Microsoft and FireEye first discussed this issue in November, 2013.
Vulnerable component: Microsoft XML Core Services
CVSSv3 score: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N/E:F/RL:O/RC:C
CWE-ID: CWE-284 - Improper Access Control
The vulnerability allows a remote attacker to obtain potentially sensitive information on the target system.
The weakness exists due to improper enforcement of cross-domain policies. A remote attacker can create a specially crafted Web page, trick the victim into visiting it using Internet Explorer, bypass cross-domain security restrictions and read local files or content from web domains the victim is authenticated with.
Successful exploitation of the vulnerability results in information disclosure on the vulnerable system.
Note: the vulnerability was being actively exploited.