The belief that cybercriminals stockpile exploits, then time their release to do the most damage gives them too much credit, according ro security researchers.
"We think that [attackers are] all highly skilled and doing careful planning," said Craig Schmugar, a security researcher for McAfee, "It's not always the case".
Schmugar compared the disclosure date of 200 zero-day vulnerabilities affecting Windows against the nearest monthly Microsoft patch day to find out if there was anything to the idea of "Exploit Wednesday" - the supposed hacker practice of releasing exploits immediately after the release of regularly-scheduled security updates from Microsoft on the second Tuesday of each month.
Under the Exploit Wednesday concept, attackers deploy threats right after a patch day to maximise the window of vulnerability, figuring that they have at least 30 days before the next round of patches is released by Microsoft.
There doesn't seem to be much to the idea, said Schmugar. "I don't see Exploit Wednesday as a strategically-timed release, but that it comes about simply because more information is being made available," he said.
Some hackers search Microsoft's vulnerability disclosures for information to put them on an exploit track, Schmugar said, while others reverse engineer an attack after comparing the patched files with their vulnerable predecessors.
The data doesn't prove that's what happens, Schmugar acknowledged, but it did discount the idea that most hackers are patient, intelligent and lucky enough to strategically launch their exploits right after a patch cycle.
In 2005, for example, 18% of the zero-day threats were disclosed within a three-day period either side of Patch Tuesday; a normal distribution for that week-long span would be 23%, Schmugar pointed out. During 2006, 31% were within 3 days of patch day; so far this year, 24% fall within the range. "The data suggests that at least in 2005 and 2007 strategic releases were not that common," Schmugar said.
"Even 2006 only showed an 8% deviation."
Hackers have to weigh any attempt at attack timing against the possibility that the zero-day vulnerability will be discovered, and patched, before they can launch it, said Schmugar. "It's like trying to sell stock at its peak price. Yes, attackers could potentially hold their vulnerabilities, but they could also shoot themselves in the foot by doing that."
Interestingly, a follow-up analysis hinted that attackers do hoard the most valuable vulnerabilities -- those originally reported as able to execute remote code. Of that zero-day subset, 41% of these most critical vulnerabilities were disclosed within 3 days either way of Patch Tuesday in 2006, and 30% so far this year, for a deviation of 18% and 7%, respectively.
Even here, however, Schmugar was suspicious, since the dates associated with active exploits are inherently inaccurate. "We don't always know right away of a zero-day vulnerability or exploit," said Schmugar. "I've been privy to information that showed by the time a patch was released, the attack had existed two or three weeks."
Schmugar was hesitant to stake out a conclusion because the data could be interpreted more than one way. "I'm somewhere in the middle between thinking attackers are and are not strategically launching exploits.
“Generally, though, because of the risk that their vulnerabilities will be found, I don't believe they think it's even worth it to hold one."