On August 28, 2019, Artifex submitted “Bug 701446: Avoid divide by zero in shading” on the master branch of Ghostscript and announced remediation of four -dSAFER sandbox bypass vulnerabilities. -dSAFER is a security sandbox used by Ghostscript for prevention of insecure PostScript operations.
Ghostscript is a free suite of software based on an interpreter for Adobe System’s PostScript and Portable Document Format (PDF) page description languages. It is widely used as a raster image processor (RIP) for raster computer printers. Currently, it has been ported from Linux to other operating systems, including UNIX, Mac OS X, VMS, Windows, OS/2, and Mac OS classic.
The four vulnerabilities disclosed this time exist respectively in .pdf_hook_DSC_Creator (CVE-2019-14811), .setuserparams2 (CVE-2019-14812), setsystemparams (CVE-2019-14813), and .pdfexectoken (CVE-2019-14817) procedures. An attacker could exploit this vulnerability to gain access to the .forceput operator via a malicious postscript file, bypass the -dSAFER protection, and execute arbitrary commands.
Scope of Impact
This vulnerability affects all applications that use Ghostscript to process PostScript contents.
Affected Ghostscript versions:
- Ghostscript before commit f531552c99a04f003412f7a83d4661e927f88d40
Unaffected Ghostscript versions:
- Ghostscript commit f531552c99a04f003412f7a83d4661e927f88d40 and later
Currently, there has been no official release to fix this vulnerability. Users can use git commands to upgrade their installations to commit f531552c99a04f003412f7a83d4661e927f88d40 or later, or directly pull the master branch code to protect against this vulnerability.
For how to fix CVE-2019-14811, CVE-2019-14812, and CVE-2019-14813 vulnerabilities, visit the following link:http://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=885444fcbe10dc42787ecb76686c8ee4dd33bf33For how to fix the CVE-2019-14817 vulnerability, visit the following link:http://git.ghostscript.com/?p=ghostpdl.git;a=commitdiff;h=cd1b1cacadac2479e291efe611979bdc1b3bdb19
Red Hat 7 and 8 distributions have been updated to fix these vulnerabilities. Users are advised to upgrade as soon as possible.
Note: Currently, Red Hat Enterprise Linux 5 and Red Hat Enterprise Linux 6 are out of support. Users of these two distributions should take temporary measures described in section 3.2 to protect against these vulnerabilities.
If users cannot upgrade to the fixed version for the time being, they can take the following measures as temporary protection against these vulnerabilities:
- Configure a security policy on ImageMagick. ImageMagick uses Ghostscript by default as the interpreter to process PostScript contents. Therefore, users can configure a security policy.xml on the ImageMagick component to disable the use of Ghostscript and the processing of PS, EPS, PDF, and XPS file formats. Users can modify the policy configuration file of ImageMagick by adding the following code to <policymap> in the /etc/ImageMagick/policy.xml directory (default location of the policy configuration 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” />
- These vulnerabilities may be triggered when Ghostscript processes files in order to generate thumbnails. To avoid this issue, users can delete or rename the /usr/bin/evince-thumbnailer executable.
- Run PDF and PS viewers, such as evince, in a SELinux sandbox using the following command:
|$ sandbox -X evince <untrusted-file.pdf>|
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