Index: src/wp-includes/capabilities.php
===================================================================
--- src/wp-includes/capabilities.php	(revision 27110)
+++ src/wp-includes/capabilities.php	(working copy)
@@ -1071,20 +1071,15 @@
 			break;
 		}
 
-		$post_author_id = $post->post_author;
-
-		// If no author set yet, default to current user for cap checks.
-		if ( ! $post_author_id )
-			$post_author_id = $user_id;
-
-		// If the user is the author...
-		if ( $user_id == $post_author_id ) {
+		// If the post author is set and the user is the author...
+		if ( $post->post_author && $user_id == $post->post_author ) {
 			// If the post is published...
 			if ( 'publish' == $post->post_status ) {
 				$caps[] = $post_type->cap->delete_published_posts;
 			} elseif ( 'trash' == $post->post_status ) {
-				if ('publish' == get_post_meta($post->ID, '_wp_trash_meta_status', true) )
+				if ( 'publish' == get_post_meta( $post->ID, '_wp_trash_meta_status', true ) ) {
 					$caps[] = $post_type->cap->delete_published_posts;
+				}
 			} else {
 				// If the post is draft...
 				$caps[] = $post_type->cap->delete_posts;
@@ -1093,10 +1088,11 @@
 			// The user is trying to edit someone else's post.
 			$caps[] = $post_type->cap->delete_others_posts;
 			// The post is published, extra cap required.
-			if ( 'publish' == $post->post_status )
+			if ( 'publish' == $post->post_status ) {
 				$caps[] = $post_type->cap->delete_published_posts;
-			elseif ( 'private' == $post->post_status )
+			} else if ( 'private' == $post->post_status ) {
 				$caps[] = $post_type->cap->delete_private_posts;
+			}
 		}
 		break;
 		// edit_post breaks down to edit_posts, edit_published_posts, or
@@ -1121,20 +1117,15 @@
 			break;
 		}
 
-		$post_author_id = $post->post_author;
-
-		// If no author set yet, default to current user for cap checks.
-		if ( ! $post_author_id )
-			$post_author_id = $user_id;
-
-		// If the user is the author...
-		if ( $user_id == $post_author_id ) {
+		// If the post author is set and the user is the author...
+		if ( $post->post_author && $user_id == $post->post_author ) {
 			// If the post is published...
 			if ( 'publish' == $post->post_status ) {
 				$caps[] = $post_type->cap->edit_published_posts;
 			} elseif ( 'trash' == $post->post_status ) {
-				if ('publish' == get_post_meta($post->ID, '_wp_trash_meta_status', true) )
+				if ( 'publish' == get_post_meta( $post->ID, '_wp_trash_meta_status', true ) ) {
 					$caps[] = $post_type->cap->edit_published_posts;
+				}
 			} else {
 				// If the post is draft...
 				$caps[] = $post_type->cap->edit_posts;
@@ -1143,10 +1134,11 @@
 			// The user is trying to edit someone else's post.
 			$caps[] = $post_type->cap->edit_others_posts;
 			// The post is published, extra cap required.
-			if ( 'publish' == $post->post_status )
+			if ( 'publish' == $post->post_status ) {
 				$caps[] = $post_type->cap->edit_published_posts;
-			elseif ( 'private' == $post->post_status )
+			} else if ( 'private' == $post->post_status ) {
 				$caps[] = $post_type->cap->edit_private_posts;
+			}
 		}
 		break;
 	case 'read_post':
@@ -1173,18 +1165,13 @@
 			break;
 		}
 
-		$post_author_id = $post->post_author;
-
-		// If no author set yet, default to current user for cap checks.
-		if ( ! $post_author_id )
-			$post_author_id = $user_id;
-
-		if ( $user_id == $post_author_id )
+		if ( $post->post_author && $user_id == $post->post_author ) {
 			$caps[] = $post_type->cap->read;
-		elseif ( $status_obj->private )
+		} else if ( $status_obj->private ) {
 			$caps[] = $post_type->cap->read_private_posts;
-		else
+		} else {
 			$caps = map_meta_cap( 'edit_post', $user_id, $post->ID );
+		}
 		break;
 	case 'publish_post':
 		$post = get_post( $args[0] );
Index: tests/phpunit/tests/user/capabilities.php
===================================================================
--- tests/phpunit/tests/user/capabilities.php	(revision 27110)
+++ tests/phpunit/tests/user/capabilities.php	(working copy)
@@ -525,6 +525,29 @@
 	}
 
 	/**
+	 * @ticket 27020
+	 */
+	function test_authorless_post() {
+
+		// Make a post without an author
+		$post = $this->factory->post->create( array( 'post_author' => 0, 'post_type' => 'post', 'post_status' => 'draft' ) );
+
+		// Add an editor and contributor
+		$editor = new WP_User( $this->factory->user->create( array( 'role' => 'editor' ) ) );
+		$contributor = new WP_User( $this->factory->user->create( array( 'role' => 'contributor' ) ) );
+
+		// editor can edit, view, and trash
+		$this->assertTrue( $editor->has_cap( 'edit_post', $post ) );
+		$this->assertTrue( $editor->has_cap( 'read_post', $post ) );
+		$this->assertTrue( $editor->has_cap( 'delete_post', $post ) );
+		// an contributor cannot
+		$this->assertFalse( $contributor->has_cap( 'edit_post', $post ) );
+		$this->assertFalse( $contributor->has_cap( 'read_post', $post ) );
+		$this->assertFalse( $contributor->has_cap( 'delete_post', $post ) );
+
+	}
+
+	/**
 	 * @ticket 16714
 	 */
 	function test_create_posts_caps() {
Index: tests/phpunit/tests/user/mapMetaCap.php
===================================================================
--- tests/phpunit/tests/user/mapMetaCap.php	(revision 27110)
+++ tests/phpunit/tests/user/mapMetaCap.php	(working copy)
@@ -232,4 +232,20 @@
 		$this->assertEquals( array( 'update_core' ), map_meta_cap( 'update_core', $this->user_id ) );
 		$this->assertEquals( array( 'edit_plugins' ), map_meta_cap( 'edit_plugins', $this->user_id ) );
 	}
+
+	/**
+	 * @ticket 27020
+	 */
+	function test_authorless_posts_capabilties() {
+
+		// Make a post without an author
+		$post_id = $this->factory->post->create( array( 'post_author' => 0, 'post_type' => 'post', 'post_status' => 'publish' ) );
+
+		// Add an editor
+		$editor = new WP_User( $this->factory->user->create( array( 'role' => 'editor' ) ) );
+
+		$this->assertEquals( array( 'edit_others_posts', 'edit_published_posts' ), map_meta_cap( 'edit_post', $editor->ID, $post_id ) );
+		$this->assertEquals( array( 'delete_others_posts', 'delete_published_posts' ), map_meta_cap( 'delete_post', $editor->ID, $post_id ) );
+
+	}
 }