Security consultants at eEye Digital Security privately reported the newly discovered flaws to the software giant on May 5 and warned that users of the dominant Internet Explorer browser and the Microsoft Outlook e-mail client were at risk of PC hijack attacks.
"A vulnerability in default installations of the affected software allows malicious code to be executed, contingent upon minimal user interaction," eEye said in a notice posted on its upcoming advisories Web page.
The company said additional miscellaneous titles are also affected by the remote code execution flaw, which is rated "high risk" and could cause remote code execution attacks.
Operating systems affected include all versions of Windows NT 4.0, fully-patched Windows 2000, and Windows XP (including Service Pack 2).
Marc Maiffret, eEyes co-founder and chief hacking officer, told Ziff Davis Internet News that the vulnerability carries the highest severity rating because of the "ease with which an attacker can compromise a vulnerable system."
"Microsoft has acknowledged these issues and were working with them to get a fix ready," Maiffret said, noting that eEyes disclosure policy is to withhold vulnerability details until a vendor-supplied patch is available.
The latest discovery follows a similar warning issued by eEye in March regarding major security holes in IE and Outlook. Details of those flaws were reported to Microsoft between March 16 and March 19. They remain unpatched.
Back then, Microsoft confirmed the existence of the flaws and promised to "take the appropriate action" to protect affected users once an investigation is completed. "At this time, Microsoft is not aware of any malicious attacks attempting to exploit the reported vulnerabilities, and there is no customer impact based on this issue," a spokesperson said.
Under normal circumstances, Microsoft patches are released on a monthly cycle, but in cases of emergency, the company could release an out-of-cycle update. Since adopting the monthly patching cycle in October 2003, Microsoft has released three out-of-cycle patches, all for "critical" IE flaws.
Separately, eEye also issued a warning for a high-severity bug in multiple versions of RealPlayer, the multi-media software program marketed by Seattle-based RealNetworks Inc.
"A vulnerability exists in Real Player that can allow the execution of arbitrary code if a user is coaxed into opening a specific file type," the company said. A successful exploit could lead to arbitrary code execution.
The flaw was reported to RealNetworks on May 4. A patch is not yet available.
According to Maiffret, the RealPlayer flaw is particularly dangerous to enterprises because an attacker can bypass firewalls and perimeter protection to launch an exploit directly on the desktop.
"When a code execution hole is found in a desktop application like RealPlayer, administrators should take special notice. The bad guys can basically walk right past your protections and launch his attack on the desktop," Maiffret added.
In the absence of mitigation guidance, Web surfers are encouraged to keep anti-virus and anti-spyware software updated and exercise caution when browsing.