The remote Debian host is missing a security-related update.
Multiple vulnerabilities have been found in MuPDF, a PDF file viewer, which may result in denial of service or the execution of arbitrary code. - CVE-2017-14685, CVE-2017-14686, and CVE-2017-14687 WangLin discovered that a crafted .xps file can crash MuPDF and potentially execute arbitrary code in several ways, since the application makes unchecked assumptions on the entry format. - CVE-2017-15587 Terry Chia and Jeremy Heng discovered an integer overflow that can cause arbitrary code execution via a crafted .pdf file.
Upgrade the mupdf packages. For the stable distribution (stretch), these problems have been fixed in version 1.9a+ds1-4+deb9u1.