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):
- Client A sends a request that starts a session and sets sensitive data:
// Request 1
session_start();
$_SESSION['secret'] = 'AliceData';
session_write_close();
- Client B (or the same client without cookies) sends a request to the same worker. This script checks
$_SESSION without starting a session:
// Request 2
// session_start() is NOT called
if (!empty($_SESSION)) {
echo "Leaked Data: " . $_SESSION['secret'];
}
- Result: Client B receives "Leaked Data: AliceData".
Workarounds
- Ensure
session_start() is called immediately at the entry point of your worker script to overwrite any residual data (though this may not cover all edge cases if middleware runs before the controller).
- Manually unset
$_SESSION at the very beginning of the worker loop, before handling the request.
References
xavierleune Remediation developer
dunglas Remediation reviewer
Summary
When running FrankenPHP in worker mode, the
$_SESSIONsuperglobal is not correctly reset between requests. This allows a subsequent request processed by the same worker to access the$_SESSIONdata of the previous request (potentially belonging to a different user) beforesession_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
$_SESSIONis stored in the Zend Engine's symbol table (EG(symbol_table)). While the standard PHP request shutdown (RSHUTDOWN) decrements the reference count of the session data, it does not remove the$_SESSIONvariable itself from the symbol table. FrankenPHP's reset logic (frankenphp_reset_super_globals) previously cleared other superglobals but failed to explicitly delete$_SESSION.Consequently, until
session_start()is called in the new request (which re-initializes the variable), the$_SESSIONarray retains the data from the previous request processed by that specific worker thread.Impact
This is a cross-request data leakage vulnerability.
$_SESSIONbefore callingsession_start(), it can access sensitive information (authentication tokens, user IDs, PII) belonging to the previous user.$_SESSIONbeing empty or unset to detect a "guest" state, or checks for specific keys in$_SESSIONprior 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):
$_SESSIONwithout starting a session:Workarounds
session_start()is called immediately at the entry point of your worker script to overwrite any residual data (though this may not cover all edge cases if middleware runs before the controller).$_SESSIONat the very beginning of the worker loop, before handling the request.References