Death By a Thousand Papercuts

Patch diffing major releases

From time to time our pentest team reviews software that we are either using or interested in acquiring. That was the case with Papercut, a multifunction printer/scanner management suite for enterprise printers. The idea behind Papercut is pretty neat, a user can submit a print job to a Papercut printer, and walk to any physical printer they are nearby and release the print job. Users don’t have to select from dozens of printers and hope they get the right one. Pretty neat! It does a lot of other stuff too, but you get the point, it’s for printing :)

Typically when starting an application security assessment I’ll start by searching for previous exploitable vulnerabilities released by other researchers. In the case of Papercut there was only one recent CVE I could find without much detail. CVE-2019–12135 stated “An unspecified vulnerability in the application server in Papercut MF and NG versions 18.3.8 and earlier and versions 19.0.3 and earlier allows remote attackers to execute arbitrary code via an unspecified vector.”

I don’t like unspecified vulnerabilities! However, this was a good opportunity to do some patch diffing, and general security research on the product. The purpose of this article will be to guide someone in attempting major release patch diffing to find an undisclosed or purposely opaque vulnerability.

Before diving into the patch diffing we also wanted to get an idea of how the application generally behaves.

Typically I’ll look for services and processes related to the target, and what those binaries try to load. Our first finding which was relatively easy to uncover was that the mobility-print.exe process attempts to load ps2pdf.exe, cmd, bat, and vbs from the windows PATH environment variable. As a developer its important to realize that this is something that could potentially be modified, which you have no control over. So loading arbitrary files from an untrusted path is not a good idea.

mobility-print.exe loading files from the PATH variable

After this finding we created a simple POC which spawned calc.exe from a path environment variable. In our case, a SQL server installation which was part of our Papercut install allowed for an unprivileged user to privilege escalate to SYSTEM due to F:\Program Files having the NTFS special permissions to write/append data.

POC bat file that spawns calc.exe
Calc.exe spawned as SYSTEM

First vulnerability down! That was easy, although it’s far from remote code execution… from the perspective of insider risk, a malicious insider with user level access to the print server could take over the print server with this vulnerability. We reported this vulnerability to Papercut and the newest release has this issue patched.

If you’ve done patch diffing of DLLs or binaries before, you know the important thing is to get the most recent version before the patch, and the version immediately after the patch. Typically a tool like BinDiff is used for comparing the patches. Unfortunately, Papercut doesn’t allow us to download a patch for their undisclosed RCE vulnerability, so the best we can do is download the point release before the vulnerability, and the point release with the patch. Unfortunately, that means that there will be a large number of updated files and the patch will be difficult to find. I made an educated guess that the remote code execution vulnerability would be an insecure deserialization vulnerability simply based on the fact that there were a lot of jar files included in the installer. The image below shows a graphical diffing view of the Papercut folder structure. The important thing here is that purple represents files that have been added.

Here we see a lot of class files added that didn’t exist before… with a lot of extraneous data filtered out.

After diffing the point release and seeing that SecureSerializationFilter was added to the codebase, the next step we took was to see where the new class is leveraged (hint it’s during serialization and deserialization of print jobs). With this information we can craft an attack payload against unpatched versions in the form of a print job.

Finally looking at the class path of the server we can see that Apache Commons Collection is included, so a Ysoserial payload should work for achieving RCE. We’ve achieved the goal of understanding the underlying root cause of the vulnerability even though the vendor did not provide any useful information in understanding the issue. But in a perfect world the vendor would have shared this information in the first place!

As a side note Papercut is one of many vendors who leverage third party libraries. MFP software represents an interesting target in that there are typically large numbers of file format parsers involved in translating image file formats and office document file formats into a format that many printers understand. Third party libraries often are leveraged for this and some may not be as vetted or secure when compared to a Microsoft developed library.

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Chris Hernandez

Red Teamer — Security Researcher — Breaking things is fun, except when its not