Flight is an extensible micro-framework for PHP. Prior to 3.18.1, the default error handler Engine::_error() writes the full exception message, exception code, and stack trace (including absolute filesystem paths) directly into the HTTP 500 response, with no debug gating. Production deployments leak internal paths, any secret interpolated into an exception message, and full module structure — giving attackers primitives for chaining other weaknesses (LFI, path traversal). This vulnerability is fixed in 3.18.1.

Affected: Packagist: flightphp/core

### Summary The default error handler `Engine::_error()` writes the full exception message, exception code, and stack trace (including absolute filesystem paths) directly into the HTTP 500 response, with no debug gating. Production deployments leak internal paths, any secret interpolated into an exception message, and full module structure — giving attackers primitives for chaining other weaknesses (LFI, path traversal).

### Affected code `flight/Engine.php` (≈ lines 678-704):

```php public function _error(Throwable $e): void { ... $msg = sprintf(<<<'HTML' <h1>500 Internal Server Error</h1> <h3>%s (%s)</h3> <pre>%s</pre> HTML, $e->getMessage(), $e->getCode(), $e->getTraceAsString() ); $this->response()->cache(0)->clearBody()->status(500)->write($msg)->send(); } ```

No `flight.debug` check, no environment gating.

### Proof of concept Any uncaught exception — including those auto-raised from `handleError()` — returns:

``` HTTP/1.1 500 Internal Server Error

<h1>500 Internal Server Error</h1> <h3>secret path /var/www/config/db.yml; token=LEAKED123 (0)</h3> <pre>#0 [internal function]: {closure}() #1 /home/user/app/vendor/flightphp/core/flight/core/Dispatcher.php(361)... #2 /home/user/app/vendor/flightphp/core/flight/Engine.php(...) ... </pre> ```

Reproduced against the live PoC app at `/poc5/error`.

### Impact - Disclosure of absolute filesystem paths (primes weaponization of LFI…