With primary election dates fast approaching in many states, officials in Pennsylvania and California issued urgent directives in recent days about a potential security risk in their Diebold Election Systems touch-screen voting machines, while other states with similar equipment hurried to assess the seriousness of the problem. It's the most severe security flaw ever discovered in a voting system," said Michael I. Shamos, a professor of computer science at Carnegie Mellon University. "This is the barn door being wide open," said Douglas W. Jones, a professor of computer science at the University of Iowa. The new concerns about Diebold's equipment were discovered by Harri Hursti, a Finnish computer expert who was working at the request of Black Box Voting. As word of Mr. Hursti's findings spread, Diebold issued a warning to recipients of thousands of its machines, saying that it had found a "theoretical security vulnerability" that "could potentially allow unauthorized software to be loaded onto the system." Aviel Rubin, a professor of computer science at Johns Hopkins University, did the first in-depth analysis of the security flaws in the source code for Diebold touch-screen machines in 2003. After studying the latest problem, he said: "I almost had a heart attack. The implications of this are pretty astounding."