The security breach, which was discovered in early June, occurred over the past three months and involved attackers gaining entry to servers that host e-commerce services for a number of small to medium-sized online stores.
Malicious code left by the hackers enabled them to intercept the personal and financial details of those that made purchases through the affected businesses.
At least 4,343 companies have been affected, Network Solutions said, with the attacks taking place between March 12th and June 8th.
Spokeswoman Susan Wade stated that the company is working with US authorities in order to determine how the hackers gained access to their system.
In related news, a recent survey by Gartner found that security is the feature most valued by those that use online banking services.