diff --git a/.github/changelog/fix-blocklist-host-normalization b/.github/changelog/fix-blocklist-host-normalization
new file mode 100644
index 000000000..44d447e9a
--- /dev/null
+++ b/.github/changelog/fix-blocklist-host-normalization
@@ -0,0 +1,4 @@
+Significance: patch
+Type: security
+
+Ensure blocked domains match regardless of letter case or a trailing dot, so they can no longer be bypassed.
diff --git a/includes/class-moderation.php b/includes/class-moderation.php
index b28164b01..2f02734a0 100644
--- a/includes/class-moderation.php
+++ b/includes/class-moderation.php
@@ -110,6 +110,10 @@ public static function activity_is_blocked_for_user( $activity, $user_id ) {
 	 * @return bool True on success, false on failure.
 	 */
 	public static function add_user_block( $user_id, $type, $value ) {
+		if ( self::TYPE_DOMAIN === $type ) {
+			$value = self::normalize_host( $value );
+		}
+
 		switch ( $type ) {
 			case self::TYPE_ACTOR:
 				return Blocked_Actors::add( $user_id, $value );
@@ -146,6 +150,10 @@ public static function add_user_block( $user_id, $type, $value ) {
 	 * @return bool True on success, false on failure.
 	 */
 	public static function remove_user_block( $user_id, $type, $value ) {
+		if ( self::TYPE_DOMAIN === $type ) {
+			$value = self::normalize_host( $value );
+		}
+
 		switch ( $type ) {
 			case self::TYPE_ACTOR:
 				return Blocked_Actors::remove( $user_id, $value );
@@ -196,6 +204,10 @@ public static function get_user_blocks( $user_id ) {
 	 * @return bool True on success, false on failure.
 	 */
 	public static function add_site_block( $type, $value ) {
+		if ( self::TYPE_DOMAIN === $type ) {
+			$value = self::normalize_host( $value );
+		}
+
 		switch ( $type ) {
 			case self::TYPE_ACTOR:
 				// Site-wide actor blocking uses the BLOG_USER_ID.
@@ -241,6 +253,10 @@ public static function add_site_blocks( $type, $values ) {
 			return;
 		}
 
+		if ( self::TYPE_DOMAIN === $type ) {
+			$values = \array_map( array( self::class, 'normalize_host' ), $values );
+		}
+
 		foreach ( $values as $value ) {
 			/**
 			 * Fired when a domain or keyword is blocked site-wide.
@@ -263,6 +279,10 @@ public static function add_site_blocks( $type, $values ) {
 	 * @return bool True on success, false on failure.
 	 */
 	public static function remove_site_block( $type, $value ) {
+		if ( self::TYPE_DOMAIN === $type ) {
+			$value = self::normalize_host( $value );
+		}
+
 		switch ( $type ) {
 			case self::TYPE_ACTOR:
 				// Site-wide actor unblocking uses the BLOG_USER_ID.
@@ -322,8 +342,9 @@ public static function is_actor_blocked( $actor_uri, $user_id = 0 ) {
 			return true;
 		}
 
-		// Check site-wide domain blocks.
-		$actor_domain = \wp_parse_url( $actor_uri, PHP_URL_HOST );
+		// Check site-wide domain blocks. Normalize the (attacker-controlled) host so a case
+		// variant or trailing dot cannot slip past a block; stored domains are normalized on insert.
+		$actor_domain = self::normalize_host( \wp_parse_url( $actor_uri, PHP_URL_HOST ) );
 		if ( $actor_domain && \in_array( $actor_domain, $site_blocks['domains'], true ) ) {
 			return true;
 		}
@@ -344,6 +365,23 @@ public static function is_actor_blocked( $actor_uri, $user_id = 0 ) {
 		return false;
 	}
 
+	/**
+	 * Normalize a host for blocklist comparison.
+	 *
+	 * DNS hosts are case-insensitive, a trailing dot denotes the same (absolute) host, and IPv6
+	 * literals may be bracketed, so lower-case, strip IPv6 brackets, and trim a trailing dot.
