Microsoft Corp. this week again changed the severity rating of a recently discovered vulnerability after a security researcher showed the company that the problem was more serious than Microsoft officials originally thought.
The company last week upgraded the severity rating of a different flaw in Internet Explorer.
The vulnerability in question this time around lies in the way that IE handles PNG (Portable Network Graphics) images. The browser fails to correctly check the parameters of PNG files when it opens them, which can result in a buffer overrun. Other Microsoft applications, including the Office products, use IE to render PNG images and exploiting this vulnerability against one of these applications would allow an attacker to run code on a users machine.
In its original advisory on Nov. 20, Microsoft gave this vulnerability a rating of "important," but on Thursday escalated it to "critical," the highest rating. The change was spurred by research from eEye Digital Security Inc., which discovered that it was possible to run code on remote users systems.
The first version of the bulletin said that an attacker could only force IE to crash by exploiting this flaw. eEye published its advisory on the BugTraq security mailing list on Wednesday.
This is the second time in about a week that Microsoft, of Redmond, Wash., has moved to upgrade the severity rating of a vulnerability.
The company on Dec. 4 released a cumulative patch for IE, which also fixes a new flaw that the company said could allow a Web site to access information on users machines.
The company rated the vulnerability as "moderate" and said that attackers could read, but not change, files on a vulnerable machine or run without parameters an executable file already present on the computer. However, a well-known security researcher posted a message to BugTraq disputing Microsofts assessment of the vulnerability and saying that the flaw is much more severe than the company let on.
Microsoft later upgraded the severity rating to "critical."