Microsoft, DHS Warn of Zero-Day Attack Targeting IE Users

A targeted attack is targeting a previously unknown vulnerability in Internet Explorer to corrupt memory and exploit victims' Windows systems, Microsoft warned in an advisory published on January 17.

The flaw, described as a scripting engine memory corruption vulnerability and designated CVE-2020-0674, allows an attacker to take control of a Windows system by forcing it to use an older version of Microsoft's JavaScript that is only present for backward compatibility. By default, Internet Explorer does not use the vulnerable dynamic library, Microsoft stated.

"The vulnerability could corrupt memory in such a way that an attacker could execute arbitrary code in the context of the current user," Microsoft stated in Advisory 200001. "An attacker who successfully exploited the 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 the vulnerability could take control of an affected system."

While the attack is serious, its impact is limited because Internet Explorer is only used by a limited number of users who want backward compatibility with older Microsoft technologies. Currently, only 2.3% of visitors use Internet Explorer 11, one of the vulnerable versions, according to W3counter.

The vulnerable library, <code>jscript.dll</code> is typically not used, so an attacker needs to control the website or have created a web page that is opened in a vulnerable browser.

"By convincing a user to view a specially crafted HTML document — [that is,] a web page [or] an email attachment — PDF file, Microsoft Office document, or any other document that supports embedded Internet Explorer scripting engine content, an attacker may be able to execute arbitrary code," Carnegie Mellon University's CERT Coordination Center stated in an advisory.

Companies that rely on Internet Explorer — a much smaller portion than a decade ago — should apply Microsoft's recommended workaround as soon as possible, says Casey Ellis, founder and chief technology officer of Bugcrowd.

"In the absence of a patch, having a workaround is crucial, and it's great that Microsoft provided alternatives to mitigate the risk to users," he says. "Since Google's Threat Analysis Group reported the vulnerability, it's unlikely that Chrome is affected by a similar bug and is safe to use."

The advice to use another browser is a more viable protection, mostly since other browsers are now much more popular than Microsoft Edge. Currently, only about 8% of web visitors use either Internet Explorer or, more likely, Microsoft Edge, according to W3Counter.

This is not the first time that Microsoft has had to scramble to contain attacks targeted its older scripting engines. If it seems like deja vu, it's because Microsoft patched a similar flaw in November. The issue, CVE-2019-1429, allowed attackers to corrupt the scripting engine's memory using a specially crafted website or an ActiveX control.

A year before that, another vulnerability, CVE-2018-8653, affected the scripting engine of Internet Explorer, allowing attackers to execute Visual Basic scripts or Microsoft's version of JavaScript.

Although Microsoft adopted a bug bounty to head off flaws, nation-state and criminal hackers continue to find ways to compromise systems, raising the question: If Microsoft's bug bounty did not convince the attacker to sell the vulnerability information to the software maker, are bug bounties effective?

Bugcrowd's Ellis defends the bounties because they raise the price of exploits and give ethical researchers another reason to disclose issues.

"This does not undermine bug bounties or crowdsourced security," he says. "The reality is that since the exploit has been used in limited targeted attacks, it is likely an offensive buyer paid more for it than Microsoft was offering or it was developed in-house for offensive use."

Ellis notes that Microsoft credited two organizations for finding the latest issue.

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