CVE-2018-16509

It was discovered that the ghostscript /invalidaccess checks fail under certain conditions. An attacker could possibly exploit this to bypass the -dSAFER protection and, for example, execute arbitrary shell commands via a specially crafted PostScript document.

Mitigation

• ImageMagick relies on ghostscript when processing certain files formats. Thus, ImageMagick can be used as an attack vector. In order to prevent ImageMagick from processing those files on Red Hat Enterprise Linux 6 and 7, you can disable the use of ghostscript and the processing of PS, EPS, PDF, and XPS file formats in ImageMagick's security policy by opening /etc/ImageMagick/policy.xml and adding the following lines to the "<policymap>" section of the file:

  <policy domain=“coder” rights=“none” pattern=“PS” />
  <policy domain=“coder” rights=“none” pattern=“EPS” />
  <policy domain=“coder” rights=“none” pattern=“PDF” />
  <policy domain=“coder” rights=“none” pattern=“XPS” />
  <policy domain=“delegate” rights=“none” pattern=“gs” />

• Additionally, this issue can be triggered when processing files in order to generate thumbnails, for example when browsing a folder containing a malicious PostScript file in Nautilus. To prevent this, remove or rename the "/usr/bin/evince-thumbnailer" executable.
  In Red Hat Enterprise Linux v.7.6 and above, the thumbnailing is done in a sandbox.

• It is possible to run PDF/PS viewers, such as evince and okular, in a SELinux sandbox using the sandbox command from the policycoreutils-sandbox package :
  $ sandbox -X evince <untrusted-file.pdf>
  The sandbox will prevent an attacker to make modifications on the file system.