That vulnerability, according to a report on the data breach by William Baird & Co., was in a popular open-source software package called Apache Struts, which is a programming framework for building web applications in Java. Two vulnerabilities in Struts have been discovered so far in 2017. One was announced in March, and another was announced earlier this week on Sept. 4. At the moment, it’s unclear which vulnerability the Baird report was referring to.

Source: The hackers who broke into Equifax exploited a flaw in open-source server software — Quartz

The bug specifically affects a popular plugin called REST, which developers use to handle web requests, like data sent to a server from a form a user has filled out. The vulnerability relates to how Struts parses that kind of data and converts it into information that can be interpreted by the Java programming language. When the vulnerability is successfully exploited, malicious code can be hidden inside of such data, and executed when Struts attempts to convert it.