Ghostscript bug could allow rogue documents to run system commands

Security

Even if you haven’t heard of the venerable Ghostscript project, you may very well have used it without knowing.

Alternatively, you may have it baked into a cloud service that you offer, or have it preinstalled and ready to go if you use a package-based software service such as a BSD or Linux distro, Homebrew on a Mac, or Chocolatey on Windows.

Ghostscript is a free and open-source implementation of Adobe’s widely-used PostScript document composition system and its even-more-widely-used PDF file format, short for Portable Document Format. (Internally, PDF files rely on PostScript code to define how to compose a document.)

For example, the popular open-source graphics program Inkscape uses Ghostscript behind the scenes to import EPS (Embedded PostScript) vector graphics files, such as you might download from an image library or receive from a design company.

Loosely put, Ghostscript reads in PostScript (or EPS, or PDF) program code, which describes how to construct the pages in a document, and converts it, or renders it (to use the jargon word), into a format more suitable for displaying or printing, such as raw pixel data or a PNG graphics file.

Unfortunately, until the latest release of Ghostscript, now at version 10.01.2, the product had a bug, dubbed CVE-2023-36664, that could allow rogue documents not only to create pages of text and graphics, but also to send system commands into the Ghostscript rendering engine and trick the software into running them.

Pipes and pipelines

The problem came about because Ghostscript’s handling of filenames for output made it possible to send the output into what’s known in the jargon as a pipe rather than a regular file.

Pipes, as you will know if you’ve ever done any programming or script writing, are system objects that pretend to be files, in that you can write to them as you would to disk, or read data in from them, using regular system functions such as read() and write() on Unix-type systems, or ReadFile() and WriteFile() on Windows…

…but the data doesn’t actually end up on disk at all.

Instead, the “write” end of a pipe simply shovels the output data into a temporary block of memory, and the “read” end of it sucks in any data that’s already sitting in the memory pipeline, as though it had come from a permanent file on disk.

This is super-useful for sending data from one program to another.

When you want to take the output from program ONE.EXE and use it as the input for TWO.EXE, you don’t need to save the output to a temporary file first, and then read it back in using the > and < characters for file redirection, like this:

 C:Usersduck> ONE.EXE > TEMP.DAT
 C:Usersduck> TWO.EXE < TEMP.DAT

There are several hassles with this approach, including these:

  • You have to wait for the first command to finish and close off the TEMP.DAT file before the second command can start reading it in.
  • You could end up with a huge intermediate file that eats up more disk space than you want.
  • You could get messed around if someone else fiddles with temporary file between the first program terminating and the second one launching.
  • You have to ensure that the temporary filename doesn’t clash with an existing file you want to keep.
  • You are left with a temporary file to clean up later that could leak data if it’s forgotten.

With a memory-based intermediate “pseudofile” in the form of a pipe, you can condense this sort of process chain into:

 C:Usersduck> ONE.EXE | TWO.EXE

You can see from this notation where the names pipe and pipeline come from, and also why the vertical bar symbol (|) chosen to represent the pipeline (in both Unix and Windows) is more commonly known in the IT world as the pipe character.

Because files-that-are-actually-pipes-at-the-operating-system-level are almost always used for communicating between two processes, that magic pipe character is generally followed not by a filename to write into for later use, but by the name of a command that will consume the output right away.

In other words, if you allow remotely-supplied content to specify a filename to be used for output, then you need to be careful if you allow that filename to have a special form that says, “Don’t write to a file; start a pipeline instead, using the filename to specify a command to run.”

When features turn into bugs

Apparently, Ghostscript did have such a “feature”, whereby you could say you wanted to send output to a specially-formatted filename starting with %pipe% or simply |, thereby giving you a chance of sneakily launching a command of your choice on the victim’s computer.

(We haven’t tried this, but we’re guessing that you can also add command-line options as well as a command name to execute, thus giving you even finer control over what sort of rogue behaviour to provoke at the other end.)

Amusingly, if that is the right word, the “sometimes patches need patches” problem popped up again in the process of fixing this bug.

In yesterday’s article about a WordPress plugin flaw, we described how the makers of the buggy plugin (Ultimate Member) have recently and rapidly gone through four patches trying to squash a privilege escalation bug:

We’ve also recently written about file-sharing software MOVEit pushing out three patches in quick succession to deal with a command injection vulnerability that first showed up as a zero-day in the hands of ransomware crooks:

In this case, the Ghostscript team first added a check like this, to detect the presence of the dangerous text %pipe... at the start of a filename:

/* "%pipe%" do not follow the normal rules for path definitions, so we
   don't "reduce" them to avoid unexpected results */

if (len > 5 && memcmp(path, "%pipe", 5) != 0) {
   . . . 

Then the programmers realised that their own code would accept a plain | character as well as the prefix %pipe%, so the code was updated to deal with both cases.

Here, instead of checking that the variable path doesn’t start with %pipe... to detect that that the filename is “safe”, the code declares the filename unsafe if it starts with either a pipe character (|) or the dreaded text %pipe...:

/* "%pipe%" do not follow the normal rules for path definitions, so we
   don't "reduce" them to avoid unexpected results */

if (path[0] == '|' || (len > 5 && memcmp(path, "%pipe", 5) == 0)) {
   . . .
Above, you’ll see that if memcmp() returns zero, it means that the comparison was TRUE, because the two memory blocks you’re comparing match exactly, even though zero in C programs is conventionally used to represent FALSE. This annoying inconsistency arises from the fact that memcmp() actually tells you the order of the two memory blocks. If the first block would sort alphanumerically before the second, you get a negative number back, so that you can tell that aardvark precedes zymurgy1. If they’re the other way around, you get a positive number, which leaves zero to denote that they’re identical. Like this:

#include <string.h>
#include <stdio.h>

int main(void) {
   printf("%dn",memcmp("aardvark","zymurgy1",8));
   printf("%dn",memcmp("aardvark","00NOTES1",8));
   printf("%dn",memcmp("aardvark","aardvark",8));
   return 0;
}
---output---
-1
1
0

What to do?

  • If you have a standalone Ghostcript package that’s managed by your Unix or Linux distro (or by a similar package manager such as the abovementioned Homebrew on macOS), make sure you’ve got the latest version.
  • If you have software that comes with a bundled version of Ghostscript, check with the provider for details on upgrading the Ghostscript component.
  • If you are a programmer, don’t accept any immediately-obvious bugfix as the beginning and end of your vulnerability-squashing work. Ask yourself, as the Ghostscript team did, “Where else could a similar sort of coding blunder have happened, and what other tricks could be used to trigger the bug we already know about.”

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