meta.php
index a9e8f987f9..1aca31a62c 100644
--- a/wp-includes/meta.php
+++ b/wp-includes/meta.php
@@ -364,12 +364,11 @@ function delete_metadata($meta_type, $object_id, $meta_key, $meta_value = '', $d
 		return false;
 
 	if ( $delete_all ) {
-		$value_clause = '';
 		if ( '' !== $meta_value && null !== $meta_value && false !== $meta_value ) {
-			$value_clause = $wpdb->prepare( " AND meta_value = %s", $meta_value );
+			$object_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s AND meta_value = %s", $meta_key, $meta_value ) );
+		} else {
+			$object_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s", $meta_key ) );
 		}
-
-		$object_ids = $wpdb->get_col( $wpdb->prepare( "SELECT $type_column FROM $table WHERE meta_key = %s $value_clause", $meta_key ) );
 	}
 
 	/**
diff --git a/wp-includes/post.php b/wp-includes/post.php
index d82e8a4d40..1dbf0c9867 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -4317,10 +4317,10 @@ function get_page_by_path( $page_path, $output = OBJECT, $post_type = 'page' ) {
 	$page_path = str_replace('%2F', '/', $page_path);
 	$page_path = str_replace('%20', ' ', $page_path);
 	$parts = explode( '/', trim( $page_path, '/' ) );
-	$parts = esc_sql( $parts );
 	$parts = array_map( 'sanitize_title_for_query', $parts );
+	$escaped_parts = esc_sql( $parts );
 
-	$in_string = "'" . implode( "','", $parts ) . "'";
+	$in_string = "'" . implode( "','", $escaped_parts ) . "'";
 
 	if ( is_array( $post_type ) ) {
 		$post_types = $post_type;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index ada4e25d77..14841aab4c 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-RC1-42050';
+$wp_version = '4.9-RC1-42056';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-includes/wp-db.php b/wp-includes/wp-db.php
index 3f7d5ac8b2..e7ec314e7b 100644
--- a/wp-includes/wp-db.php
+++ b/wp-includes/wp-db.php
@@ -1106,20 +1106,22 @@ class wpdb {
 	function _real_escape( $string ) {
 		if ( $this->dbh ) {
 			if ( $this->use_mysqli ) {
-				return mysqli_real_escape_string( $this->dbh, $string );
+				$escaped = mysqli_real_escape_string( $this->dbh, $string );
 			} else {
-				return mysql_real_escape_string( $string, $this->dbh );
+				$escaped = mysql_real_escape_string( $string, $this->dbh );
 			}
+		} else {
+			$class = get_class( $this );
+			if ( function_exists( '__' ) ) {
+				/* translators: %s: database access abstraction class, usually wpdb or a class extending wpdb */
+				_doing_it_wrong( $class, sprintf( __( '%s must set a database connection for use with escaping.' ), $class ), '3.6.0' );
+			} else {
+				_doing_it_wrong( $class, sprintf( '%s must set a database connection for use with escaping.', $class ), '3.6.0' );
+			}
+			$escaped = addslashes( $string );
 		}
 
-		$class = get_class( $this );
-		if ( function_exists( '__' ) ) {
-			/* translators: %s: database access abstraction class, usually wpdb or a class extending wpdb */
-			_doing_it_wrong( $class, sprintf( __( '%s must set a database connection for use with escaping.' ), $class ), '3.6.0' );
-		} else {
-			_doing_it_wrong( $class, sprintf( '%s must set a database connection for use with escaping.', $class ), '3.6.0' );
-		}
-		return addslashes( $string );
+		return $this->add_placeholder_escape( $escaped );
 	}
 
