CVE-2026-75021 in fastify-cli
Summary
by MITRE • 09/08/2026
fastify-cli starts the Node.js Inspector when a debug flag is used, but it ignores the explicit bind address the user supplies and binds the Inspector to a broadly reachable address instead of the intended loopback. As a result the debugging interface can be exposed beyond the local machine, and because the Inspector protocol allows arbitrary code evaluation, a remote party that reaches it can achieve remote code execution on the developer's machine. This affects fastify-cli from 1.5.0 up to 8.0.1. Users should upgrade to fastify-cli 8.0.1, which honors the configured Inspector bind address.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability in fastify-cli represents a critical misconfiguration of the Node.js debugging interface that exposes sensitive development tools to network-based attacks. When developers utilize the debug flag within this command-line tool, the application is intended to launch with an active inspection protocol for remote debugging purposes. However, the software fails to correctly process or respect explicit bind address parameters provided by the user. Instead of restricting access to the local loopback interface which limits connectivity strictly to the host machine, the inspector binds to a broadly reachable network address. This architectural oversight effectively bypasses standard localhost security boundaries, making the debugging port accessible from external networks and potentially the public internet depending on the underlying network configuration and firewall rules.
The operational impact of this flaw is severe due to the inherent capabilities of the Node.js Inspector protocol. The inspector allows for interactive code evaluation, meaning that any entity with network access to the exposed port can execute arbitrary JavaScript commands within the context of the running application process. This capability translates directly into Remote Code Execution (RCE) on the developer's machine. An attacker who discovers or scans for this open debugging port can inject malicious scripts, exfiltrate sensitive data such as environment variables and session tokens, modify application logic in real-time, or use the compromised server as a pivot point to attack other systems within the internal network infrastructure. This risk is particularly acute because development environments often run with elevated privileges or contain proprietary source code and credentials that are not present in production deployments.
From a classification perspective, this vulnerability aligns closely with CWE-201 which describes exposure of sensitive information through an unintended data channel, specifically regarding the leakage of control capabilities via debugging interfaces. It also maps to ATT&CK technique T1059 Command and Scripting Interpreter where attackers leverage built-in system tools for execution without deploying additional malware payloads. The failure to restrict network binding addresses is a common pattern in development utilities that prioritize convenience over security defaults, leading to CWE-284 Improper Access Control where the application fails to properly enforce restrictions on authorized users or processes accessing resources.
To mitigate this risk, immediate action is required for all systems running affected versions of fastify-cli ranging from 1.5.0 up through 8.0.1. The primary remediation step is to upgrade the tooling environment to version 8.0.2 or later where the bug has been corrected and the inspector correctly honors the configured bind address settings. In environments where upgrading immediately is not feasible, administrators should implement strict network segmentation rules that block external access to debugging ports typically used by Node.js inspectors such as port 9229. Additionally, developers should avoid exposing development servers on public-facing interfaces and utilize firewall rules or container networking policies to ensure that inspection protocols remain confined to localhost unless explicitly required for specific remote debugging scenarios with proper authentication mechanisms in place.