phar-stream-wrapper

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PHP Phar Stream Wrapper

Abstract & History

Based on Sam Thomas’ findings concerning insecure deserialization in combination with obfuscation strategies allowing to hide Phar files inside valid image resources, the TYPO3 project decided back then to introduce a PharStreamWrapper to intercept invocations of the phar:// stream in PHP and only allow usage for defined locations in the file system.

Since the TYPO3 mission statement is inspiring people to share, we thought it would be helpful for others to release our PharStreamWrapper as standalone package to the PHP community.

The mentioned security issue was reported to TYPO3 on 10th June 2018 by Sam Thomas and has been addressed concerning the specific attack vector and for this generic PharStreamWrapper in TYPO3 versions 7.6.30 LTS, 8.7.17 LTS and 9.3.1 on 12th July 2018.

With PHP 8.0.0 the default behavior changed and meta-data is not deserialized automatically anymore:

License

In general the TYPO3 core is released under the GNU General Public License version 2 or any later version (GPL-2.0-or-later). In order to avoid licensing issues and incompatibilities this PharStreamWrapper is licenced under the MIT License. In case you duplicate or modify source code, credits are not required but really appreciated.

Credits

Thanks to Alex Pott, Drupal for creating back-ports of all sources in order to provide compatibility with PHP v5.3.

Installation

The PharStreamWrapper is provided as composer package typo3/phar-stream-wrapper and has minimum requirements of PHP v5.3 (v2 branch) and PHP v7.0 (master branch).

Installation for PHP v7.0

composer require typo3/phar-stream-wrapper ^3.0

Installation for PHP v5.3

composer require typo3/phar-stream-wrapper ^2.0

Example

The following example is bundled within this package, the shown PharExtensionInterceptor denies all stream wrapper invocations files not having the .phar suffix. Interceptor logic has to be individual and adjusted to according requirements.

\TYPO3\PharStreamWrapper\Manager::initialize(
    (new \TYPO3\PharStreamWrapper\Behavior())
        ->withAssertion(new \TYPO3\PharStreamWrapper\Interceptor\PharExtensionInterceptor())
);

if (in_array('phar', stream_get_wrappers())) {
    stream_wrapper_unregister('phar');
    stream_wrapper_register('phar', \TYPO3\PharStreamWrapper\PharStreamWrapper::class);
}

Interceptors

The following interceptor is shipped with the package and ready to use in order to block any Phar invocation of files not having a .phar suffix. Besides that individual interceptors are possible of course.

class PharExtensionInterceptor implements Assertable
{
    /**
     * Determines whether the base file name has a ".phar" suffix.
     *
     * @param string $path
     * @param string $command
     * @return bool
     * @throws Exception
     */
    public function assert(string $path, string $command): bool
    {
        if ($this->baseFileContainsPharExtension($path)) {
            return true;
        }
        throw new Exception(
            sprintf(
                'Unexpected file extension in "%s"',
                $path
            ),
            1535198703
        );
    }

    /**
     * @param string $path
     * @return bool
     */
    private function baseFileContainsPharExtension(string $path): bool
    {
        $baseFile = Helper::determineBaseFile($path);
        if ($baseFile === null) {
            return false;
        }
        $fileExtension = pathinfo($baseFile, PATHINFO_EXTENSION);
        return strtolower($fileExtension) === 'phar';
    }
}

ConjunctionInterceptor

This interceptor combines multiple interceptors implementing Assertable. It succeeds when all nested interceptors succeed as well (logical AND).

\TYPO3\PharStreamWrapper\Manager::initialize(
    (new \TYPO3\PharStreamWrapper\Behavior())
        ->withAssertion(new ConjunctionInterceptor([
            new PharExtensionInterceptor(),
            new PharMetaDataInterceptor(),
        ]))
);

PharExtensionInterceptor

This (basic) interceptor just checks whether the invoked Phar archive has an according .phar file extension. Resolving symbolic links as well as Phar internal alias resolving are considered as well.

\TYPO3\PharStreamWrapper\Manager::initialize(
    (new \TYPO3\PharStreamWrapper\Behavior())
        ->withAssertion(new PharExtensionInterceptor())
);

PharMetaDataInterceptor

This interceptor is actually checking serialized Phar meta-data against PHP objects and would consider a Phar archive malicious in case not only scalar values are found. A custom low-level Phar\Reader is used in order to avoid using PHP’s Phar object which would trigger the initial vulnerability.

\TYPO3\PharStreamWrapper\Manager::initialize(
    (new \TYPO3\PharStreamWrapper\Behavior())
        ->withAssertion(new PharMetaDataInterceptor())
);

Reader

$reader = new Phar\Reader('example.phar');
var_dump($reader->resolveContainer()->getManifest()->deserializeMetaData());

Helper

Security Contact

In case of finding additional security issues in the TYPO3 project or in this PharStreamWrapper package in particular, please get in touch with the TYPO3 Security Team.

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