source: http://www.securityfocus.com/bid/26707/info

Joomla! is prone to multiple SQL-injection vulnerabilities because it fails to sufficiently sanitize user-supplied data before using it in an SQL query.

Exploiting these issues could allow an attacker to compromise the application, access or modify data, or exploit latent vulnerabilities in the underlying database.

Joomla! 1.5 RC3 is vulnerable; other versions may also be affected.

UPDATE (December 10, 2007): The validity of the issues is being disputed on the Joomla! Bug Tracker. Please see the references for details. Reports indicate that the related message was posted by a Joomla! developer, but this has not been confirmed. 

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