CVE 2004-1019
The deserialization code in PHP before 4.3.10 and PHP 5.x up to 5.0.2 allows remote attackers to cause a denial of service and execute arbitrary code via untrusted data to the unserialize function that may trigger "information disclosure, double-free and negative reference index array underflow" results.
See the 
CVE page on Mitre.org
for more details.
