Improper Input Validation
The mailto (aka com_mailto) component in Joomla! 1.5 before 1.5.7 sends e-mail messages without validating the URL, which allows remote attackers to transmit spam.
CWE-20 - Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.