Remote Command Execution on RemotePC for Windows

During an audit we executed in 2019, we had to test a deployment where a third-party company had to remotely connect to special purpose computers to perform maintenance. At the time, they had chosen a software called RemotePC to remotely login into these special purpose computers rather than relying on RDP. RemotePC is a a remote desktop software that lets a support agent remotely connect to a computer and take control of keyboard, mouse, and screen. It's quite similar to TeamViewer.

Transport security fell into the scope of this specific audit so we covered all communication channels established by RemotePC clients. Turns out RemotePC Windows client does not properly validates SSL certificates, allowing a man-in-the-middle attacker to:

  • capture credentials when user logs in with remotepc account
  • observe the remotely accessed desktop, inject keystrokes and mouse events
  • hijack the auto-update mechanism in order to get the RemotePC client to execute an arbitrary executable, leading to remote command execution

All versions prior to 7.6.26-28/04/20 are vulnerable. We strongly recommend anyone using RemotePC to update to the latest version available at

Understanding DTLS Usage in VoIP Communications

When working on a mobile application security assessment we noted an unusual traffic flow. This was a DTLS handshake coming from a remote server to the mobile application listener. As we always pay close attention to transport security implementation in the applications we test, we were about to verify if certificates are properly validated in the observed case. However, it was not that simple.

The desire to test client-side TLS certificates validation led us to: development of custom tools, interception and digging through various types of network traffic, reading through RFC documents and even patching of binary files. Now we understand that we were facing DTLS handshake initiated to derive keys to secure media communications between parties, following WebRTC recommendations.

In this article we describe what we did, how we did that and what obstacles we had to overcome. This exercise helped us to formulate a methodology to test some aspects of WebRTC-capable applications from a security standpoint.

Introducing Qsslcauditproxy

The Qsslcaudit tool developed by my colleague Pavel provides an easy way for testing a client SSL implementation. To make testing of multiple connections even more straightforward, I created a proxy wrapper for this tool.

How it works

The user configures qsslcauditproxy as a proxy server on the device under test, for example a mobile device.

Client-side TLS Implementation Assessment with Qsslcaudit - The WPS Office Case

In this post we demonstrate how to use our tool to assess client-side TLS implementation: qsslcaudit. qsslcaudit helps determine if a TLS client (mobile application, standalone application, web service) properly validates server's certificate and if only secure protocols are supported. Issues in TLS implementation can be abused by attackers to intercept victim's traffic: extract sensitive information, alter client's requests or server's responses and so on.

Basic information on how to use the tool can be found in its README file. However, we believe that the best demo is real-world test scenario against existing and widely used application. For this demo, we chose to target KingSoft WPS Office Android mobile application.

The issues described here are still not fixed at the time of this writing. We reported them to KingSoft via HackerOne, issues were confirmed but not fixed. Then we reported them to Google, got a reply that this is a known problem as it was reported earlier. Given that Kingsoft developers are aware of it but chose not to fix it, and that Google and some bug bounty hunters also know about this issue, we decided to report it publicly. At least WPS Office users (over 1.3 billion users per Google Play Store estimates) will be aware of the risks of using it.


CVE-2020-0601: theory and practice

On the 14th of January 2020, Microsoft fixed CVE-2020-0601, a high severity vulnerability affecting the way Windows CryptoAPI (Crypt32.dll) validates Elliptic Curve Cryptography (ECC) certificates. Corresponding advisories were issued by NSA and CERT on the same day. On the next day it got a few nicknames such as Chain of Fools, or CurveBall. Some internal details were given by Thomas Ptacek and Kudelski Security. The latter one inspired us to look at this vulnerability from a more practical point of view.

qsslcaudit release v0.8.1, CVE-2020-0601 test included

This is a new release of our tool designed to assess TLS clients security (certificates validation, protocols and ciphers support): v0.8.1.

The corresponding packages for various Ubuntu versions are prepared in ppa:gremwell/qsslcaudit. Packaging for Kali is handled by Kali maintainers.


Office 365 User Enumeration Reloaded

During a recent engagement, we tried to enumerate email accounts by abusing previously reported user enumeration issue affecting Office 365, but found out it no longer works.

In the past, sending authentication requests to ActiveSync with Basic HTTP authentication mechanism would return different status code disclosing the user's existence. A 404 meant the user did not exist, a 401 meant the user existed. We don't know exactly when Microsoft released that specific update, but it now returns a 401 whether the user exists or not.

We therefore had to find another way. By looking at HTTP responses from ActiveSync, we've identified that it still leaks information about the user existence. Whenever the HTTP response header X-MailboxGuid is set, that means the user exists.

We packed everything in a Python3 script that will read usernames from a text file and output the users and validity as CSV. You can find it at It also includes a user enumeration technique based on login page.

qsslcaudit release v0.6.0

This is a new release of our tool designed to assess TLS clients security (certificates validation, protocols and ciphers support): v0.6.0.

The single huge feature added: support of assessing DTLS clients.

Pentesting Meteor Applications with Burp Suite

The following post will cover some techniques to test Meteor applications with Burp Suite. This can also be applied to other protocols that run over WebSockets.
To test this yourself, the example Meteor application “Todos” can be downloaded here.

Test Fallback to XHR


Subscribe to RSS - blogs