allowed_classes_callback option for unserialize()

> I'm writing a this email to propose adding allowed_classes_callback > option to unserialize(). > > The class name as parsed from the serialized string is passed as a > parameter to the callback, it should return a boolean. true would allow > the class, false would block it. This blocks classes the same way > 'allowed_classes' would, but by callback instead. The callback will be > triggered *after* allowed_classes is evaluated (if present). Blocking > will have the same effect as allowed_classes, using > __PHP_Incomplete_Class. > > This callback would solve a few problems where allowed_classes is not > sufficient: > > - This would also allow for fixing legacy applications where it is not > exactly clear what is being unserialized. In my use-case the callable > returns a true value but a E_USER_DEPRECATION is triggered. This way > data can be collected about what classes to allow through monitoring > these deprecations, providing a non-disrupting way to secure > unserialize calls. This is especially helpful in very generic > unserialize usages like caches. > > - It would allow for an is_subclass_of() check where for example an > interface can be added to classes that are safe to get unserialized. > Current allowed_classes array only matches the exact class, not it's > children. > > Note that these problems are not resolvable by using > unserialize_callback_func because that call only happens for unloaded > classes, where the PHP Object Injection vulnerability > https://owasp.org/www-community/vulnerabilities/PHP_Object_Injection > affects classes that might already have been loaded. > > In de pull request Jakub wrote: >> Personally I don't see any issue here and it seems quite self contained and small so I wouldn't mind to get it merged without RFC. It might be worth to email internals first to double check that there are no objections first. > > Out of scope: I understand that __PHP_Incomplete_Class is not very > 2025. Adding an option throw_for_unknown_classes has been suggested on > this list. I'm happy to implement that, but for now I'd like to keep it > at allowed_classes_callback. > > Feedback is very much appreciated. > > The change and some more details live here: > https://github.com/php/php-src/pull/19087