Server-Side Request Forgery in vas3k TaxHacker

Executive Summary

This is a Server-Side Request Forgery (SSRF) vulnerability in the TaxHacker application affecting versions up to 0.8.5. The flaw exists in the testLLMProviderAction function where unsanitized parameters like provider, apiKey, model, and baseUrl are passed to the OpenAI client. When provider is set to openai_compatible, the baseUrl is used directly, allowing the server to make HTTP requests to arbitrary endpoints. The vulnerability is exploitable without authentication and can be used to probe internal networks or read responses from internal services.

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Detection Guidance

To detect this SSRF vulnerability in TaxHacker, check if the testLLMProviderAction endpoint is accessible without authentication. Use curl to send a request with controlled parameters like provider=openai_compatible and baseUrl pointing to internal or external targets. Example: curl -X POST http://<target>/apps/settings/actions.ts -d '{"provider":"openai_compatible","baseUrl":"http://169.254.169.254/latest/meta-data/"}'

Monitor server logs for unexpected outbound HTTP POST requests to internal or external IPs. Check if responses from internal services are reflected back in API responses.

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Impact Analysis

An attacker could exploit this to access internal systems, scan internal networks, or retrieve sensitive data from internal services like cloud metadata endpoints. In self-hosted mode, the impact is severe as no session is required. Even in SaaS mode, insufficient route-level protection may allow exploitation. The vulnerability enables port scanning, internal service fingerprinting, and potential data exfiltration.

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Compliance Impact

This SSRF vulnerability could lead to unauthorized access to internal systems, potentially exposing sensitive financial or personal data processed by TaxHacker. For GDPR, this may violate principles of data protection by design and default, risking unauthorized data breaches. Under HIPAA, if the app handles protected health information, the lack of access controls could result in unauthorized disclosures, violating security and privacy requirements.

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Mitigation Strategies

Immediately add session validation to the testLLMProviderAction endpoint to ensure only authenticated users can access it. Implement a strict allowlist for baseUrl to block private IP ranges and non-HTTPS protocols.

Disable the echoing of raw connection errors or response bodies in API responses. Replace them with generic failure messages to prevent information leakage.

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