UpGuard's Cyber Risk Team said Dow Jones had put the figure of users who were at risk at 2.2 million.
The security firm said the data that was accessible included names, addresses, account information, email addresses, and the last four digits of credit card numbers of subscribers.
Dow Jones is the parent company of The Wall Street Journal and Barron's.
The data repository resided on an Amazon Web Services S3 bucket and had been configured using permissions to allow any AWS “Authenticated Users” to download the data via the URL of the repository.
"The revelation of this cloud leak speaks to the sustained danger of process error as a cause of data insecurity, with improper security settings allowing the leakage of the sensitive information of millions of Dow Jones customers," UpGuard said in a statement.
"The data exposed in this cloud leak could be exploited by malicious actors employing a number of attack vectors already known to have been successful in the past.
"Finally, the aversion of Dow Jones and Company to notifying affected customers of this data exposure denies consumers the ability to swiftly act to protect their own personal information."
The leak was discovered by UpGuard's director of cyber risk research Chris Vickery on 30 May when he found an Amazon S3 cloud-based data repository accessible to AWS authenticated users under the subdomain "dj-skynet."
Vickery began to download the contents from 1 June onwards and the repository was secured on 6 June. He found several directories within the subdomain; one, titled "cust_subscription", contained four compressed Apache Avro files which were 771MB in size. The smallest of the four files was 89MB and expanded to 2Gb when decompressed.
"Once decompressed, these files are revealed to be four large text logs composed entirely of Dow Jones customer data, prepared in a format that could easily be fed into a database for internal record-keeping," UpGuard said.
"Among the fields populated with data throughout the text files are customer names, internal Dow Jones customer IDs, home and business addresses, and account details, such as the promotional offer under which a customer signed up for a subscription.
"Perhaps most critical was the inclusion of the last four digits of customer credit cards in the files, as well as customer email addresses also used to login to their accounts online. A small percentage of customers also had their phone numbers exposed in the files."