PHP: rfc:phar_stop_autoloading_metadata

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====== PHP RFC: Don't automatically unserialize Phar metadata outside getMetadata() ======
  * Version: 0.4
  * Date: 2020-07-07
  * Author: Tyson Andre <tandre@php.net>
  * Status: Implemented
  * First Published at: https://wiki.php.net/rfc/phar_stop_autoloading_metadata
  * Implementation: https://github.com/php/php-src/pull/5855

===== Introduction =====

In 2018, various vulnerabilities were found in php web frameworks due to the phar stream wrappers. A call such as ''%%file_exists("phar://...somepath.extension/file/within/phar.ext")%%'' will lead to unserializing the metadata found in the phar. As https://php.net/unserialize notes, "unserialization can result in code being loaded and executed due to object instantiation and autoloading, and a malicious user may be able to exploit this."

Various blog posts have been written explaining the resulting vulnerabilities,
such as https://www.ixiacom.com/company/blog/exploiting-php-phar-deserialization-vulnerabilities-part-1

===== Proposal =====

Don't unserialize the metadata automatically when a phar file is opened by php. Make PHP unserialize the metadata **only** if ''%%Phar->getMetadata()%%'' or ''%%PharFile->getMetadata()%%'' is called directly.

Additionally, add an ''array $unserialize_options = []'' parameter to both getMetadata() implementations, defaulting to the current default ''unserialize()'' behavior such as allowing any classes. (As an implementation detail, if ''$unserialize_options'' is set to anything other than the default, the resulting metadata won't be cached and this won't return the value from the cache. E.g. ''%%getMetaData(['allowed_classes' => []])%%'' after ''setMetadata(new stdClass())'' will likely trigger a ''%%unserialize(['allowed_classes' => []])%%'' call internally.)

This implements one possible solution for https://bugs.php.net/bug.php?id=76774

===== Backward Incompatible Changes =====

Any side effects from ''%%__wakeup()%%'', ''%%__destruct()%%'', etc. that were triggered during/after unserialization of metadata when the phar is loaded will stop happening, and will only happen when ''getMetadata()'' is directly called.

Eliminating the side effects is the goal of this RFC, for security reasons. Typical phars use scalars and associative arrays for any metadata, and many phars don't need metadata at all.

===== Proposed PHP Version(s) =====

8.0

===== RFC Impact =====
==== To SAPIs ====
This affects stream wrapper calls such as ''file_exists()'' (they will no longer call ''unserialize()'' if there is metadata), as well as direct calls to ''%%Phar->getMetadata()%%'' (they will call ''unserialize()'' if there is metadata, instead of using the data from the time the phar file was loaded)

===== Vote =====

Yes/No, requiring 2/3 majority to stop automatically unserializing metadata.
Voting started on 2020-07-21 and ended 2020-08-04.

<doodle title="Stop automatically unserializing Phar metadata outside direct getMetadata() calls" auth="tandre" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== References =====

https://externals.io/message/110871 '[RFC] Don't automatically unserialize Phar metadata outside getMetadata()'

https://externals.io/message/110856 'Including "Disable the ability to use concrete types in PHAR metadata" in PHP 8.0?'

https://externals.io/message/105271 'PHP deserialization techniques offer rich pickings for security researchers'

https://bugs.php.net/bug.php?id=76774 'Request #76774 Disable the ability to use concrete types in PHAR metadata'

===== Rejected Features =====

I'd considered continuing to call ''unserialize()'' all of the time to validate phars earlier, but with ''allowed_classes=[]'' instead (controlled by an ini setting). This seemed like an unnecessary ini setting that could be avoided by just not automatically loading the metadata.

===== Changelog =====

0.4: Change from ''getMetadata($allowed_classes = ...)'' to ''getMetadata(array $unserialize_options = [])'' in this document. I forgot about max_depth being added in php 8.0 and the usefulness of being able to support future options added to unserialize() without changing the signature of getMetadata. Elaborate on implementation details ''$unserialize_options'' would lead to when setMetaData is called before ''%%$pharFileOrEntry->getMetadata(['allowed_classes' => $classes])%%''

0.3: Clarify wording, add link to RFC announcement thread. Remove inapplicable ini defaults section.

0.2: Link to implementation.
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