MiKTeX 2.9.6840 has been released on September 30, 2018.
Fixed security issues
This applies to MiKTeX (TeX Live is an upstream source):
An issue was discovered in t1_check_unusual_charstring functions in writet1.c files in TeX Live before 2018-09-21. A buffer overflow in the handling of Type 1 fonts allows arbitrary code execution when a malicious font is loaded by one of the vulnerable tools: pdflatex, pdftex, dvips, or luatex.
- 199: miktex searches the whole file system if grffile is used