diff --git a/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-api-update-updater.php b/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-api-update-updater.php
index c438c8e50b..fba93c07d9 100644
--- a/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-api-update-updater.php
+++ b/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-api-update-updater.php
@@ -221,11 +221,21 @@ public static function update_single_plugin( $plugin_slug ) {
 	 * @return string The served version, or '' when the plugin isn't in `update_source`.
 	 */
 	public static function get_served_version( $plugin_slug ) {
+		return (string) ( self::get_served_release( $plugin_slug )->version ?? '' );
+	}
+
+	/**
+	 * The release currently served from `update_source`.
+	 *
+	 * @param string $plugin_slug The plugin slug.
+	 * @return object|null Row with `version` and `stable_tag`, or null when the plugin isn't in `update_source`.
+	 */
+	public static function get_served_release( $plugin_slug ) {
 		global $wpdb;
 
-		return (string) $wpdb->get_var(
+		return $wpdb->get_row(
 			$wpdb->prepare(
-				"SELECT version FROM {$wpdb->prefix}update_source WHERE plugin_slug = %s",
+				"SELECT version, stable_tag FROM {$wpdb->prefix}update_source WHERE plugin_slug = %s",
 				$plugin_slug
 			)
 		);
diff --git a/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-plugin-scan-gandalf.php b/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-plugin-scan-gandalf.php
index a0b58c457d..68274af6ec 100644
--- a/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-plugin-scan-gandalf.php
+++ b/wordpress.org/public_html/wp-content/plugins/plugin-directory/jobs/class-plugin-scan-gandalf.php
@@ -7,6 +7,7 @@
 
 namespace WordPressdotorg\Plugin_Directory\Jobs;
 
+use WordPressdotorg\Plugin_Directory\Plugin_Directory;
 use WordPressdotorg\Plugin_Directory\Template;
 use WP_Error;
 use WP_Http;
@@ -74,6 +75,19 @@ public static function dispatch_from_import_context( $plugin, $import_context )
 		$previous_version     = get_post_meta( $plugin->ID, 'last_version', true ) ?: null;
 		$previous_zip_url     = null;
 
+		// A blocked or cooling-down release never reaches `update_source`, so diffing against the served release keeps a re-tagged payload from becoming its own baseline.
+		$served = API_Update_Updater::get_served_release( $plugin->post_name );
+		if ( $served && substr( $version, 0, 128 ) !== $served->version ) {
+			$previous_release_ref = $served->stable_tag ?: null;
+			$previous_version     = $served->version ?: null;
+		}
+
+		// A blocked release must not be the baseline either way — dropping it makes the scanner run a full scan.
+		if ( $previous_release_ref && API_Update_Updater::is_release_blocked( Plugin_Directory::get_release( $plugin, $previous_release_ref ) ) ) {
+			$previous_release_ref = null;
+			$previous_version     = null;
+		}
+
 		if ( $previous_release_ref && $previous_release_ref !== $release_ref && 'trunk' !== $previous_release_ref ) {
 			$previous_zip_url = Template::download_link( $plugin, $previous_release_ref );
 		}
diff --git a/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/Gandalf_Scan_Baseline_Test.php b/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/Gandalf_Scan_Baseline_Test.php
new file mode 100644
index 0000000000..8e6cd296cc
--- /dev/null
+++ b/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/Gandalf_Scan_Baseline_Test.php
@@ -0,0 +1,259 @@
+<?php
+/**
+ * Tests for the security scan diff baseline.
+ *
+ * @package WordPressdotorg\Plugin_Directory\Tests
+ */
+
+declare( strict_types = 1 );
+
+use PHPUnit\Framework\Attributes\Group;
+use PHPUnit\Framework\TestCase;
+use WordPressdotorg\Plugin_Directory\Jobs\Plugin_Scan_Gandalf;
+use WordPressdotorg\Plugin_Directory\Plugin_Directory;
+
+/**
+ * Tests that scan dispatches baseline against the served release, so a release
+ * blocked by a scan can't become the diff baseline for its own re-release.
+ *
+ * Extends the plain PHPUnit TestCase: WP_UnitTestCase is not compatible with
+ * the PHPUnit 11 runner used by this suite. Isolation comes from giving every
+ * test its own plugin post instead of per-test transactions.
