CVE-2026-82416: 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 <?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: ```sh 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`).