Pennsylvania election officials are urging some counties to take added safety measures before Tuesday's primary because one touch-screen voting machine model contains a security vulnerability that could lead to fraud.

The machine, Diebold Election System's AccuVote-TSX, is being used by 16 counties, including five in Western Pennsylvania: Armstrong, Clarion, Somerset, Warren and Washington.

According to election officials, a computer expert with unfettered access to the ATM-like machine could, in theory, change its software with relative ease and potentially alter election results.

"It's an awful flaw. But it's easy to fix," said Michael Shamos, a Carnegie Mellon University computer science professor who tests voting machines for the state.

Secretary of State Pedro A. Cortes, who oversees elections in Pennsylvania, sent a directive May 2 to counties using the Diebold machines, telling local elections departments to reinstall authorized software on each TSX and keep the units locked up, a step normally taken by every county.

Mr. Cortes described the probability of a security breach as "low," and his directive said Diebold would correct the flaw.

David Bear, a Diebold spokesman, said the North Canton, Ohio-based company plans to adjust the machines so they can more easily recognize unauthorized software.

But he defended the security of the machines in their current design, saying any tampering would require the cooperation of "corrupt" election officials.

The trustworthiness of the nation's election systems has been under intense scrutiny since Florida's botched 2000 presidential elections, which prompted Congress to enact the Help America Vote Act.

The law, known as HAVA, is forcing local governments across the country to buy new voting machines this year.

Diebold has been a leading supplier of new equipment. Allegheny County officials had considered buying the TSX before they eventually settled on another touch-screen machine, Election Systems & Software's iVotronic.

The Diebold flaw was discovered earlier this year by a researcher for Black Box Voting, a nonprofit organization in Washington state that closely follows computer security issues in electronic voting.

The organization contacted Dr. Shamos and he confirmed the problems, which, he said, could not have been detected by the normal certification process in Pennsylvania.

California and Iowa have since followed the state's lead in calling on local election departments to take additional precautions with their Diebold machines.

Yesterday, Black Box Voting released a new report, completed with the help of a Finnish computer expert, that says Diebold's equipment contains even deeper flaws.

"Someone could have gone in at any point in the machine's life and put something in there to alter votes. It's a permanent contamination," said Bev Harris, director and founder of Black Box Voting. "Diebold needs to pull them off the shelf and redesign them."

To date, there has been no proof of wrongdoing, but Ms. Harris said undetected tampering could take place.

Mr. Bear said groups like Black Box Voting are trying to "confuse or frighten" voters.

"This system has been more thoroughly reviewed than any system in the country," he said.

Dr. Shamos said Pennsylvania is addressing concerns raised by Ms. Harris' organization. He said someone with considerable knowledge of Diebold's computer systems could install dangerous software, but gaining access would be very difficult if local election officials follow normal security procedures.

Ronn Kuzmovich, director of elections for Clarion County, said his office performed new "logic and accuracy" tests on the county's 120 Diebold machines and then locked them away.

In Somerset County, officials reinstalled software in 234 machines. Tina Pritts, director of voter registration and elections in the county, said she didn't have any concerns about using Diebold equipment, while Mr. Kuzmovich said the company's service has been "outstanding."