Several vulnerabilities have been found in the TIFF library, which may result in denial of service or the execution of arbitrary code if malformed image files are processed.
For the oldstable distribution (stretch), these problems have been fixed in version 4.0.8-2+deb9u5.
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/