diff --git a/src/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryService.php b/src/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryService.php
index d1cd8383..42c42bf2 100644
--- a/src/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryService.php
+++ b/src/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryService.php
@@ -14,6 +14,7 @@
 use ProfessionalWiki\NeoWiki\GraphDatabasePlugins\Neo4j\Application\Exception\QueryException;
 use ProfessionalWiki\NeoWiki\GraphDatabasePlugins\Neo4j\Application\Exception\QueryTimeoutException;
 use ProfessionalWiki\NeoWiki\GraphDatabasePlugins\Neo4j\Application\Exception\WriteQueryRejectedException;
+use ProfessionalWiki\NeoWiki\Domain\GraphDatabase\BackendFailureMessage;
 use ProfessionalWiki\NeoWiki\GraphDatabasePlugins\Neo4j\Persistence\Neo4jResultNormalizer;
 use Throwable;
 
@@ -41,7 +42,11 @@ public function execute( Neo4jQueryRequest $request ): Neo4jQueryResult {
 			// not a misleading "backend unavailable".
 			throw $this->translateNeo4jException( $e );
 		} catch ( Throwable $e ) {
-			throw new BackendUnavailableException( $e->getMessage(), 0, $e );
+			throw new BackendUnavailableException(
+				BackendFailureMessage::withoutCredentials( $e->getMessage() ),
+				0,
+				$e
+			);
 		}
 
 		if ( !$allowed ) {
@@ -59,7 +64,11 @@ public function execute( Neo4jQueryRequest $request ): Neo4jQueryResult {
 		} catch ( Neo4jException $e ) {
 			throw $this->translateNeo4jException( $e );
 		} catch ( Throwable $e ) {
-			throw new BackendUnavailableException( $e->getMessage(), 0, $e );
+			throw new BackendUnavailableException(
+				BackendFailureMessage::withoutCredentials( $e->getMessage() ),
+				0,
+				$e
+			);
 		}
 
 		$durationMs = (int)( microtime( true ) * 1000 ) - $startedAt;
diff --git a/tests/phpunit/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryServiceTest.php b/tests/phpunit/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryServiceTest.php
index 8638d830..012ece34 100644
--- a/tests/phpunit/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryServiceTest.php
+++ b/tests/phpunit/GraphDatabasePlugins/Neo4j/Application/Neo4jQueryServiceTest.php
@@ -31,6 +31,13 @@
  */
 class Neo4jQueryServiceTest extends TestCase {
 
+	/**
+	 * How the driver reports an unreachable server: by quoting the Bolt URI it dialed, credentials
+	 * included. The query surfaces hand this message to callers, anonymous ones included.
+	 */
+	private const string UNREACHABLE_SERVER_MESSAGE =
+		"Cannot connect to any server on alias: default with Uris: ('bolt://neowiki_read:S3cr3t@graph.example:7687')";
+
 	public function testReturnsRowsAsListWithColumnsFromProtocolKeys(): void {
 		$service = $this->newService(
 			$this->stubEngineWithRows( [
@@ -207,6 +214,44 @@ public function queryIsAllowed( string $cypher ): bool {
 		$service->execute( $this->newRequest( 'MATCH (n) RETURN n' ) );
 	}
 
+	public function testEngineBackendFailureDoesNotCarryCredentials(): void {
+		$service = $this->newService(
+			$this->stubEngineThrowing( new RuntimeException( self::UNREACHABLE_SERVER_MESSAGE ) )
+		);
+
+		$message = $this->executeAndCatch( $service )->getMessage();
+
+		$this->assertStringNotContainsString( 'S3cr3t', $message );
+		$this->assertStringContainsString( 'graph.example', $message );
+	}
+
+	public function testValidatorBackendFailureDoesNotCarryCredentials(): void {
+		$throwingValidator = new class( self::UNREACHABLE_SERVER_MESSAGE ) implements CypherQueryValidator {
+			public function __construct( private string $message ) {
+			}
+
+			public function queryIsAllowed( string $cypher ): bool {
+				throw new RuntimeException( $this->message );
+			}
+		};
+		$service = $this->newService( $this->stubEngineWithRows( [] ), validator: $throwingValidator );
+
+		$message = $this->executeAndCatch( $service )->getMessage();
+
+		$this->assertStringNotContainsString( 'S3cr3t', $message );
+		$this->assertStringContainsString( 'graph.example', $message );
+	}
+
+	private function executeAndCatch( Neo4jQueryService $service ): BackendUnavailableException {
+		try {
+			$service->execute( $this->newRequest( 'MATCH (n) RETURN n' ) );
+		} catch ( BackendUnavailableException $e ) {
+			return $e;
+		}
+
+		$this->fail( 'Expected a BackendUnavailableException.' );
+	}
+
 	public function testValidatorNeo4jSyntaxErrorBecomesCypherSyntaxException(): void {
 		$throwingValidator = new class implements CypherQueryValidator {
 			public function queryIsAllowed( string $cypher ): bool {