GHSA-m7v2-7gxm-vc2v

### Description

`Symfony\Bridge\Monolog\Command\ServerLogCommand` (the `server:log` console command) is a development-time helper that opens a TCP listener and displays log records pushed to it by the application's logging pipeline. Two unsafe defaults combine into a remotely reachable PHP object-deserialization sink:

1. The listener binds to `0.0.0.0:9911` by default; it accepts connections on every interface, not only loopback. 2. Each received frame is processed as `unserialize(base64_decode($message))` without an `allowed_classes` allowlist, without authentication, and without any integrity check. The decoded value is then passed to `displayLog(..., array $record)` which assumes (without validating) that the result is an array.

Any host that can reach TCP port 9911 on a machine running `server:log` can therefore submit attacker-chosen serialized PHP payloads. The minimum impact is an unauthenticated denial of service (sending a non-array, e.g. `serialize(new stdClass())`, crashes the listener with a type error). Object injection with magic-method side effects (`__wakeup()` / `__destruct()` / etc.) is reachable before the array type-check fires; full remote code execution is environment-dependent and contingent on usable gadget chains in the autoload set of the target process.

### Resolution

The `server:log` command no longer binds to all interfaces by default: the default `--host` is now `127.0.0.1:9911`, requiring explicit opt-in to accept off-host traffic. Message decoding is gated by an `unserialize()` allowlist restricted to the `Symfony\Component\VarDumper\Caster\*` and `Symfony\Component\VarDumper\Cloner\*` classes that legitimately appear inside dumped log records; any other class is rejected and the record discarded.

The patch for this issue is available [here](https://github.com/symfony/symfony/commit/0891b2f293896c488e26943dc034334364b77fc4) for branch 5.4.

### Credits

Symfony would like to thank Toàn Thắng and Sam Sanoop for reporting the issue and Nicolas Grekas for fixing it.