Microsoft has issued an emergency update for its Internet Explorer browser to patch a critical vulnerability attackers are actively exploiting to install malware on targeted computers.

CVE-2015-2502, as the remote code-execution flaw is indexed, can be exploited when vulnerable computers visit booby-trapped websites or possibly when they open malicious HTML-based e-mails. The bug involves the way IE stores objects in memory and results in an error that corrupts memory contents. The vulnerability, which is present in all supported versions of IE, carries Microsoft's top severity of critical for all desktop versions of Windows. The rating is one step lower for server OSes because IE on those versions runs in a restricted mode known as enhanced security configuration.

In an advisory posted Tuesday afternoon, Microsoft officials wrote:

A remote code execution vulnerability exists when Internet Explorer improperly accesses objects in memory. This vulnerability could corrupt memory in such a way that an attacker could execute arbitrary code in the context of the current user. An attacker could host a specially crafted website that is designed to exploit this vulnerability through Internet Explorer, and then convince a user to view the website. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements by adding specially crafted content that could exploit this vulnerability. In all cases, however, an attacker would have no way to force users to view the attacker-controlled content. Instead, an attacker would have to convince users to take action, typically by getting them to click a link in an instant messenger or email message that takes users to the attacker's website, or by getting them to open an attachment sent through email. An attacker who successfully exploited this vulnerability could gain the same user rights as the current user. If the current user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Systems where Internet Explorer is used frequently, such as workstations or terminal servers, are at the most risk from this vulnerability.

The advisory went on to say the vulnerability is being actively exploited, but provided no other details. A separate summary of vulnerabilities patched this month said only that exploits had been detected.

Readers should install the update as soon as possible. Those who cannot do so may find some protection using the Enhanced Mitigation Experience Toolkit (EMET), which helps mitigate exploits.