Zoom zero-day discovery makes calls safer, hackers $200,000 richer

Two Dutch white-hat security specialists entered the annual computer hacking contest Pwn2Own, managed to find a Remote Code Execution (RCE) flaw in Zoom and are $200,000 USD better off than they were before.


Pwn2Own is a high profile event organized by the Zero Day Initiative that challenges hackers to find serious new vulnerabilities in commonly used software and mobile devices. The event is held to demonstrate that popular software and devices come with flaws and vulnerabilities, and offers a counterweight to the underground trade in vulnerabilities.

The “targets” volunteer their software and devices and offer a reward for successful attacks. Fans are treated to a hacking spectacle, successful hackers get kudos and no small amount of cash (in this case the reward was a whopping $200,000 USD), and vendors find nasty bugs that might otherwise be sold to criminals.

Pwn2Own 2021 runs from 6 April to 8 April. The full schedule for this year can be found on their site. This year the event has focused on software and devices used when working from home (WFH), including Microsoft Teams and Zoom, for obvious reasons.

The white hats

Keuper and Alkemade, who are employed by cybersecurity company Computest, combined three vulnerabilities to take over a remote system on the second day of the Pwn2wn event. The vulnerabilities require no interaction of the victim. They just need to be on a Zoom call.

The vulnerability

In the light of responsible disclosure, the full details of the method have been kept under wraps. What we do know is that it was Remote Code Execution (RCE) flaw: As a class of software security flaws that allow a malicious actor to execute code of their choosing on a remote machine over a LAN, WAN, or the Internet.

We also know that the method works on the Windows and Mac version of the Zoom software, but does not affect the browser version. It is unclear whether the iOS- and Android-apps are vulnerable since Keuper and Alkemade did not look into those.

The Pwn2Own organization have tweeted a gif demonstrating the vulnerability in action. You can see the attacker open the calculator on the system running Zoom. Calc.exe is often used as the program that hackers open on a remote system to show that they can run code on the affected machine.

A Zoom RCE being used to open the Windows calculator

Not patched yet

Understandably, Zoom has not yet had the time to issue a patch for the vulnerability. They have 90 days to do so before details of the flaw are released, but they are expected to do it way before that period is over. The fact that the researchers came out on the second day of the Pwn2Own event with this vulnerability does not mean they figured it out in those two days. They will have put in months of research to find the different flaws and combine them into an RCE attack.

Security done right

This event, and the procedures and protocols that surround it, demonstrate very nicely how white-hat hackers work, and what responsible disclosure means. Keep the details to yourself until protection in the form of a patch is readily available for everyone involved (with the understanding that vendors will do their part and produce a patch quickly).


For now, the two hackers and Zoom are the only ones that know how the vulnerability works. As long as it stays that way there is not much that Zoom users have to worry about. For those that worry anyway, the browser version is said to be safe from this vulnerability. For anyone else, keep your eyses peeled for the patch and update at earliest convenience after it comes out.

Update April 9

Zoom responded to the articles about the Pwn2Own event:

“We thank the Zero Day Initiative for allowing us to sponsor and participate in Pwn2Own Vancouver 2021, an event highlighting the critical and skillful work performed by security researchers. We take security very seriously and greatly appreciate the research from Computest. 

We are working to mitigate this issue with respect to Zoom Chat, our group messaging product. In-session chat in Zoom Meetings and Zoom Video Webinars are not impacted by the issue. The attack must also originate from an accepted external contact or be a part of the target’s same organizational account. 

As a best practice, Zoom recommends that all users only accept contact requests from individuals they know and trust. If you think you’ve found a security issue with Zoom products, please send a detailed report to our Vulnerability Disclosure Program in our Trust Center.”

Stay safe, everyone!


Pieter Arntz

Malware Intelligence Researcher

Was a Microsoft MVP in consumer security for 12 years running. Can speak four languages. Smells of rich mahogany and leather-bound books.