In notification letters made public Thursday, the bank said it had redacted sensitive information in Chapter 13 bankruptcy proof-of-claim forms that were filed electronically, but that the information turned out to be viewable "as a result of the deficiency in the software used to save imaged documents."

An HSBC spokeswoman declined to elaborate on the cause of the problem, but said "a limited number of customers" were affected. HSBC has "no reason to believe customers' personal information may have been compromised," she added via e-mail. The company sent letters to affected customers in October and is offering them one year of free credit monitoring.

Some customers of the following HSBC companies are affected: HSBC Taxpayer Financial Services, Beneficial New Hampshire and Household Finance Corporation.

According to filed Thursday with the New Hampshire Attorney General's office, HSBC failed to completely redact information on forms that were filed between May 1, 2007, and Oct. 17, 2008. HSBC said it learned of the problem on July 9, 2009.

The data disclosed may have covered HSBC credit card, line-of-credit or mortgage information, the company said.