+ *
+ * The group is declared as an attribute as well as `@group`: PHPUnit 11 ignores
+ * a class-level `@group` docblock, while older runners ignore the attribute.
+ *
+ * @group jobs
+ */
+#[Group( 'jobs' )]
+class Gandalf_Scan_Baseline_Test extends TestCase {
+
+	/** The version the update_source row serves — the last release users actually got. */
+	private const SERVED_VERSION = '1.4.3';
+
+	/** The version whose release was blocked by a scan — the previous import. */
+	private const BLOCKED_VERSION = '1.4.4';
+
+	/** The version being imported and scanned. */
+	private const NEW_VERSION = '1.4.5';
+
+	/**
+	 * Counter to give every test plugin a unique slug.
+	 *
+	 * @var int
+	 */
+	private static int $plugin_count = 0;
+
+	/**
+	 * The plugin post under test.
+	 *
+	 * @var \WP_Post
+	 */
+	private \WP_Post $plugin;
+
+	/**
+	 * The scan request body captured from the dispatch, or null when none was sent.
+	 *
+	 * @var array|null
+	 */
+	private ?array $request_body = null;
+
+	/**
+	 * The pre_http_request callback capturing the dispatch.
+	 *
+	 * @var \Closure
+	 */
+	private \Closure $http_mock;
+
+	/**
+	 * Create a published plugin importing NEW_VERSION after a blocked BLOCKED_VERSION.
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+
+		wp_cache_flush();
+
+		if ( ! defined( 'WP_GANDALF_SCAN_SHARED_SECRET' ) ) {
+			define( 'WP_GANDALF_SCAN_SHARED_SECRET', 'test-shared-secret' );
+		}
+
+		$plugin = Plugin_Directory::create_plugin_post(
+			array(
+				'post_name'   => 'baseline-test-' . ( ++self::$plugin_count ),
+				'post_title'  => 'Scan Baseline Test Plugin',
+				'post_status' => 'publish',
+			)
+		);
+
+		$this->assertInstanceOf( \WP_Post::class, $plugin );
+		$this->plugin = $plugin;
+
+		/*
+		 * The stub update_source table survives across runs — the WP test
+		 * installer only drops core tables — so clear leftovers that would
+		 * collide with this run's plugin ID or read as a served version.
+		 */
+		global $wpdb;
+		$wpdb->delete( $wpdb->prefix . 'update_source', array( 'plugin_id' => $this->plugin->ID ) );
+		$wpdb->delete( $wpdb->prefix . 'update_source', array( 'plugin_slug' => $this->plugin->post_name ) );
+
+		update_post_meta( $this->plugin->ID, 'version', self::NEW_VERSION );
+		update_post_meta( $this->plugin->ID, 'last_version', self::BLOCKED_VERSION );
+		update_post_meta( $this->plugin->ID, 'last_stable_tag', self::BLOCKED_VERSION );
+
+		$this->request_body = null;
+		$this->http_mock    = function ( mixed $preempt, array $parsed_args, string $url ): array {
+			$this->assertSame( Plugin_Scan_Gandalf::ENDPOINT, $url );
+			$this->request_body = json_decode( $parsed_args['body'], true );
+
+			return array(
+				'headers'  => array(),
+				'body'     => wp_json_encode( array( 'scan_id' => $this->request_body['scan_id'] ) ),
+				'response' => array(
+					'code'    => 200,
+					'message' => 'OK',
+				),
+			);
+		};
+		add_filter( 'pre_http_request', $this->http_mock, 10, 3 );
+	}
+
+	/**
+	 * Unhook the HTTP mock.
+	 */
+	protected function tearDown(): void {
+		remove_filter( 'pre_http_request', $this->http_mock, 10 );
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Insert an update_source row serving a version.
+	 *
+	 * @param string $version    The served version.
+	 * @param string $stable_tag The served stable tag.
+	 */
+	private function stage_served_release( string $version, string $stable_tag ): void {
+		global $wpdb;
+
+		$wpdb->insert(
+			$wpdb->prefix . 'update_source',
+			array(
+				'plugin_id'        => $this->plugin->ID,
+				'plugin_slug'      => $this->plugin->post_name,
+				'available'        => 1,
+				'version'          => $version,
+				'stable_tag'       => $stable_tag,
+				'plugin_name'      => $this->plugin->post_title,
+				'requires_plugins' => '',
+				'last_updated'     => $this->plugin->post_modified,
+			)
+		);
+	}
+
+	/**
+	 * Dispatch a scan for NEW_VERSION and return the captured request body.
