Authentication Bypass and SSRF in FastChat via Worker Registration

Executive Summary

FastChat has an authentication bypass flaw in the /register_worker endpoint. Unauthenticated attackers can register fake worker addresses and perform server-side request forgery. This lets them impersonate valid workers to intercept user prompts, images, responses, or scan internal network ports across the worker mesh.

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Detection Guidance

Check for unauthorized worker registrations by monitoring the /register_worker endpoint logs for unexpected POST requests. Inspect network traffic for unusual server-side request forgery attempts targeting internal ports. Verify worker addresses registered in FastChat configurations match expected trusted sources.

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Impact Analysis

Attackers could capture sensitive user data like prompts, images, or responses by registering malicious workers under legitimate model names. They might also probe internal systems to find other vulnerabilities or exfiltrate data from the worker network.

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Compliance Impact

This vulnerability could lead to unauthorized access to personal or sensitive data, violating GDPR's data protection requirements and HIPAA's safeguards for protected health information. Organizations using FastChat may face compliance breaches and legal penalties.

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Mitigation Strategies

Disable the /register_worker endpoint if not required. Implement strict authentication for worker registration. Restrict network access to the FastChat worker mesh. Update FastChat to the latest patched version. Monitor for suspicious worker registrations or internal network probes.

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