 	/**
@@ -1201,14 +1203,14 @@ class wpdb {
 	 *
 	 * All placeholders MUST be left unquoted in the query string. A corresponding argument MUST be passed for each placeholder.
 	 *
+	 * For compatibility with old behavior, numbered or formatted string placeholders (eg, %1$s, %5s) will not have quotes
+	 * added by this function, so should be passed with appropriate quotes around them for your usage.
+	 *
 	 * Literal percentage signs (%) in the query string must be written as %%. Percentage wildcards (for example,
 	 * to use in LIKE syntax) must be passed via a substitution argument containing the complete LIKE string, these
 	 * cannot be inserted directly in the query string. Also see {@see esc_like()}.
 	 *
-	 * This method DOES NOT support sign, padding, alignment, width or precision specifiers.
-	 * This method DOES NOT support argument numbering or swapping.
-	 *
-	 * Arguments may be passed as individual arguments to the method, or as a single array containing all arguments. A combination 
+	 * Arguments may be passed as individual arguments to the method, or as a single array containing all arguments. A combination
 	 * of the two is not supported.
 	 *
 	 * Examples:
@@ -1225,8 +1227,9 @@ class wpdb {
 	 * @return string|void Sanitized query string, if there is a query to prepare.
 	 */
 	public function prepare( $query, $args ) {
-		if ( is_null( $query ) )
+		if ( is_null( $query ) ) {
 			return;
+		}
 
 		// This is not meant to be foolproof -- but it will catch obviously incorrect usage.
 		if ( strpos( $query, '%' ) === false ) {
@@ -1237,8 +1240,10 @@ class wpdb {
 		$args = func_get_args();
 		array_shift( $args );
 
-		// If args were passed as an array (as in vsprintf), move them up
+		// If args were passed as an array (as in vsprintf), move them up.
+		$passed_as_array = false;
 		if ( is_array( $args[0] ) && count( $args ) == 1 ) {
+			$passed_as_array = true;
 			$args = $args[0];
 		}
 
@@ -1249,28 +1254,62 @@ class wpdb {
 			}
 		}
 
-		$query = str_replace( "'%s'", '%s', $query ); // in case someone mistakenly already singlequoted it
-		$query = str_replace( '"%s"', '%s', $query ); // doublequote unquoting
-		$query = preg_replace( '|(?<!%)%f|' , '%F', $query ); // Force floats to be locale unaware
-		$query = preg_replace( '|(?<!%)%s|', "'%s'", $query ); // quote the strings, avoiding escaped strings like %%s
-		$query = preg_replace( '/%(?:%|$|([^dsF]))/', '%%\\1', $query ); // escape any unescaped percents
+		/*
+		 * Specify the formatting allowed in a placeholder. The following are allowed:
+		 *
+		 * - Sign specifier. eg, $+d
+		 * - Numbered placeholders. eg, %1$s
+		 * - Padding specifier, including custom padding characters. eg, %05s, %'#5s
+		 * - Alignment specifier. eg, %05-s
+		 * - Precision specifier. eg, %.2f
+		 */
+		$allowed_format = '(?:[1-9][0-9]*[$])?[-+0-9]*(?: |0|\'.)?[-+0-9]*(?:\.[0-9]+)?';
 
-		// Count the number of valid placeholders in the query
-		$placeholders = preg_match_all( '/(^|[^%]|(%%)+)%[sdF]/', $query, $matches );
+		/*
+		 * If a %s placeholder already has quotes around it, removing the existing quotes and re-inserting them
+		 * ensures the quotes are consistent.
+		 *
+		 * For backwards compatibility, this is only applied to %s, and not to placeholders like %1$s, which are frequently
+		 * used in the middle of longer strings, or as table name placeholders.
+		 */
+		$query = str_replace( "'%s'", '%s', $query ); // Strip any existing single quotes.
+		$query = str_replace( '"%s"', '%s', $query ); // Strip any existing double quotes.
+		$query = preg_replace( '/(?<!%)%s/', "'%s'", $query ); // Quote the strings, avoiding escaped strings like %%s.
 
