FrankenPHP: Cross-request PHP session ID leak in worker mode when the session extension is a shared module

Summary

In worker mode under concurrency, FrankenPHP can hand a PHP session id from one request to another when PHP's session extension is loaded as a shared module instead of being statically linked into the FrankenPHP binary. A request presenting Cookie: session=A can have session_id() resolve to B (a different concurrent client's id) or to a stale id from a previous request served by the same worker thread.

Impact

• Security: cross-user session exposure / session hijacking in worker mode. Same impact class as CVE-2026-24894.
• Functional: users are intermittently logged out; their cookie's session is ignored and they receive a foreign or empty session.

Affected / not affected

Exposure is determined by how the session extension is linked in the PHP build FrankenPHP is compiled against, not by the FrankenPHP release:

• Affected: any build where the session extension is not visible to FrankenPHP at compile time, i.e. loaded as a shared / out-of-tree module (extension=session in an ini). This includes the RPM and Debian packages and other packaged php-zts distributions that ship session as a shared extension under conf.d.
• Not affected: builds where session is statically compiled into the PHP that FrankenPHP links against, such as the official static binaries and Docker images. There, the reset code below is compiled in and clears the id every request.

The C session-reset code is unchanged across 1.11.x-1.12.x, so this is not a code regression between releases. A "1.12.2 works / 1.12.3 leaks" observation on the same host reflects two differently-built packages, not a source change. The persistent-zval helpers (#2366) are compiled only under FRANKENPHP_TEST and are unrelated.

Root cause

FrankenPHP resets per-request session state in frankenphp_reset_session_state(), which releases PS(id) so the next session_start() re-reads the request cookie. That whole function is guarded by #ifdef HAVE_PHP_SESSION, a macro defined only when the session extension headers are present at FrankenPHP compile time (session statically enabled in the PHP build). When session is a shared module, HAVE_PHP_SESSION is undefined, the reset is compiled out, and PS(id) survives the request boundary.

PHP's session_start() reads the request cookie only when PS(id) is NULL (ext/session/session.c, try_find_session_id_in_cookies(): if (!PS(use_cookies) || PS(id)) return;). With a stale PS(id), the cookie is ignored and the worker reuses the previous request's id, exposing it to whichever client is served next.

Reproduction

Minimal worker script (index.php):

<?php
$handler = static function () {
    session_start();
    $sent = $_COOKIE['session'] ?? '(none)';
    $id   = session_id();
    header('Content-Type: text/plain');
    echo ($sent === '(none)' || $sent === $id) ? "OK   sent=$sent id=$id\n"
                                               : "LEAK sent=$sent id=$id\n";
    session_write_close();
};
while (\frankenphp_handle_request($handler)) {}

Run under worker mode with num 2 (≥2 workers), session.use_strict_mode=1, on a PHP build where session is a shared module. Pre-create two valid sessions A and B, then fire concurrent requests alternating the two cookies:

seq 600 | xargs -P32 -I{} sh -c '
  c=$([ $(( {} % 2 )) -eq 0 ] && echo A || echo B);
  curl -s -H "Cookie: session=$c" http://127.0.0.1:8080/' | grep -c LEAK

On an affected build a large fraction of responses are LEAK (session_id() differs from the cookie sent, and A and B sometimes resolve to the same id). On a statically-linked build the count is always 0.

Fix

When HAVE_PHP_SESSION is undefined, FrankenPHP now resets the session module through the module registry on every worker request (its RSHUTDOWN releases PS(id), RINIT re-initialises the globals), so the per-request cleanup always runs regardless of how the extension is linked. Statically-linked builds are unchanged and keep using frankenphp_reset_session_state().

Verified on PHP 8.6-dev ZTS, 2 workers, session.use_strict_mode=1, 600 alternating-cookie concurrent requests: before the fix 519/600 leaked; after the fix 0/600. Statically-linked builds stayed at 0/600.

Workarounds

• Use a FrankenPHP build with session statically compiled (official static binaries or Docker images).
• Or, in the worker loop, after session_write_close() call session_id('') so the next session_start() re-reads the request cookie (with session.use_strict_mode=1).