+	 * Applied to incoming hosts before comparison and to domains on insert, keeping both sides
+	 * canonical.
+	 *
+	 * @param string|null $host The host.
+	 * @return string The normalized host.
+	 */
+	private static function normalize_host( $host ) {
+		$host = \strtolower( (string) $host );
+		$host = \rtrim( $host, '.' );
+		return \trim( $host, '[]' );
+	}
+
 	/**
 	 * Check activity against blocklists.
 	 *
@@ -374,11 +412,12 @@ private static function check_activity_against_blocks( $activity, $blocked_actor
 			}
 		}
 
-		// Check blocked domains.
+		// Check blocked domains. Normalize the (attacker-controlled) hosts so a case variant
+		// or trailing dot cannot slip past a block; stored domains are normalized on insert.
 		$urls = array(
-			\wp_parse_url( $actor_id, PHP_URL_HOST ),
-			\wp_parse_url( $activity->get_id(), PHP_URL_HOST ),
-			\wp_parse_url( object_to_uri( $activity->get_object() ) ?? '', PHP_URL_HOST ),
+			self::normalize_host( \wp_parse_url( $actor_id, PHP_URL_HOST ) ),
+			self::normalize_host( \wp_parse_url( $activity->get_id(), PHP_URL_HOST ) ),
+			self::normalize_host( \wp_parse_url( object_to_uri( $activity->get_object() ) ?? '', PHP_URL_HOST ) ),
 		);
 		foreach ( $blocked_domains as $domain ) {
 			if ( \in_array( $domain, $urls, true ) ) {
diff --git a/tests/phpunit/tests/includes/class-test-moderation.php b/tests/phpunit/tests/includes/class-test-moderation.php
index c9ac2bcbd..7b98da92b 100644
--- a/tests/phpunit/tests/includes/class-test-moderation.php
+++ b/tests/phpunit/tests/includes/class-test-moderation.php
@@ -1075,4 +1075,144 @@ public function test_is_actor_blocked_mixed_scenarios() {
 		Moderation::remove_site_block( 'domain', $domain );
 		Moderation::remove_user_block( $this->test_user_id, 'actor', $actor_uri3 );
 	}
+
+	/**
+	 * Domains are stored normalized so case and trailing-dot variants cannot bypass a block.
+	 *
+	 * @covers ::add_site_block
+	 * @covers ::get_site_blocks
+	 * @covers ::add_user_block
+	 * @covers ::get_user_blocks
+	 * @covers ::add_site_blocks
+	 */
+	public function test_domain_blocks_are_normalized_on_insert() {
+		Moderation::add_site_block( 'domain', 'Example.COM.' );
+		$this->assertSame( array( 'example.com' ), Moderation::get_site_blocks()['domains'] );
+
+		Moderation::add_user_block( $this->test_user_id, 'domain', 'Spam.Example.COM' );
+		$this->assertSame( array( 'spam.example.com' ), Moderation::get_user_blocks( $this->test_user_id )['domains'] );
+
+		Moderation::add_site_blocks( 'domain', array( 'Bulk.Example.COM.', 'OTHER.example.com' ) );
+		$this->assertSame(
+			array( 'example.com', 'bulk.example.com', 'other.example.com' ),
+			Moderation::get_site_blocks()['domains']
+		);
+	}
+
+	/**
+	 * A normalized stored block can be removed regardless of the case/trailing dot supplied.
+	 *
+	 * @covers ::remove_site_block
+	 * @covers ::remove_user_block
+	 */
+	public function test_domain_blocks_are_normalized_on_remove() {
+		Moderation::add_site_block( 'domain', 'example.com' );
+		Moderation::remove_site_block( 'domain', 'Example.COM.' );
+		$this->assertSame( array(), Moderation::get_site_blocks()['domains'] );
+
+		Moderation::add_user_block( $this->test_user_id, 'domain', 'spam.example.com' );
+		Moderation::remove_user_block( $this->test_user_id, 'domain', 'Spam.Example.COM.' );
+		$this->assertSame( array(), Moderation::get_user_blocks( $this->test_user_id )['domains'] );
+	}
+
+	/**
+	 * Case and trailing-dot host variants in an actor URI cannot slip past a domain block.
