A security researcher says he can force Adobe Systems' widely used PDF readers to execute potentially malicious commands despite an emergency security fix the company released earlier this week.

The update Adobe added to its Reader and Acrobat applications contained a patch designed to prevent attackers from using the apps to launch potentially dangerous commands or files on end users' machines. But Le Manh Tung, a senior security researcher at Viet Nam–based Bkis Internet Security, said he can bypass the fix by doing nothing more than putting quotation marks around the command he wants a targeted machine to remotely execute.

The weakness was first demonstrated by researcher Didier Stevens and later expanded by Jeremy Conway and others. Adobe had said it wanted to find a way to eliminate the threat without removing powerful functionality relied on by some users.

On Thursday, Tung published this proof-of-concept, showing how a booby-trapped PDF file can still be used to override settings designed to block the auto-launch feature and open the Windows calculator. It works by using the command “calc.exe” rather than calc.exe. His blog post is here.

Tung said a related vulnerability, which allowed an attacker to alter the warning Adobe displays before a command is executed, appears to be patched properly. That means it will be harder to trick a victim into clicking the Open button, which is required for an exploit to be successful. Still, with Adobe's apps installed on well over 90 percent of computers, we're sure there are users who would fall prey to scams.

It's unclear if attackers can bypass the setting to execute only code that's already installed on a targeted machine, or if they can also embed malicious payloads into PDF files as before. An Adobe spokeswoman wasn't immediately available for comment, and neither were Tung nor Stevens. ®