Don't automatically unserialize Phar metadata outside getMetadata() View as Markdown

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.

Vote: Stop automatically unserializing Phar metadata outside direct getMetadata() calls — Yes: 25 · No: 0 (closed)

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.