New vulnerability impacts Internet Explorer users

Microsoft is investigating new, public reports of a vulnerability in all supported versions of Internet Explorer. The main impact of the vulnerability is remote code execution. This advisory contains workarounds and mitigations for this issue.

The vulnerability exists due to an invalid flag reference within Internet Explorer. It is possible under certain conditions for the invalid flag reference to be accessed after an object is deleted. In a specially-crafted attack, in attempting to access a freed object, Internet Explorer can be caused to allow remote code execution.

At this time, Microsoft is aware of targeted attacks attempting to use this vulnerability and is monitoring the threat landscape and take action against malicious sites that attempt to exploit this vulnerability.

Mitigating factors

Data Execution Prevention (DEP) helps protect against attacks that result in code execution and is enabled by default in Internet Explorer 8 on the following Windows operating systems: Windows XP Service Pack 3, Windows Vista Service Pack 1, Windows Vista Service Pack 2, and Windows 7.

Protected Mode in Internet Explorer on Windows Vista and later Windows operating systems helps to limit the impact of the vulnerability as an attacker who successfully exploited this vulnerability would have very limited rights on the system. An attacker who successfully exploited this vulnerability on Internet Explorer 7 or Internet Explorer 8 could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less affected than users who operate with administrative user rights.

In a Web-based attack scenario, an attacker could host a Web site that contains a Web page that is used to exploit this vulnerability. In addition, compromised Web sites and Web sites that accept or host user-provided content or advertisements could contain specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to visit these Web sites. Instead, an attacker would have to convince users to visit the Web site, typically by getting them to click a link in an e-mail message or Instant Messenger message that takes users to the attacker’s Web site.