Critical vulnerabilities were found in Joomla!. All are due to improper verification or sanitisation of input data. Details were not specified but it is known that a successful attack allows to steal informations and execute arbitrary HTML and script code. Flaws were found in versions 1.7.3 and prior. Users should upgrade to version 1.7.4 or 2.5.0.

Source: Secunia
26.01.2012