fix(telemetry): detect intercepted uploads on client side#70
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The agent currently treats any HTTP 200 from the presigned-URL PUT as proof the upload reached S3 and immediately calls notify. A TLS- inspecting proxy, DLP appliance, or outbound-filtering firewall on a customer network can terminate that PUT mid-flight and synthesize a 200 response without forwarding the bytes — the run is then reported as a silent success despite no object ever existing in S3. Two checks, in order: 1. After the PUT returns 200, require x-amz-request-id and/or x-amz-id-2 on the response. Real S3 always sets these; a proxy fake-200 can't reproduce them. Missing both → non-retryable failure with the response Server header captured for forensics. 2. Before calling notify, POST /telemetry/confirm-upload to ask the backend whether the object actually exists in S3. A definitive uploaded=false is fatal (bail with reason). 404 (old backend without the endpoint), non-200, or transport errors fall through to notify, whose own server-side precheck remains the safety net. On success the agent prints a definitive "Upload confirmed in S3 (N bytes)" line, separate from "Backend processing started", so the user can tell upload-landed from processing-kicked-off. Adds tests for: real-S3 happy path, synthetic-200 PUT rejection, confirm=false fatal path, confirm 404 fallback, confirm 5xx fallback. Signed-off-by: Swarit Pandey <swarit@stepsecurity.io>
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ashishkurmi
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May 13, 2026
| proxyHint := putResp.Header.Get("Server") | ||
| _, _ = io.Copy(io.Discard, putResp.Body) | ||
| _ = putResp.Body.Close() | ||
| return fmt.Errorf("S3 PUT returned 200 but the response is not from AWS (missing x-amz-request-id and x-amz-id-2; Server=%q) — the upload likely did not reach S3, possibly intercepted by a TLS-inspecting proxy or outbound firewall on this network", proxyHint) |
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we should log instead of returning an error.
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Pull request overview
This PR hardens telemetry S3 uploads against “synthetic 200 OK” responses (e.g., from TLS-inspecting proxies) and adds a backend-assisted confirmation step to detect cases where an upload appeared successful but the object did not actually persist in S3.
Changes:
- Rejects S3 PUT
200 OKresponses that are missing bothx-amz-request-idandx-amz-id-2, treating them as non-retryable upload failures. - Adds a best-effort
/telemetry/confirm-uploadcall prior to notifying the backend, failing the run only when the backend definitively reportsuploaded=false. - Expands unit test coverage to validate the new interception detection and confirm-upload compatibility behaviors (success, uploaded=false fatal, 404/5xx fallthrough).
Reviewed changes
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| File | Description |
|---|---|
| internal/telemetry/telemetry.go | Adds client-side detection of non-AWS “200 OK” PUT responses and introduces a backend confirmation step before notify. |
| internal/telemetry/telemetry_test.go | Adds tests covering synthetic 200 rejection and confirm-upload behavior across success/failure/compat scenarios. |
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What does this PR do?
Type of change
Testing
./stepsecurity-dev-machine-guard --verbose./stepsecurity-dev-machine-guard --json | python3 -m json.toolmake lintmake testRelated Issues