Fix: Preserve bare < characters in feed titles

src/wp-includes/default-filters.php

@@ -254,9 +254,11 @@
 add_filter( 'wp_get_custom_css', 'wp_replace_insecure_home_url' );
 
 // RSS filters.
-add_filter( 'the_title_rss', 'strip_tags' );
+
 add_filter( 'the_title_rss', 'ent2ncr', 8 );
-add_filter( 'the_title_rss', 'esc_html' );
+add_filter( 'the_title_rss', 'wp_encode_bare_lt', 9 );
+add_filter( 'the_title_rss', 'strip_tags', 10 );
+add_filter( 'the_title_rss', 'esc_html', 11 );
 add_filter( 'the_content_rss', 'ent2ncr', 8 );
 add_filter( 'the_content_feed', 'wp_staticize_emoji' );
 add_filter( 'the_content_feed', '_oembed_filter_feed_content' );

src/wp-includes/formatting.php

@@ -5509,6 +5509,29 @@ function normalize_whitespace( $str ) {
 	return $str;
 }
 
+/**
+ * Encodes bare `<` characters (those not starting an HTML tag) as `&lt;`.
+ *
+ * @since x.x.x
+ *
+ * @param string $text The text to encode.
+ * @return string Text with bare `<` characters encoded.
+ */
+function wp_encode_bare_lt( $text ) {
+	/*
+	 * A valid HTML tag begins with `<` followed by:
+	 * - a letter  (opening tag, e.g. `<strong>`)
+	 * - `/`       (closing tag, e.g. `</strong>`)
+	 * - `!`       (comment or doctype, e.g. `<!-- … -->`)
+	 * - `?`       (processing instruction, e.g. `<?xml … ?>`)
+	 *
+	 * Any `<` not followed by one of these is a bare less-than sign and is
+	 * encoded as `&lt;` so that strip_tags() does not silently discard it.
+	 */
+	$encoded = preg_replace( '#<(?![a-zA-Z/!?])#', '&lt;', $text );
+	return null !== $encoded ? $encoded : $text;
+}
+
 /**
  * Properly strips all HTML tags including 'script' and 'style'.
  *

tests/phpunit/tests/feed/atom.php

@@ -294,4 +294,30 @@ public function test_atom_enclosure_with_extended_url_length_type_parsing() {
 			}
 		}
 	}
+
+	/**
+	 * Tests that a bare `<` in a post title is encoded inside the Atom feed CDATA block, not stripped.
+	 *
+	 * Atom wraps `the_title_rss()` output in `<![CDATA[…]]>`. This test verifies
+	 * the encoded `<` character survives into that block by writing the raw title
+	 * directly to the database, bypassing kses sanitization on insert.
+	 *
+	 * @ticket 9993
+	 *
+	 * @covers ::get_the_title_rss
+	 */
+	public function test_atom_title_encodes_bare_lt_in_cdata() {
+		global $wpdb;
+
+		$post_id = self::factory()->post->create();
+
+		$wpdb->update( $wpdb->posts, array( 'post_title' => '& > test <' ), array( 'ID' => $post_id ) );
+		clean_post_cache( $post_id );
+
+		$this->go_to( '/?feed=atom&p=' . $post_id );
+		$feed = $this->do_atom();
+
+		$this->assertStringContainsString( 'test &lt;', $feed, 'Bare `<` was stripped from the Atom feed title CDATA block.' );
+		$this->assertStringNotContainsString( 'test ]]>', $feed, 'Bare `<` must not be stripped (leaving title truncated before `]]>`).' );
+	}
 }

tests/phpunit/tests/feed/rss2.php

@@ -629,4 +629,42 @@ function ( $headers ) use ( $today ) {
 
 		$this->go_to( '/?feed=rss2&withcomments=1' );
 	}
+
+	/**
+	 * Tests that the `the_title_rss` filter encodes bare `<` characters rather than stripping them.
+	 *
+	 * @ticket 9993
+	 * @dataProvider data_title_rss_encodes_special_characters
+	 *
+	 * @covers ::get_the_title_rss
+	 */
+	public function test_title_rss_encodes_special_characters( $title, $expected ) {
+		$this->assertSame( $expected, apply_filters( 'the_title_rss', $title ) );
+	}
+
+	/**
+	 * Data provider for test_title_rss_encodes_special_characters().
+	 *
+	 * @return array[]
+	 */
+	public static function data_title_rss_encodes_special_characters() {
+		return array(
+			'bare less-than at end is encoded not stripped' => array(
+				'& > test <',
+				'&amp; &gt; test &lt;',
+			),
+			'bare less-than in middle is encoded'        => array(
+				'a < b',
+				'a &lt; b',
+			),
+			'html tags are still stripped by strip_tags' => array(
+				'<strong>bold</strong>',
+				'bold',
+			),
+			'plain text passes through unchanged'        => array(
+				'Hello World',
+				'Hello World',
+			),
+		);
+	}
 }