The OOXML parsing of the file indexer does not disable external entity resolution. A crafted xlsx or pptx document placed in an indexed directory can cause local files to be read or outbound HTTP requests to be performed, with the retrieved content being written to the search index.
Improper Restriction of XML External Entity Reference The product processes an XML document that can contain XML entities with URIs that resolve to documents outside of the intended sphere of control, causing the product to embed incorrect documents into its output.