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Test a WebSocket API in layers: first verify the HTTP Upgrade handshake, then check authentication and authorization, message behavior, security controls, and recovery after disconnects. A successful socket connection proves only that the protocol handshake worked; it does not prove that users can perform only permitted actions or that the application handles missed events correctly.
Keep protocol behavior separate from application rules. RFC 6455 defines how a WebSocket connection is established and closed. Your API contract defines which messages are valid, what they mean, and how the client should restore subscriptions or state after reconnecting.
What to verify in a WebSocket test
A repeatable test should check both the connection and the behavior carried over it. Record the expected result for each stage rather than treating “socket opened” as the whole test.
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- Identity and permissions: Were credentials accepted, and can the connected identity perform only its authorized actions?
- Messages: Are valid messages handled correctly, while malformed or unauthorized messages are rejected without unintended side effects?
- Connection lifecycle: Are normal closes distinguished from abnormal transport loss, and does retry behavior follow the client’s policy?
- Application recovery: After reconnecting, does the client restore subscriptions, resume from a cursor, replay events, or fetch a snapshot as specified by the API?
The protocol does not decide the application’s recovery rule. Make that rule explicit in the API contract and assert it in tests.
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How to test the opening handshake
A WebSocket connection begins as an HTTP request. The client sends an HTTP GET with Upgrade headers; the server must return 101 Switching Protocols and the required handshake headers before the client treats the connection as established. See RFC 6455.
Check a successful upgrade
For each endpoint, specify the URI scheme (ws:// or wss://), path, query parameters, and required headers. On success, verify all of the following:
- The HTTP status is
101. - The
Upgradeheader iswebsocket. - The
Connectionheader containsUpgrade. Sec-WebSocket-Acceptmatches the value expected from the client’s handshake key.- Any selected
Sec-WebSocket-Protocolwas offered by the client. - Any negotiated extension was among those requested by the client.
Preserve the handshake request and response in test output where your client exposes them. That makes it possible to distinguish a negotiation problem from a later message-level failure.
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Test failed and boundary handshakes
Include negative cases: a wrong path, missing or invalid credentials, an unsupported subprotocol, a rejected origin, an expired or invalid session, and a server-side refusal. A non-101 response is still an HTTP failure, not an established WebSocket. Assert its status and body according to your service contract; there is no universal status code or error payload for every rejection.
For each failure, record the response, selected protocol if any, client-visible error, and close outcome when available. This helps identify whether the request was refused during the HTTP handshake or whether the connection failed after establishment.
How to test authentication, authorization, and security
WebSocket can use authentication during the handshake, but the protocol itself does not provide application authentication or authorization. Test the mechanism your service actually uses, such as cookies, HTTP authentication, or TLS authentication, and separately verify permissions for each message or action. See the RFC and OWASP Web Security Testing Guide v4.2.
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Exercise credential and permission cases
At connection time, try valid, absent, malformed, expired, and revoked credentials as applicable. After connection, test each protected action with an authorized identity, an authenticated but unauthorized identity, and no or expired identity where the service permits those states. Change identifiers in message payloads to check that a user cannot retrieve or modify another user’s data.
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Check origin, transport, and payload handling
For browser clients, test both accepted and rejected Origin values against the deployed policy. RFC 6455 allows a server to use the origin when deciding whether to accept a connection, and OWASP identifies missing origin validation as a cross-origin risk. Do not assume every non-browser client sends an Origin header.
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For sensitive traffic, verify that the endpoint uses wss://, the certificate is valid, and the TLS configuration matches deployment requirements. Send invalid schema values, unexpected message types, boundary-size payloads, and encoded or otherwise untrusted content. Check that validation is consistent and that rejected input does not cause unintended state changes. Run fuzzing or replay only against systems you are authorized to test.
How to test reconnection and recovery
Distinguish a normal close from a transport loss or abnormal closure. For abnormal closures, RFC 6455 recommends delaying retries with a randomized initial wait and increasing the delay after repeated failures. It describes a random initial delay in the 0–5 second range as reasonable guidance, not a universal mandatory setting, and suggests increasingly longer delays such as truncated binary exponential backoff. See RFC 6455, Section 7.2.3.
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Test the retry policy as a sequence, not as a single reconnect:
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- Induce a transient connection failure and observe the time until the first retry.
- Keep the service unavailable through repeated attempts; verify that delays increase and respect the client’s configured cap.
- Restore the service and confirm the client reconnects without many clients retrying in lockstep.
- Assert the application’s recovery behavior: for example, subscription restoration, cursor-based resume, missed-event replay, or fetching a full snapshot.
The required state-recovery behavior is API-specific. State it as an expected result before running the test; the WebSocket protocol does not prescribe whether a client must replay missed events or request a fresh snapshot.
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For each scenario, fix the initial state, inputs, timing window, and expected result. Capture connection status, handshake details, sent and received messages, timestamps, and close code or reason where the test client exposes them.
| Scenario | Assertions to define |
|---|---|
| Connect successfully | Handshake status and required headers; negotiated subprotocol and extensions. |
| Send a valid message | Expected response or server event, including its content and timing window. |
| Send malformed or unauthorized input | Expected rejection and confirmation of no unintended side effect. |
| Receive a server-pushed event | Expected event arrives without the client polling or sending a triggering request, where the contract requires push behavior. |
| Close normally | Expected close handling and any specified close code or reason. |
| Drop transport and reconnect | Retry timing, eventual connection, and the API-specific state-recovery rule. |
| Run concurrent clients or sustained sessions | Handshake success, message latency, errors, and close behavior under the scenario’s defined load. |
A passing handshake is not evidence that authorization, message semantics, or recovery works. Keep those assertions separate so a failure points to the layer that needs attention.
Choose a tool for the kind of test
Different tools support different parts of the workflow; the available documentation does not establish a complete feature-by-feature comparison.
| Approach | Useful for | Limits to account for |
|---|---|---|
| Postman raw WebSocket request | Interactive exploration: create a raw WebSocket request, enter a ws:// or wss:// URL, connect, inspect behavior, and disconnect. See Postman’s WebSocket request documentation. |
The cited documentation describes interactive requests; it does not establish a complete automated assertion workflow. |
| Grafana k6 | Scripted WebSocket scenarios and load testing. Its documentation includes event handlers and a check for handshake status 101. See k6 WebSockets documentation. |
For new tests, the documentation recommends k6/websockets and says the experimental WebSocket module is deprecated. Consult the module documentation for current scripting details because APIs can change. |
| OWASP ZAP guidance | Security-oriented interception, replay, and fuzzing of WebSocket messages. The OWASP Testing Guide v4 discusses the ZAP WebSocket tab. | This is an archived guide and should be treated as orientation, not definitive current procedure. See the archived OWASP WebSocket testing page. |
Use interactive tools to explore request and response behavior, scripted tests for repeatable contract or load scenarios, and security tooling for authorized replay or fuzz work. Select based on the assertions and execution environment you need; do not assume one tool covers every layer.
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