FrankenPHP leaks session data between requests in worker mode

Summary

When running FrankenPHP in worker mode, the $_SESSION superglobal is not correctly reset between requests. This allows a subsequent request processed by the same worker to access the $_SESSION data of the previous request (potentially belonging to a different user) before session_start() is called.

Details

In standard PHP execution, the environment is torn down completely after every request. In FrankenPHP's worker mode, the application stays in memory, and superglobals are manually reset between requests.

The vulnerability exists because $_SESSION is stored in the Zend Engine's symbol table (EG(symbol_table)). While the standard PHP request shutdown (RSHUTDOWN) decrements the reference count of the session data, it does not remove the $_SESSION variable itself from the symbol table. FrankenPHP's reset logic (frankenphp_reset_super_globals) previously cleared other superglobals but failed to explicitly delete $_SESSION.

Consequently, until session_start() is called in the new request (which re-initializes the variable), the $_SESSION array retains the data from the previous request processed by that specific worker thread.

Impact

This is a cross-request data leakage vulnerability.

• Confidentiality: If an application reads $_SESSION before calling session_start(), it can access sensitive information (authentication tokens, user IDs, PII) belonging to the previous user.
• Logic Errors / Impersonation: If application logic relies on $_SESSION being empty or unset to detect a "guest" state, or checks for specific keys in $_SESSION prior to session initialization, a malicious actor (or accidental race condition) could trigger privilege escalation or user impersonation.

This affects only users running FrankenPHP in worker mode and not session_start() for each request, which is done by default by most frameworks.

PoC

The following steps demonstrate the issue (derived from the regression tests added in the fix):

1. Client A sends a request that starts a session and sets sensitive data:

// Request 1
session_start();
$_SESSION['secret'] = 'AliceData';
session_write_close();

2. (or the same client without cookies) sends a request to the same worker. This script checks starting a session: