A Pfizer employee who installed unauthorised file-sharing software on a company laptop has exposed the Social Security numbers and other personal data belonging to about 17,000 current and former employees at the drug maker.

Of that group, about 15,700 individuals actually had their data accessed and copied by an unknown number of persons on a peer-to-peer network, the company said in letters sent to affected employees and to state attorneys general alerting them of the breach.

Pfizer officials could not be immediately reached for comment. But copies of the letters were posted on several sites, including Pharnalot, a blog covering the pharmaceutical industry.

The incident has prompted an investigation by Connecticut Attorney General Richard Blumenthal; some 305 Pfizer employees in that state were affected by the breach. In a June 6 letter, Blumenthal asked Pfizer to provide details on the measures in place prior to the breach to protect against data compromises, when the company discovered the breach and how it responded.

Blumenthal's letter also asked Pfizer to describe how it was able to make a distinction between the data that was actually compromised and data that might only potentially have been accessed. Blumenthal's letter gave Pfizer until June 22 to respond.

According to Pfizer's description of the incident in its letter to employees, the compromise stemmed from the use of unauthorised file-sharing software on an employee's laptop.

The June 1 letter signed by Pfizer general counsel Lisa Goldman did not mention how the company discovered the breach. But as soon as it did, the company recovered the laptop from the employee and the file-sharing software was disabled, she said. Because the system was being used to access the Internet from outside of Pfizer's own network, no other data was compromised. Goldman also apologised to the affected individuals for the inconvenience.

Pfizer has contracted for a "support and protection" package with credit reporting agency Experian for all affected individuals, Goldman said. The packages include a year's worth of free credit monitoring service and a $25,000 insurance policy covering costs that individuals might incur as a result of the breach, Goldman noted.

Such incidents highlight the importance of implementing controls for preventing either accidental or deliberate data leaks via file sharing or other applications such as Instant Messaging, said Devin Redmond, director of the security products group at security vendor Websense. Such controls should include measures such as content filtering at network gateways, strong access controls to sensitive data and preventing access to file-sharing applications, he said.

News of the Pfizer breach coincides with the release of a study by Dartmouth University's Tuck School of Business that looked into the dangers posed by file-sharing applications. The study examined data involving P2P searches and files related to the top 30 U.S. banks over a seven-week period between December 2006 and February 2007. A surprisingly high number of people sharing music and other files on peer-to-peer systems are inadvertently exposing all sorts of bank account and similar personal information on their computers to criminals lurking on the networks to harvest data.