Microsoft has confirmed the presence of a zero-day vulnerability impacting Internet Explorer (IE) versions 6 and 7.
The software giant said in an advisory issued late Monday that the flaw "exists as a null pointer reference of Internet Explorer" and involves the way the browser handles cascading style sheets objects. Attackers may be able to execute code on victims' machines if they can persuade them to visit a malicious website.
In addition, cybercriminals can compromise normally trusted websites to perpetrate the attack, said Michael Sutton, vice president of security research at security firm Zscaler.
“Internet Explorer versions 6 and 7 account for approximately 41 percent of web browsers in use today, so this vulnerability will be an enticing one for attackers,” he said. “Attacks such as these are also prime candidates for targeting otherwise legitimate websites as an attack vector. The exploit can be triggered simply via HTML code, so attackers can inject code into websites with weak security protections.”According to Symantec experts, reliable proof-of-concept code has yet to be published, but it is expected.
There have been no reports of in-the-wild exploits, according to the advisory. IE 8 is not affected.
Microsoft listed a number of suggestions that users can take to lessen the possibility of being hit by an exploit. They include mitigating actions, such as running IE 7 in "Protected Mode," and workarounds, including setting internet and intranet security zone settings to "high" before running ActiveX controls.
Company engineers are working on a patch and plan to release one once it is cleared for widespread distribution. Microsoft's next scheduled security update release is Dec. 8, but the firm occasionally issues out-of-band fixes for emergency vulnerabilities.
In the meantime, Microsoft and its internet partners plan to patrol the web for instances of active attacks.
"Together with our partners, we will continue to monitor the threat landscape and will take action against any websites that seek to exploit this vulnerability," Jerry Bryant, senior security program manager at Microsoft, said in a blog post Monday.