CODE RED • SANDBOX ESCAPE • RCE

Windows Under Attack: Critical Deno Vulnerability (CVE-2025-61787) Enables Command Injection on All Windows Devices

By CyberDudeBivash • October 09, 2025 • V6 “Leviathan” Deep Dive

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Definitive Guide: Table of Contents

Part 1: The Executive Briefing — A Crisis for a “Secure” Runtime

This is a critical alert for all developers and organizations using the Deno runtime on the Windows platform. A high-severity **command injection** vulnerability, tracked as **CVE-2025-61787**, has been discovered, and a patch is now available. This flaw allows a malicious Deno script to **escape the sandbox** and execute arbitrary commands on the host operating system, leading to a full system compromise.

Deno’s primary value proposition is its “secure-by-default” architecture. A vulnerability of this nature, which completely breaks the core security model of the platform, is a catastrophic failure. For any company using Deno to run backend services, CLI tools, or web applications on Windows, this is a “stop everything and patch now” event. Active exploitation is expected to begin immediately.

Business Impact:

**Server Compromise:** Any Deno-based web server or API running on Windows is at risk of a full takeover, leading to data theft or ransomware.
**Developer Workstation Compromise:** A developer tricked into running a malicious Deno script can have their machine fully compromised, leading to the theft of source code, credentials, and a beachhead into the corporate network.
**Supply Chain Risk:** Malicious Deno scripts could be published to third-party registries, leading to a widespread supply chain attack.

Part 2: Technical Deep Dive — The Deno Sandbox, Windows APIs, and the Command Injection Flaw

Deno’s Security Model: The Sandbox

Unlike Node.js, Deno executes code in a secure sandbox by default. A Deno script has no access to the file system, the network, or the environment unless the user explicitly grants it permission via command-line flags (e.g., `–allow-net`, `–allow-read`). The vulnerability in CVE-2025-61787 is a **sandbox escape**, which means it bypasses this entire security model.

The Flaw: Improper Handling of UNC Paths

The vulnerability exists in the core Deno runtime’s C++/Rust code that interfaces with the Windows operating system for file operations. Specifically, it is a command injection flaw in how Deno parses and handles specially crafted **UNC (Universal Naming Convention)** paths.

The Exploit Chain:

An attacker crafts a malicious Deno script that calls a file-accessing API (e.g., `Deno.open()`).
The file path provided is not a normal path, but a malicious UNC path that includes shell metacharacters (like `&` or `|`). For example: `\\malicious-server\share\file.txt & calc.exe`.
The Deno runtime fails to properly sanitize this input before passing it to an underlying Windows API call that is executed via the command shell.
The Windows shell interprets the `&` as a command separator. It attempts to access the network path, and then it **executes `calc.exe`** (or any other command the attacker chooses) on the local machine, with the full permissions of the Deno process. The sandbox has been bypassed.

Part 3: The Defender’s Playbook — A Guide to Patching, Hardening, and Hunting

1. PATCH YOUR DENO RUNTIME IMMEDIATELY

This is your highest priority. The Deno project has released an emergency security patch. You must update your Deno runtime on all Windows machines by running the following command:

deno upgrade

2. HARDEN Your Scripts with Least Privilege

This incident is a powerful reminder of Deno’s core security philosophy. You should always run Deno scripts with the absolute minimum permissions necessary. For example, a script that only needs to make network requests should be run with `deno run –allow-net=example.com script.ts`, and **not** with broad flags like `–allow-all` or `–allow-run`.

3. HUNT for Post-Compromise Behavior

You must assume you have been targeted. Your SOC team needs to hunt for the “golden signal” of a sandbox escape: the Deno process spawning anomalous child processes.

The Golden Query for Your EDR:

ParentProcessName: deno.exe
AND ProcessName IN ('cmd.exe', 'powershell.exe', 'cscript.exe', 'wscript.exe')

Unless you have a specific, authorized script that is supposed to do this, any hit on this query is a critical indicator of compromise that requires immediate investigation.

Detect the Evasive Behavior: A modern **XDR platform** is essential for detecting sandbox escapes. It can see the anomalous parent-child process relationship and other behavioral TTPs that signature-based tools would miss.

Part 4: The Strategic Takeaway — The Illusion of “Perfect Security”

For CISOs and technology leaders, this incident is a critical lesson. Even modern, “secure-by-default” languages and runtimes like Deno are not a silver bullet. They are complex pieces of software, written by humans, and they will have vulnerabilities. While their security models provide a significant and valuable layer of defense, you cannot become complacent.

A resilient **DevSecOps** program still requires a defense-in-depth approach. This means that even when you are using a “secure” runtime, you must still have robust endpoint security (EDR), network segmentation, and proactive threat hunting to act as your essential safety net for when the built-in security inevitably fails.

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About the Author

CyberDudeBivash is a cybersecurity strategist with 15+ years in application security, DevSecOps, and incident response, advising CISOs across APAC. [Last Updated: October 09, 2025]

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