It was found that the superexec operator was available in the internal dictionary. A specially crafted PostScript file could use this flaw in order to, for example, have access to the file system outside of the constrains imposed by -dSAFER.
Please refer to the "Mitigation" section of CVE-2018-16509 : <https://access.redhat.com/security/cve/cve-2018-16509>