The employee, Cody Sultenfuss, said he had no warning before his details were posted.
"They took money I did not have," he told The Associated Press in a series of emails, which did not specify the amount taken. "I think 'Why me?' I am not rich."
But the breach doesn't necessarily pose a risk to owners of the credit cards. A card user who suspects fraudulent activity on his or her card can contact the credit card company to dispute the charge.
Stratfor said in an email to members, signed by Stratfor Chief Executive George Friedman and passed on to AP by subscribers, that it had hired a "leading identity theft protection and monitoring service" on behalf of the Stratfor members affected by the attack. The company said it will send another email on services for affected members by Wednesday.
Stratfor acknowledged that an "unauthorized party" had revealed personal information and credit card data of some of its members.
The company had sent another email to subscribers earlier in the day saying it had suspended its servers and email after learning that its website had been hacked.
One member of the hacking group, who uses the handle AnonymousAbu on Twitter, claimed that more than 90,000 credit cards from law enforcement, the intelligence community and journalists -- "corporate/exec accounts of people like Fox" News -- had been hacked and used to "steal a million dollars" and make donations.
It was impossible to verify where credit card details were used. Fox News was not on the excerpted list of Stratfor members posted online, but other media organizations including MSNBC and Al-Jazeera English appeared in the file.
Anonymous warned it has "enough targets lined up to extend the fun fun fun of LulzXmas through the entire next week."
The group has previously claimed responsibility for attacks on credit card companies Visa Inc. and MasterCard Inc., eBay Inc.'s PayPal, as well as other groups in the music industry and the Church of Scientology.