CVE-2026-42552

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):

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.

Patch (fixed in 3.18.1, commit b8dd23a)

A new flight.debug setting (default false) gates the verbose output. In production the handler now emits only <h1>500 Internal Server Error</h1>. Developers can set flight.debug = true in local environments to restore the full trace output.

Credit

Discovered by @Rootingg.

• Disclosure of absolute filesystem paths (primes weaponization of LFI / path-traversal vulnerabilities in the same application).
• Disclosure of secrets (DB credentials, API tokens) when exceptions are constructed with interpolated configuration values.
• Enumeration of installed vendor packages and internal application structure.

CVE-2026-42552 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.18.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42552? CVE-2026-42552 is a high-severity security vulnerability in flightphp/core (composer), affecting versions < 3.18.1. It is fixed in 3.18.1.
2. How severe is CVE-2026-42552? CVE-2026-42552 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of flightphp/core are affected by CVE-2026-42552? flightphp/core (composer) versions < 3.18.1 is affected.
4. Is there a fix for CVE-2026-42552? Yes. CVE-2026-42552 is fixed in 3.18.1. Upgrade to this version or later.
5. Is CVE-2026-42552 exploitable, and should I be worried? Whether CVE-2026-42552 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42552 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42552? Upgrade flightphp/core to 3.18.1 or later.