More than 9 out of every 10 Windows users are vulnerable to the recent Flash zero-day vulnerability according to a Danish security company. Adobe has said that it won't patch the flaw until this Thursday.

Secunia said that 92 percent of the 900,000 users who have recently run the company's Personal Software Inspector (PSI) utility have Flash Player 10 on their PCs, while 31 percent have Flash Player 9. (The total exceeds 100 percent because some users have installed both.)

The most-current versions of Flash Player - 9.0.159.0 and 10.0.22.87) - are vulnerable to hackers conducting drive-by attacks hosted on malicious and legitimate-but-compromised sites. Anti-virus vendors have reported hundreds, in some cases thousands, of sites launching drive-bys against Flash.

Secunia's PSI also pegged the installed base of the current Adobe Reader 9.1.2 and Abode Acrobat 9.1.2 at 48 percent and 2 percent, respectively. Because both include an interpreter to handle Flash content embedded in PDF files, they also can be exploited. The initial attacks, in fact, were based on rigged PDFs.

Adobe has acknowledged that Flash, Reader and Acrobat contain a critical bug. Last Wednesday, it kicked its security process into high gear, promising it would deliver patches for Flash by 30 July, and fixes for Reader and Acrobat by 31 July.

Until then, users have few options other than to delete, disable or rename the flawed component, "authplay.dll;" Adobe has posted terse instructions in a security bulletin, as have other organisations, including the US Computer Emergency Response Team ( US-CERT).

The bug at the root of the vulnerability was first logged in Adobe's bug tracking database nearly seven months ago, at the end of 2008.

PSI scans Windows systems for installed applications, then compares their version numbers to the most up-to-date editions; if they're different, it makes note, then provides a link to the patch update. "[A] PC user with vulnerabilities in his installed software, is like a house owner with open or unlocked doors," said Mikkel Winther, the manager of Secunia's PSI partner programme. "Maybe nobody will rob his house or compromise his system, but it is indeed possible and he hasn't secured himself against it."

Adobe has been faced with one security emergency after another this year. In mid-March, it patched several Reader vulnerabilities, then followed that with two more updates in May and June. Also last week, after Secunia noticed that Adobe continued to provide an outdated edition of Reader for download from its website, Adobe said it might change how its software updater worked.