+	 *
+	 * @covers ::is_actor_blocked
+	 */
+	public function test_is_actor_blocked_normalizes_host() {
+		Moderation::add_site_block( 'domain', 'blocked.example.com' );
+
+		$bypass_uris = array(
+			'https://Blocked.Example.com/@user',  // Upper-case host.
+			'https://BLOCKED.EXAMPLE.COM/@user',  // All caps.
+			'https://blocked.example.com./@user', // Trailing FQDN dot.
+			'https://Blocked.Example.COM./@user', // Mixed case + trailing dot.
+		);
+
+		foreach ( $bypass_uris as $uri ) {
+			$this->assertTrue( Moderation::is_actor_blocked( $uri ), "URI should be blocked: $uri" );
+		}
+
+		// A genuinely different domain still passes.
+		$this->assertFalse( Moderation::is_actor_blocked( 'https://blocked.example.org/@user' ) );
+
+		Moderation::remove_site_block( 'domain', 'blocked.example.com' );
+	}
+
+	/**
+	 * Case and trailing-dot host variants in an activity cannot slip past a domain block.
+	 *
+	 * @covers ::activity_is_blocked
+	 * @covers ::activity_is_blocked_site_wide
+	 * @covers ::check_activity_against_blocks
+	 */
+	public function test_activity_blocking_normalizes_host() {
+		Moderation::add_site_block( 'domain', 'evil.example.com' );
+
+		$bypass_actors = array(
+			'https://Evil.Example.com/@user',
+			'https://EVIL.EXAMPLE.COM/@user',
+			'https://evil.example.com./@user',
+		);
+
+		foreach ( $bypass_actors as $actor ) {
+			/* @var Activity $activity Activity. */
+			$activity = Activity::init_from_array(
+				array(
+					'type'   => 'Create',
+					'actor'  => $actor,
+					'object' => array(
+						'id'      => 'https://example.org/note/1',
+						'type'    => 'Note',
+						'content' => 'Test',
+					),
+				)
+			);
+
+			$this->assertTrue( Moderation::activity_is_blocked( $activity ), "Actor should be blocked: $actor" );
+		}
+
+		Moderation::remove_site_block( 'domain', 'evil.example.com' );
+	}
+
+	/**
+	 * A blocked domain is matched on the activity id and object URI hosts, not just the actor.
+	 *
+	 * @covers ::activity_is_blocked
+	 * @covers ::check_activity_against_blocks
+	 */
+	public function test_activity_blocking_normalizes_id_and_object_hosts() {
+		Moderation::add_site_block( 'domain', 'evil.example.com' );
+
+		// Clean actor, but the activity id is on a blocked domain (case variant).
+		/* @var Activity $by_id Activity. */
+		$by_id = Activity::init_from_array(
+			array(
+				'id'     => 'https://EVIL.example.com/activities/1',
+				'type'   => 'Create',
+				'actor'  => 'https://good.example.org/@user',
+				'object' => array(
+					'id'      => 'https://good.example.org/note/1',
+					'type'    => 'Note',
+					'content' => 'Test',
+				),
+			)
+		);
+		$this->assertTrue( Moderation::activity_is_blocked( $by_id ), 'Blocked id host should block the activity.' );
+
+		// Clean actor and id, but the object URI is on a blocked domain (trailing dot).
+		/* @var Activity $by_object Activity. */
+		$by_object = Activity::init_from_array(
+			array(
+				'id'     => 'https://good.example.org/activities/2',
+				'type'   => 'Announce',
+				'actor'  => 'https://good.example.org/@user',
+				'object' => 'https://evil.example.com./note/2',
+			)
+		);
+		$this->assertTrue( Moderation::activity_is_blocked( $by_object ), 'Blocked object host should block the activity.' );
+
+		Moderation::remove_site_block( 'domain', 'evil.example.com' );
+	}
 }