Microsoft words it:
Vulnerability in Microsoft Font Driver Could Allow Remote Code Execution (3079904).
But later makes the danger a little clearer:
A remote code execution vulnerability exists in Microsoft Windows when the Windows Adobe Type Manager Library improperly handles specially crafted OpenType fonts. An attacker who successfully exploited this vulnerability could take complete control of the affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.
There are multiple ways an attacker could exploit this vulnerability, such as by convincing a user to open a specially crafted document, or by convincing a user to visit an untrusted webpage that contains embedded OpenType fonts. The update addresses the vulnerability by correcting how the Windows Adobe Type Manager Library handles OpenType fonts.
When this security bulletin was issued, Microsoft had information to indicate that this vulnerability was public but did not have any information to indicate this vulnerability had been used to attack customers. Our analysis has shown that exploit code could be created in such a way that an attacker could consistently exploit this vulnerability. (emphasis added)
Show of hands: How many of you visit untrusted sites with embedded OpenType fonts?
Microsoft rates this critical and for all versions of Windows.
No patch has been issued for Windows XP or Windows Server 2003.