Fleet: IP spoofing allows bypassing API rate limiting

Summary

A vulnerability in Fleet's IP extraction logic allows unauthenticated attackers to bypass API rate limiting by spoofing client IP headers. This may allow brute-force login attempts or other abuse against Fleet instances exposed to the public internet.

Impact

Fleet extracted client IP addresses from request headers (True-Client-IP, X-Real-IP, X-Forwarded-For) without validating that those headers originate from a trusted proxy. The extracted IP is used as the key for rate limiting and IP ban decisions.

As a result, an attacker could rotate the value of these headers on each request, causing Fleet to treat each attempt as coming from a different client. This effectively bypasses per-IP rate limits on sensitive endpoints such as the login API, enabling unrestricted brute-force or credential stuffing attacks.

This issue primarily affects Fleet instances that are directly exposed to the internet without a reverse proxy that overwrites forwarded-IP headers. Instances behind a properly configured proxy or WAF are less affected.

Workarounds

If an immediate upgrade is not possible, administrators should ensure Fleet is deployed behind a reverse proxy (e.g., nginx, Cloudflare, AWS ALB) that overwrites X-Forwarded-For with the true client IP, and apply rate limiting at the proxy or WAF layer.

For more information

If you have any questions or comments about this advisory:
Email us at security@fleetdm.com
Join #fleet in osquery Slack

Credits

We thank @fuzzztf for responsibly reporting this issue.