Adobe is set to release a patch today for an older version of the Reader PDF viewer to stymie attacks like those aimed at major defence contractors earlier this month.

Nine days ago, the company confirmed a critical bug in Reader and promised to fix the flaw in Reader and Acrobat 9.x this week.

The exploits uncovered by security researchers were aimed specifically at Reader 9.x using malformed PDF documents attached to bogus emails.

A day after Adobe acknowledged the vulnerability, researchers at Symantec confirmed that attacks had targeted defence contractors , as well as individuals working in the telecommunications, manufacturing, computer hardware and chemical sectors. The attacks spiked on 1 December, Symantec said.

The attackers may have been hoping to steal confidential information from the targeted firms.

If opened by the recipient, the malicious PDF hijacked the Windows PC, then infected those machines with "Sykipot," a general-purpose backdoor Trojan that was first spotted being used in March 2010 as the payload in attacks exploiting a then-unpatched bug in Microsoft's IE6 and IE7.

Later research by Symantec and others found hints of Chinese involvement: Code remnants were in the Simplified Chinese character set, and the malware's command-and-control server was traced to a Chinese IP address.

But unlike Symantec, independent security researcher Brandon Dixon didn't think a national government or other well-funded organisation was behind the Sykipot attacks that exploited the Reader flaw.

"The tool used to create this malicious PDF document has little modularity or sophistication. For this reason alone I have a hard time believing this attack was created by a nation-state government," Dixon said in a blog post last weekend, one of three in which he analysed the threat.

"Instead, I think this was done by a small group of people whose motivation would be to support their government and send data back to them. This sort of behaviour fits the Chinese hacker model and gives a bit more value to the traits identified within the document and dropper."

Adobe today again told users - as it did last week - that it will not deliver patches for Reader and Acrobat 10 on Windows, or for any version of those applications on Mac OS X and Unix, until January 10, 2012.

It has justified the delay by pointing out that Reader 10 includes an anti-exploit "sandbox" which blocks the in-circulation exploit, and that it has seen no sign of attacks targeting Mac or Linux machines.

The patched versions of Reader and Acrobat 9.x will be available tomorrow from Adobe's website . Alternately, users will be able to run the programs' integrated update tool or wait for the software to prompt them that a new version is available.

Adobe has not disclosed what time it will issue the Reader and Acrobat 9.x updates.

Friday's fix will be the sixth security update for Reader this year.