Improper Restriction of XML External Entity Reference
codelibs fess versions 12.1.3 through 12.2.2 and 12.3.0 through 12.3.1 before commit faa265b contains a XML External Entity (XXE) vulnerability in GSA XML file parser that can result in Disclosure of confidential data, denial of service, SSRF, port scanning. This attack appear to be exploitable via specially crafted GSA XML files. This vulnerability appears to have been fixed in after commit faa265b.
CWE-611 - Improper Restriction of XML External Entity Reference
Listed 4th in the 'OWASP Top Ten', XML External Entities (XXE) vulnerability allows attackers to provide an XML input that contains an external entity. When the XML is parsed, it can cause data extraction and manipulation, execution of commands, denial-of-service attacks, and server-side request forgery.