Microsoft Outlook is a popular mail client.
A vulnerability exists in Outlook which allows execution of arbitrary code when a victim user views a web page or an e-mail message created by an attacker.
During Outlook installation, a mailto: URL handler is registered to the system. When a mailto: URL is opened, the system starts OUTLOOK.EXE with the following arguments:
OUTLOOK.EXE -c IPM.Note /m "mailto:email@address"
A web page or e-mail message exploiting this flaw may contain for instance an IMG tag to refer to a mailto: URL. The victim user need not click on a link.
The issue is not a standard "cross site scripting" vulnerability, but a different kind of injection attack. The exploit can inject command line switches and arguments to OUTLOOK.EXE because quote symbols in the URL aren't escaped or otherwise processed. This can be considered a new vulnerability category, and further investigation has shown that similar attacks can be carried out against other software which register a URL handler.
According to Microsoft the affected supported versions are Microsoft Office XP SP2 and Microsoft Outlook 2002 SP 2.
Microsoft was informed on July 21st, 2003 and has released an update to correct the problem. A bulletin describing the update can be seen here.
The vulnerability was discovered and researched by Jouko Pynnönen (firstname.lastname@example.org), Finland.