I regularly find XML eXternal Entity (XXE) vulnerabilities while performing penetration tests. These are particularly often present in Java-based systems, where the default for most XML parsers still is parsing and acting upon inline DTDs, even though I have not seen a single use case where this was really neceassary. While the vulnerability is useful for file disclosures (and Java is nice enough to also provide directory listings) or even process listings (via /proc/pid/cmdline), recently I stumbled over another interesting attack vector when using a Java XML parser.

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