Fleet has a rate limiting bypass via untrusted client IP headers

Impact

Fleet trusted client-supplied IP address headers when determining the source IP for incoming requests. This allowed authenticated and unauthenticated clients to spoof their apparent IP address and bypass per-IP rate limiting controls.

Fleet determines a client’s public IP address using HTTP headers such as:

• X-Forwarded-For
• X-Real-IP
• True-Client-IP

These headers were trusted without validation. An attacker could supply arbitrary values in these headers, causing Fleet to treat each request as originating from a different IP address.

This could allow an attacker to bypass per-IP rate limits and increase the effectiveness of brute-force or password-spraying attempts against authentication endpoints.

This issue does not allow authentication bypass, privilege escalation, data exposure, or remote code execution on its own.

Workarounds

Run Fleet behind a trusted reverse proxy or load balancer that overwrites client IP headers.

For more information

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Credits

We thank @fuzzztf for responsibly reporting this issue.