-		if ( count ( $args ) !== $placeholders ) {
-			wp_load_translations_early();
-			_doing_it_wrong( 'wpdb::prepare',
-				/* translators: 1: number of placeholders, 2: number of arguments passed */
-				sprintf( __( 'The query does not contain the correct number of placeholders (%1$d) for the number of arguments passed (%2$d).' ),
-					$placeholders,
-					count( $args ) ),
-				'4.9.0'
-			);
+		$query = preg_replace( "/(?<!%)(%($allowed_format)?f)/" , '%\\2F', $query ); // Force floats to be locale unaware.
+
+		$query = preg_replace( "/%(?:%|$|(?!($allowed_format)?[sdF]))/", '%%\\1', $query ); // Escape any unescaped percents.
+
+		// Count the number of valid placeholders in the query.
+		$placeholders = preg_match_all( "/(^|[^%]|(%%)+)%($allowed_format)?[sdF]/", $query, $matches );
+
+		if ( count( $args ) !== $placeholders ) {
+			if ( 1 === $placeholders && $passed_as_array ) {
+				// If the passed query only expected one argument, but the wrong number of arguments were sent as an array, bail.
+				wp_load_translations_early();
+				_doing_it_wrong( 'wpdb::prepare', __( 'The query only expected one placeholder, but an array of multiple placeholders was sent.' ), '4.9.0' );
+
+				return;
+			} else {
+				/*
+				 * If we don't have the right number of placeholders, but they were passed as individual arguments,
+				 * or we were expecting multiple arguments in an array, throw a warning.
+				 */
+				wp_load_translations_early();
+				_doing_it_wrong( 'wpdb::prepare',
+					/* translators: 1: number of placeholders, 2: number of arguments passed */
+					sprintf( __( 'The query does not contain the correct number of placeholders (%1$d) for the number of arguments passed (%2$d).' ),
+						$placeholders,
+						count( $args ) ),
+					'4.8.3'
+				);
+			}
 		}
 
 		array_walk( $args, array( $this, 'escape_by_ref' ) );
-		return @vsprintf( $query, $args );
+		$query = @vsprintf( $query, $args );
+
+		return $this->add_placeholder_escape( $query );
 	}
 
 	/**
@@ -1893,6 +1932,64 @@ class wpdb {
 		}
 	}
 
+	/**
+	 * Generates and returns a placeholder escape string for use in queries returned by ::prepare().
+	 *
+	 * @since 4.8.3
+	 *
+	 * @return string String to escape placeholders.
+	 */
+	public function placeholder_escape() {
+		static $placeholder;
+
+		if ( ! $placeholder ) {
+			// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
+			$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
+			// Old WP installs may not have AUTH_SALT defined.
+			$salt = defined( 'AUTH_SALT' ) ? AUTH_SALT : rand();
+
+			$placeholder = '{' . hash_hmac( $algo, uniqid( $salt, true ), $salt ) . '}';
+		}
+
+		/*
+		 * Add the filter to remove the placeholder escaper. Uses priority 0, so that anything
+		 * else attached to this filter will recieve the query with the placeholder string removed.
+		 */
+		if ( ! has_filter( 'query', array( $this, 'remove_placeholder_escape' ) ) ) {
+			add_filter( 'query', array( $this, 'remove_placeholder_escape' ), 0 );
+		}
+
+		return $placeholder;
+	}
+
+	/**
+	 * Adds a placeholder escape string, to escape anything that resembles a printf() placeholder.
+	 *
+	 * @since 4.8.3
+	 *
+	 * @param string $query The query to escape.
+	 * @return string The query with the placeholder escape string inserted where necessary.
+	 */
+	public function add_placeholder_escape( $query ) {
+		/*
+		 * To prevent returning anything that even vaguely resembles a placeholder,
+		 * we clobber every % we can find.
+		 */
+		return str_replace( '%', $this->placeholder_escape(), $query );
+	}
+
+	/**
+	 * Removes the placeholder escape strings from a query.
+	 *
+	 * @since 4.8.3
+	 *
+	 * @param string $query The query from which the placeholder will be removed.
+	 * @return string The query with the placeholder removed.
+	 */
+	public function remove_placeholder_escape( $query ) {
+		return str_replace( $this->placeholder_escape(), '%', $query );
+	}
+
 	/**
 	 * Insert a row into a table.
 	 *
@@ -2064,7 +2161,7 @@ class wpdb {
 		$conditions = implode( ' AND ', $conditions );
 
 		$sql = "UPDATE `$table` SET $fields WHERE $conditions";
-		
+
 		$this->check_current_query = false;
 		return $this->query( $this->prepare( $sql, $values ) );
 	}