This may have come too late in the comments for anyone to see it, but if the exploit is active on your system, adding a key to ARDAgent's Info.plist makes the problem go away without disabling ARDAgent altogether. (Whether or not ARDAgent is a security vulnerability itself is another story.)

<key>NSAppleScriptEnabled</key>
        <string>YES</string>

That "YES" is not a typo; setting it to "NO" does not fix the problem. AFAICT this makes osascript expect that ARDAgent will implement more of its own AppleScript handlers...which of course, it doesn't.

P.S. I searched for other, similar problem setuid apps, and turned up check_afp.app (which someone else posted already) and, surprisingly, GoogleUpdaterInstaller. Fortunately, even though these apps run setuid, they won't respond to the "do shell script" attack.