There is a new and unpatched vulnerability with exploit code in the wild that affects the latest version of IE. The exploit works by including an abnormally large (a couple thousand) number of script actions inside a single HTML tag. This will cause a memory array to write out of bounds and cause an immediate or eventual browser crash. Both McAfee and Symantec have released signatures to detect this exploit. While this is only a DoS vulnerability at the moment, there is ongoing attempts to try to use this as a vector for remote code execution.
New IE 0-Day Exploit in the Wild - SANS