Authentication Bypass in 9Router via Unprotected /codex Endpoint

Compliance Impact

The vulnerability in 9router allows unauthenticated remote attackers to bypass API-key authentication and use the operator's stored LLM provider credentials to make upstream calls. This unauthorized access can lead to unexpected billing and exposure of model behavior.

Such unauthorized access and potential data exposure could negatively impact compliance with standards and regulations like GDPR and HIPAA, which require strict access controls and protection of sensitive data. The bypass of authorization mechanisms increases the risk of data misuse and unauthorized processing, which are critical compliance concerns.

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Executive Summary

CVE-2026-55638 is a vulnerability in the 9router npm package that allows remote unauthenticated attackers to bypass API-key authentication by sending requests to the /codex/* endpoint. This happens because the middleware that enforces API-key checks only verifies the original request path before it is rewritten by Next.js configuration. The rewrite maps /codex/* to the protected backend endpoint /api/v1/responses, but the authorization check is skipped for /codex/* requests.

As a result, attackers can make the server perform upstream calls to the LLM provider using the operator's stored credentials without authorization.

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Impact Analysis

This vulnerability can lead to unauthorized use of the LLM proxy service, allowing attackers to consume provider credits and cause unexpected billing charges for the operator.

Additionally, it may expose the behavior of the underlying language model, potentially leaking sensitive information or model responses.

Because the attack requires no privileges or user interaction and can be performed remotely over the network, it poses a high risk.

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Detection Guidance

This vulnerability can be detected by checking if your 9router instance allows unauthenticated requests to the /codex/* endpoint, which bypasses the API-key authentication gate.

You can test this by sending an HTTP request to the /codex/* path and observing if the server processes the request without requiring authentication.

• Use curl to send a request to the vulnerable endpoint, for example: curl -v http://<router-ip>:<port>/codex/test
• Check server logs for any unauthorized access attempts or upstream provider calls triggered by requests to /codex/*.
• Verify if the server is bound to 0.0.0.0 (all interfaces), which exposes it to external networks, by running commands like: netstat -tuln | grep <port> or ss -tuln | grep <port>

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Mitigation Strategies

The primary mitigation step is to upgrade the 9router package to version 0.5.2 or later, where this vulnerability is fixed.

If immediate upgrade is not possible, restrict network access to the 9router service by limiting exposure to trusted networks only and avoid binding the server to 0.0.0.0.

Implement firewall rules or network policies to block unauthorized external access to the /codex/* endpoint.

Monitor server logs for suspicious requests to /codex/* and investigate any unexpected upstream provider calls.

Consider applying additional security controls such as size limits, timeout settings for remote media fetching, and host blocklists to reduce risk of related attacks.

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