CVE-2026-55638: 9router Unauthenticated LLM Proxy Access via /codex Rewrite Authorization Bypass
Summary
CVE-2026-55638: 9router: Unauthenticated LLM proxy access via /codex rewrite authorization bypass. Affects 9router versions < 0.5.2. Fixed in 0.5.2. How to fix ↓
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Background
9router is an open-source package that provides an OpenAI/Anthropic-compatible LLM (Large Language Model) proxy, allowing users to interact with various LLM services. It is designed to facilitate seamless communication with LLM providers while ensuring secure access through API key checks. However, a vulnerability has been identified that allows unauthenticated access to the LLM proxy, compromising the intended security measures.
Vulnerability Detail
The vulnerability arises from a flaw in the middleware authorization logic of 9router. Specifically, the middleware checks for API keys based on the original incoming request path before applying any rewrite rules. Since the path '/codex/*' is not included in the protected API prefixes, requests to this path bypass the API key check entirely. This allows unauthenticated remote attackers to access the LLM proxy and make upstream provider calls using the operator's stored credentials. The potential ramifications include unauthorized usage of the LLM proxy, unexpected billing impacts, and exposure of sensitive model behavior.
How To Fix
Latest published version: 0.5.109 as of 23 Oct 2026
npm install 9router@0.5.109Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
0.5.2
This is the first release that contains the fix.
npm install 9router@0.5.2Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, you can modify the middleware configuration to include '/codex' in the protected API prefixes. This can be done by updating the PUBLIC_PREFIXES array in the middleware file (src/dashboardGuard.js) to include '/codex':
PUBLIC_PREFIXES = ["/v1", "/v1beta", "/api/v1", "/api/v1beta", "/codex"];
Conclusion
In summary, CVE-2026-55638 presents a significant security risk for applications using the 9router package due to an authorization bypass vulnerability. It is crucial to update to the patched version or implement the provided workaround to mitigate the risk of unauthorized access to the LLM proxy.
See every vulnerability recorded for 9router →
FAQ
CVE-2026-55638 is a vulnerability in the 9router package that allows unauthenticated access to its LLM proxy through a specific path rewrite.
If your application uses 9router, an attacker could exploit this vulnerability to access the LLM proxy without authentication, leading to unauthorized usage and potential billing issues.
Implement the suggested workaround by modifying the middleware configuration to include '/codex' in the protected API prefixes until you can update to the patched version.
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The CVSS score for this vulnerability is 8.6, indicating a high severity level.
References
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