Detailed Technical Analysis of "PPRCE2: Postscript Platform Remote Code Execution (CVE-2016-4809)"

Overview:
This write-up by Artsploit details a PostScript (PS) execution vulnerability within Ghostscript, leveraging this to achieve Remote Code Execution (RCE). The vulnerability is documented under CVE-2016-4809 and exploits the Ghostscript interpreter used widely across platforms for handling PostScript files.

Key Technical Details:

1. PostScript Overview:
   PostScript is a scripting language used for rendering vector graphics and text, commonly found in printing and document processing. Ghostscript is an open-source interpreter for PostScript and PDF files, extensively used in many systems and applications.

2. Vulnerability Discovery:
   This exploitation arises from Ghostscript’s processing of user-controlled PostScript files. The vulnerability lies in the handling of PostScript operators, allowing arbitrary command execution when rendering a malicious PostScript file.

3. Malformed PostScript Files: By crafting specific PostScript functions, an attacker can manipulate stack-based operations to control the interpreter. PostScript’s capability to invoke system commands can directly translate to executing arbitrary code.

4. Exploit Mechanism:
   The attack leverages specific Ghostscript operators (e.g., .forceput and .systemdict) to override intrinsic security restrictions (like readonly or executable). By systematically breaking down these limitations, the attacker can execute arbitrary commands.

5. Bypassing Execution Restrictions: (payload) (key) .forceput This sequence can reassign execution privileges within Ghostscript's environment, modifying system-level dictionaries.

6. Command Injection: Once control is achieved, the attacker deploys typical shell execution payloads: postscript userdict /Startup setdatasource

   This example shows how user dictionaries and data sources are manipulated to call OS-level commands, leading to arbitrary code execution.

7. Real-World Application:
   Imagine a scenario where a web application allows users to upload PostScript files for document processing. By submitting a malicious PostScript document, an attacker can exploit the Ghostscript vulnerability on the server-side, leading to RCE. This vector is potent on web servers, print servers, and any service incorporating Ghostscript for document rendering.

8. Mitigation Strategies:

9. Security Patches: Applying updates to Ghostscript is critical. Ensure that the latest version, addressing CVE-2016-4809, is deployed across systems.

10. Sandboxing: Operating Ghostscript within a restricted environment (e.g., chroot, Docker containers) can limit the scope of potential damage.

11. Input Validation/Sanitization: Implement rigorous validation for all input files. Although challenging, ensuring scripts don’t contain harmful operations can mitigate the risks.

12. Disable Dangerous Features: By default, disable or restrict the features in Ghostscript that allow script execution and command invocations from within PostScript files.

Key Takeaways:

• Importance of Updates: This vulnerability highlights how crucial it is to keep third-party libraries and tools updated.
• Complexity of Input Files: File types typically perceived as harmless (e.g., images, documents) can contain scripts and executable commands, posing significant security risks.
• Sandboxing and Isolation: Running critical services within isolated environments can dramatically reduce the impact of potential vulnerabilities.
• Awareness and Training: Developers and administrators must be aware of the potential risks associated with file parsing and script execution features in common utilities like Ghostscript.

Conclusion:

The detailed analysis of CVE-2016-4809 presented in the Artsploit blog underscores a critical security gap in a widely-used utility. The ability to execute system commands by manipulating PostScript's flexible and powerful scripting capabilities demonstrates a sophisticated exploitation technique. This serves as a cautionary tale on the potential dangers inherent in scriptable document formats and the necessity of robust security practices.

For full details, read the original blog post here.