The list of serious unpatched vulnerabilities in Microsofts Internet Explorer browser keeps getting longer and longer.
Less than a week after researcher Michal Zalewski went public with a new zero-day vulnerability that could be used in code execution attacks, the software maker has acknowledged yet another flaw affecting fully patched Windows systems.
The new IE flaw was discovered by Secunia researcher Andreas Sandblad during code analysis into the Zalewski warning.
In fact, Secunia initially reported its findings to Microsoft as a "successful exploit" of that bug, but according to Microsofts internal investigations, Secunia actually found a new problem.
"This is potentially a new, privately reported, exploitable vulnerability," a Microsoft spokesperson told eWEEK.
Secunia has since updated its advisory with a note that its discovery is a "variant" of the bug reported by Zalewski.
The flaw is due to a memory corruption error when processing a specially crafted HTML script that contains malformed "object" tags, and could be exploited by attackers to remotely take complete control of an affected system by convincing a user to visit a specially crafted Web page.
"[We have] confirmed code execution on a fully patched system with Internet Explorer 6.0 and Microsoft Windows XP SP2. Other versions may also be affected. Details about this variant will not be publicly disclosed at present, but have been sent to Microsoft, who is currently working on a patch," the company said.
Secunias discovery is particularly worrisome because it was found during an investigation into a publicly reported issue.
It is likely that someone else with malicious intent could make a similar discovery and turn it into a zero day attack, says Matthew Murphy, an independent security researcher who has himself reported IE flaws to Microsoft recently.
"Its likely that theyre dealing with two very similar attacks and two distinct code flaws. That wouldnt be all that surprising, because code that has one major bug will often suffer from some similar defects elsewhere," Murphy said.
Murphy commended Microsoft for acknowledging that the flaw can turn into an exploit.
"I think making it known that the issue is exploitable is a good step, because people need to know that to assess risk. However, now that we know its exploitable, more people are going to be looking to find out how to do it," he added.
Secunia has slapped a "highly critical" rating on the flaw, warning that it can be used in PC takeover attacks.
Based on 85 advisories published by Secunia between 2003 and 2005, about 25 percent of IE bugs remain unpatched. More than 40 percent of those advisories are serious enough to be used in system compromise attacks.