diff --git a/wp-activate.php b/wp-activate.php
index 7c59656045..db0d951496 100644
--- a/wp-activate.php
+++ b/wp-activate.php
@@ -26,7 +26,9 @@ $activate_cookie = 'wp-activate-' . COOKIEHASH;
 $key    = '';
 $result = null;
 
-if ( ! empty( $_GET['key'] ) ) {
+if ( isset( $_GET['key'] ) && isset( $_POST['key'] ) && $_GET['key'] !== $_POST['key'] ) {
+	wp_die( __( 'A key value mismatch has been detected. Please follow the link provided in your activation email.' ), __( 'An error occurred during the activation' ), 400 );
+} elseif ( ! empty( $_GET['key'] ) ) {
 	$key = $_GET['key'];
 } elseif ( ! empty( $_POST['key'] ) ) {
 	$key = $_POST['key'];
diff --git a/wp-admin/includes/screen.php b/wp-admin/includes/screen.php
index 4161a8e807..aec26c97b4 100644
--- a/wp-admin/includes/screen.php
+++ b/wp-admin/includes/screen.php
@@ -449,7 +449,9 @@ final class WP_Screen {
 
 			switch ( $base ) {
 				case 'post' :
-					if ( isset( $_GET['post'] ) )
+					if ( isset( $_GET['post'] ) && isset( $_POST['post_ID'] ) && (int) $_GET['post'] !== (int) $_POST['post_ID'] )
+						wp_die( __( 'A post ID mismatch has been detected.' ), __( 'Sorry, you are not allowed to edit this item.' ), 400 );
+					elseif ( isset( $_GET['post'] ) )
 						$post_id = (int) $_GET['post'];
 					elseif ( isset( $_POST['post_ID'] ) )
 						$post_id = (int) $_POST['post_ID'];
diff --git a/wp-admin/post.php b/wp-admin/post.php
index 8e2717cd8f..885bbddfec 100644
--- a/wp-admin/post.php
+++ b/wp-admin/post.php
@@ -16,7 +16,9 @@ $submenu_file = 'edit.php';
 
 wp_reset_vars( array( 'action' ) );
 
-if ( isset( $_GET['post'] ) )
+if ( isset( $_GET['post'] ) && isset( $_POST['post_ID'] ) && (int) $_GET['post'] !== (int) $_POST['post_ID'] )
+	wp_die( __( 'A post ID mismatch has been detected.' ), __( 'Sorry, you are not allowed to edit this item.' ), 400 );
+elseif ( isset( $_GET['post'] ) )
  	$post_id = $post_ID = (int) $_GET['post'];
 elseif ( isset( $_POST['post_ID'] ) )
  	$post_id = $post_ID = (int) $_POST['post_ID'];
@@ -82,6 +84,10 @@ function redirect_post($post_id = '') {
 	exit;
 }
 
+if ( isset( $_POST['post_type'] ) && $post && $post_type !== $_POST['post_type'] ) {
+	wp_die( __( 'A post type mismatch has been detected.' ), __( 'Sorry, you are not allowed to edit this item.' ), 400 );
+}
+
 if ( isset( $_POST['deletepost'] ) )
 	$action = 'delete';
 elseif ( isset($_POST['wp-preview']) && 'dopreview' == $_POST['wp-preview'] )
diff --git a/wp-includes/class-wp.php b/wp-includes/class-wp.php
index 4edcaf7307..de489fafcc 100644
--- a/wp-includes/class-wp.php
+++ b/wp-includes/class-wp.php
@@ -265,6 +265,8 @@ class WP {
 		foreach ( $this->public_query_vars as $wpvar ) {
 			if ( isset( $this->extra_query_vars[$wpvar] ) )
 				$this->query_vars[$wpvar] = $this->extra_query_vars[$wpvar];
+			elseif ( isset( $_GET[ $wpvar ] ) && isset( $_POST[ $wpvar ] ) && $_GET[ $wpvar ] !== $_POST[ $wpvar ] )
+				wp_die( __( 'A variable mismatch has been detected.' ), __( 'Sorry, you are not allowed to view this item.' ), 400 );
 			elseif ( isset( $_POST[$wpvar] ) )
 				$this->query_vars[$wpvar] = $_POST[$wpvar];
 			elseif ( isset( $_GET[$wpvar] ) )
diff --git a/wp-includes/ms-deprecated.php b/wp-includes/ms-deprecated.php
index 5c7bfe2a82..b73a8d4fea 100644
--- a/wp-includes/ms-deprecated.php
+++ b/wp-includes/ms-deprecated.php
@@ -240,10 +240,13 @@ function wpmu_admin_do_redirect( $url = '' ) {
 	_deprecated_function( __FUNCTION__, '3.3' );
 
 	$ref = '';
-	if ( isset( $_GET['ref'] ) )
-		$ref = $_GET['ref'];
-	if ( isset( $_POST['ref'] ) )
-		$ref = $_POST['ref'];
+	if ( isset( $_GET['ref'] ) && isset( $_POST['ref'] ) && $_GET['ref'] !== $_POST['ref'] ) {
+		wp_die( __( 'A variable mismatch has been detected.' ), __( 'Sorry, you are not allowed to view this item.' ), 400 );
+	} elseif ( isset( $_POST['ref'] ) ) {
+		$ref = $_POST[ 'ref' ];
+	} elseif ( isset( $_GET['ref'] ) ) {
+		$ref = $_GET[ 'ref' ];
+	}
 
 	if ( $ref ) {
 		$ref = wpmu_admin_redirect_add_updated_param( $ref );
@@ -256,7 +259,9 @@ function wpmu_admin_do_redirect( $url = '' ) {
 	}
 
 	$url = wpmu_admin_redirect_add_updated_param( $url );
-	if ( isset( $_GET['redirect'] ) ) {
+	if ( isset( $_GET['redirect'] ) && isset( $_POST['redirect'] ) && $_GET['redirect'] !== $_POST['redirect'] ) {
+		wp_die( __( 'A variable mismatch has been detected.' ), __( 'Sorry, you are not allowed to view this item.' ), 400 );
+	} elseif ( isset( $_GET['redirect'] ) ) {
 		if ( substr( $_GET['redirect'], 0, 2 ) == 's_' )
 			$url .= '&action=blogs&s='. esc_html( substr( $_GET['redirect'], 2 ) );
 	} elseif ( isset( $_POST['redirect'] ) ) {