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.
Exploitability
AV:NAC:LAT:PPR:HUI:NVulnerable System
VC:HVI:NVA:NSubsequent System
SC:LSI:NSA:N5.9/CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:N/VC:H/VI:N/VA:N/SC:L/SI:N/SA:NInput Validation