+	 *
+	 * @return array The captured scan request body.
+	 */
+	private function dispatch_scan(): array {
+		// phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_set_error_handler -- A successful dispatch logs an E_USER_NOTICE; keep it out of the test output.
+		set_error_handler( static fn (): bool => true, E_USER_NOTICE );
+
+		try {
+			$dispatched = Plugin_Scan_Gandalf::dispatch_from_import_context(
+				$this->plugin,
+				array(
+					'stable_tag'       => self::NEW_VERSION,
+					'old_stable_tag'   => self::BLOCKED_VERSION,
+					'changed_svn_tags' => array( self::NEW_VERSION ),
+				)
+			);
+		} finally {
+			restore_error_handler();
+		}
+
+		$this->assertTrue( $dispatched );
+		$this->assertIsArray( $this->request_body );
+
+		return $this->request_body;
+	}
+
+	/**
+	 * With a version held out of update_source, the follow-up release is diffed
+	 * against the served release, not the held one.
+	 */
+	public function test_served_release_is_baseline(): void {
+		$this->stage_served_release( self::SERVED_VERSION, self::SERVED_VERSION );
+
+		$request = $this->dispatch_scan();
+
+		$this->assertSame( self::NEW_VERSION, $request['version'] );
+		$this->assertSame( self::SERVED_VERSION, $request['previous_version'] );
+		$this->assertSame( self::SERVED_VERSION, $request['previous_release_ref'] );
+		$this->assertSame(
+			'https://downloads.wordpress.org/plugin/' . $this->plugin->post_name . '.' . self::SERVED_VERSION . '.zip',
+			$request['previous_zip_url']
+		);
+	}
+
+	/**
+	 * When the row already serves the scanned version — imports without a
+	 * cooldown write it before the scan dispatches — the last imported release
+	 * remains the baseline.
+	 */
+	public function test_falls_back_to_last_release_when_scanned_version_is_served(): void {
+		$this->stage_served_release( self::NEW_VERSION, self::NEW_VERSION );
+
+		$request = $this->dispatch_scan();
+
+		$this->assertSame( self::BLOCKED_VERSION, $request['previous_version'] );
+		$this->assertSame( self::BLOCKED_VERSION, $request['previous_release_ref'] );
+	}
+
+	/**
+	 * A plugin with no update_source row falls back to the last imported release.
+	 */
+	public function test_falls_back_to_last_release_without_served_release(): void {
+		$request = $this->dispatch_scan();
+
+		$this->assertSame( self::BLOCKED_VERSION, $request['previous_version'] );
+		$this->assertSame( self::BLOCKED_VERSION, $request['previous_release_ref'] );
+	}
+
+	/**
+	 * A baseline candidate that is itself a blocked release is dropped: with
+	 * no served row, the last imported release would be the baseline, but the
+	 * scan must run without one — a full scan — to surface its content.
+	 */
+	public function test_no_baseline_when_last_release_is_blocked(): void {
+		Plugin_Directory::add_release(
+			$this->plugin,
+			array(
+				'tag'           => self::BLOCKED_VERSION,
+				'version'       => self::BLOCKED_VERSION,
+				'release_block' => array( 'blocked_at' => time() ),
+			)
+		);
+
+		$request = $this->dispatch_scan();
+
+		$this->assertNull( $request['previous_version'] );
+		$this->assertNull( $request['previous_release_ref'] );
+		$this->assertNull( $request['previous_zip_url'] );
+	}
+
+	/**
+	 * A trunk-served release has no retrievable previous ZIP — its build was
+	 * overwritten — so the scan carries no baseline.
+	 */
+	public function test_no_baseline_when_serving_from_trunk(): void {
+		$this->stage_served_release( self::SERVED_VERSION, 'trunk' );
+
+		$request = $this->dispatch_scan();
+
+		$this->assertNull( $request['previous_version'] );
+		$this->assertNull( $request['previous_release_ref'] );
+		$this->assertNull( $request['previous_zip_url'] );
+	}
+}