Microsoft has warned that attackers are targeting Internet Explorer (IE) with an exploit of a critical unpatched vulnerability in all current versions of the browser.
Only IE9, which is still in beta, is unaffected.
Microsoft and others confirmed that attacks are circulating in the wild, primarily targeting IE6, the nine-year-old browser that Microsoft's been trying to kill for more than a year.
"So far, the attacks we have seen only target Internet Explorer 6 and would not have been successful against Internet Explorer 8," said Andrew Roths, Jonathan Ness and Chengyun Chu, three engineers who work on the Microsoft Security Response Centre team.
Microsoft downplayed the threat, saying it has seen only "extremely limited" attacks thus far.
The exploit relies on a heap spray to take down IE, said Roths, Ness and Chu. Hackers can hijack Windows PCs by getting users to visit a malicious site, making the threat a classic "drive-by" attack that can instantly commandeer a machine with a vulnerable version of IE.
Although the newer IE8 contains the vulnerability, it's immune to the current round of attacks because it switches on DEP, or data execution prevention, by default. DEP is one of two key defensive measures within Windows -- the other is ASLR, or address space layout randomization -- designed to block attacks, or at least make the hacker work harder.
Antivirus vendor Symantec said that it had first seen exploits aimed at the IE bug several days ago when it came across spam that had been sent to select individuals within some organizations. The messages posed as hotel reservation notifications.
"Within the e-mail, the perpetrators added a link to a specific page hosted on an otherwise legitimate site," said Symantec researcher Vikram Thakur in an entry on his company's blog . "The hackers had gotten access to the Web site account and uploaded content without the owners knowing."
Anyone visiting the hacked site with IE6 or IE7 -- the former doesn't support DEP, while the latter doesn't enable it by default -- is infected with malware that opens a "backdoor" on the compromised computer, then downloads a number of files containing additional commands
Symantec said it reported the bug to Microsoft and reached out to the owners of the server hosting the attack page and malware. That server has since been taken offline.
"The files on this server had been accessed by people in lots of organizations in multiple industries across the globe," said Thakur. "[But] very few of them were seen accessing the payload file, which means that most users were using a browser which wasn't vulnerable or targeted."
Microsoft did not say when it would patch the bug, and urged users to protect themselves by upgrading to IE9's beta or implementing one of several workarounds. Among the latter: Turn on DEP in IE7; apply a custom cascading style sheet (CSS) for formatting documents loaded in IE; and deploy and configure EMET (Enhanced Mitigation Experience Toolkit), a free utility available for download from Microsoft's site .
EMET is a stop-gap designed to keep older applications secure until companies upgrade to up-to-date, and theoretically safer, versions of those programs. The tool lets IT administrators, and consumers willing to take the plunge, switch on several Windows defenses -- including ASLR and DEP -- for applications whose developers didn't turn them on by default.
Microsoft also said that it's unlikely that the exploit could be bundled with another to sidestep DEP. "The current techniques for bypassing DEP cannot be directly applied because the memory corruption is a partial vtable pointer overwrite," said Roths, Ness and Chu.
Attacks able to work around DEP have become more popular of late. Last March, Dutch researcher Peter Vreugdenhil exploited a vulnerability in IE8 running on Windows 7 with attack code that evaded both DEP and ASLR to win $10,000 at the fourth-annual Pwn2Own contest. Several months later, Ruben Santamarta, a researcher at the Spanish security firm Wintercore, published attack code for a critical vulnerability in IE8 that he said bypassed DEP and ASLR.
The next regularly-scheduled Patch Tuesday for Microsoft is 9 November, but if past practice is any clue, it's very unlikely a fix will be available by then. A better bet would be 14 December, which will probably contain a cumulative update for IE because Microsoft has taken to patching its browser on even-numbered months.
So far, the bug doesn't meet the bar for issuing an emergency, or "out-of-band" update, Microsoft spokesman Jerry Bryant said.