The flaw stems from error in the handling of redirections for URLs with the "mhtml:" URI handler. Security notification firm Secunia reports that the same bug was discovered six months ago in IE6 but remains unresolved. In any case, the flaw has managed to find its way into the code-base of IE7.
The vulnerability - rated as "less critical" by Secunia, the first to warn of the problem - might be used to access documents served from another web site. So the security bug lends itself to possible misuse in various scam and phishing attacks, Thomas Kristensen, CTO of security notification firm Secunia, told El Reg.
Secunia has produced up a test, featuring proof-of-concept code, in order to illustrate the problem
Since IE7 has just been released, and therefore has a negligible user base, real-life exploitation is somewhat unlikely, at least for now, and the IE6 version of the exploit hasn't been used much in anger, to the best of our knowledge at least. But discovery of the flaw suggests that IE7, just like IE6 and previous versions of Microsoft's browser software, will require regular patching despite much-touted (and not inconsequential) security improvements.
Some security bugs in IE7 were discovered during the development process while others are (doubtless) awaiting discovery. In February, security researcher Tom Ferris discovered a means to crash preview (beta 2) versions of IE7. Microsoft promised to fix the problem. ®