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MOPS-2010-061: PHP SplObjectStorage Deserialization Use-After-Free Vulnerability

Note: This article was originally published at Planet PHP on 26 June 2010.
Planet PHP

A use-after-free vulnerability was discovered in the deserialization of SPLObjectStorage objects that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected versions

Affected is PHP 5.2 Affected is PHP 5.3




This vulnerability was disclosed by Stefan Esser of SektionEins GmbH during the SyScan Singapore 2010 security conference.

Detailed information

PHP's unserialize() function has had many memory corruption and use-after-free vulnerabilities in the past. Therefore it should be obvious by now that exposing it to user supplied input is not a good idea. However many widespread PHP applications directly unserialize() the content of cookies or POST requests. Especially closed source PHP applications developed for websites often use serialized user input.

In addition to that the APIs of popular services/applications like Wordpress transfer serialized data over insecure HTTP connections, which makes them vulnerable to unserialize() exploits via man-in-the-middle-attacks. Even more applications deserialize the content of database fields which means SQL injection vulnerabilities can be used to launch attacks against unserialize(). As demonstrated by the MOPS-2010-060 vulnerability even simple arbitrary writes to the $_SESSION variable can result in attacks against unserialize(), too. And the story does not stop here because many more applications deserialize the content of cache files, so arbitrary file overwrite vulnerabilities can be used to launch attacks against unserialize() and lead to arbitrary code execution although everything except the cache files is not writable.

While the core of the unserialize() function was audited very heavily during the last years the SPL objects shipping with PHP and supporting deserialization have not been audited very much. Therefore it was no suprise to find a use-after-free vulnerability in the SPLObjectStorage implementation that is very similar to a vulnerability in the unserialize() core that was fixed in 2004 and disclosed by us, too.

In PHP 5.3.x the actual vulnerability is caused by the spl_object_storage_attach() function removing previously inserted extra data if the same object is inserted twice.

void spl_object_storage_attach(spl_SplObjectStorage *intern, zval *obj, zval *inf TSRMLS_DC"/

Truncated by Planet PHP, read more at the original (another 11001 bytes)