External risk intelligence

Elysia Prototype Pollution Vulnerability Allows Remote Code Execution

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2025-66456

Elysia is a framework used to build web applications, APIs, and network services. Because these applications are commonly deployed as public-facing web or API endpoints to handle client-server communication and request validation, the vulnerable code is likely to be reachable from the internet in typical deployments.

Elysiajs Elysia

1.4.0 to before 1.4.17

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This advisory addresses a critical vulnerability found in the Elysia Typescript framework, which is used for request validation and communication. The issue could allow attackers to execute arbitrary code on affected systems. The main concern is confirming if this framework is used within your environment and, if so, assessing the potential exposure.

Code vulnerability in a web framework.
Affects request validation and server communication.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could reach this vulnerability by sending a specially crafted request to a web application built with Elysia. If the application uses a specific validation schema merging technique, the attacker can exploit a flaw in how the `mergeDeep` function handles data, potentially leading to code execution when combined with another vulnerability.

Exposed web service required.
Vulnerable schema merge triggered.
Allows remote code execution.

Live Threat

Current exploitation, exposure, and threat context

When used in certain configurations, this vulnerability in the `mergeDeep` function could allow an attacker to manipulate the application's behavior by polluting its prototype. This could lead to unintended consequences, especially when combined with other vulnerabilities as described in the advisory.

Application logic and behavior.
Prototype pollution via crafted input.
Unintended service state or execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

Application owners are responsible for addressing this critical vulnerability within the Elysia framework, which is used for request validation and server communication. The initial step is to identify all instances of the affected Elysia versions, determine their exposure, and confirm business criticality to prioritize remediation efforts.

Identify accountable application owners.
Verify external exposure and business impact.
Plan vendor coordination or remediation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the Elysia framework?

Elysia is a TypeScript web framework designed for building performant server-side applications. Developers use it to manage API routing, perform complex request validation, handle type inference, and generate OpenAPI documentation for client-server communication.

How does CVE-2025-66456 work as a prototype pollution vulnerability?

This vulnerability, classified as CWE-1321, occurs when the framework's 'mergeDeep' function insecurely merges data structures. It allows an attacker to inject properties into the base JavaScript prototype. By overwriting these prototype properties, the attacker can influence the application's logic, which may be chained with other flaws to gain full control.

What triggers this vulnerability in Elysia?

The flaw is triggered when an application merges two schema validation results that share a key, specifically involving an 'any' type guard. It does not occur in configurations that avoid this specific merging pattern or that do not utilize the vulnerable 'mergeDeep' logic during request validation.

How does Halo Surface Signal categorize the risk of this CVE?

Halo Surface Signal identifies this as 'Likely' to be reachable from the internet. Because Elysia is commonly used to build public-facing APIs and network services that process incoming client data, the vulnerable validation paths are frequently exposed to remote network traffic.

How do I address this Elysia security issue?

You should first identify all applications running Elysia versions 1.4.0 through 1.4.16. The primary fix is to update your project dependency to version 1.4.17 or later. As a temporary workaround if an update is not immediately possible, ensure your request bodies are sanitized to remove the '__proto__' key.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.