Multiple vulnerabilities were discovered in MuPDF, a PDF, XPS, and e-book viewer which could result in denial of service or the execution of arbitrary code if malformed documents are opened.
For the stable distribution (stretch), these problems have been fixed in version 1.9a+ds1-4+deb9u4.
We recommend that you upgrade your mupdf packages.
Further information about Debian Security Advisories, how to apply these updates to your system and frequently asked questions can be found at: https://www.debian.org/security/