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Test a WebSocket API in layers: first verify the HTTP Upgrade handshake, then check authentication and permissions, message behavior, and recovery after a disconnect. A successful connection proves only that the protocol switch worked; it does not prove that the application authorizes a user or handles messages correctly.
What a WebSocket test must prove
WebSocket testing covers two distinct layers. Protocol checks establish whether the client and server complete the connection and follow WebSocket rules. Application checks establish whether particular identities may send particular messages, what responses or events follow, and how the client recovers application state after interruption. Keep those assertions separate so a successful handshake cannot mask an authorization or message-handling failure.
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For each scenario, define the starting state, inputs, timing window, expected messages or events, and expected connection or close outcome. Capture timestamps, handshake details, messages sent and received, and the close code and reason when the test client exposes them.
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A WebSocket connection begins as an HTTP GET Upgrade request. Before treating the connection as established, verify a successful 101 Switching Protocols response and the required handshake fields. RFC 6455 specifies the handshake and the calculation of Sec-WebSocket-Accept from the client key: RFC 6455.
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- Check the
ws://orwss://scheme, endpoint path, query parameters, and required request headers. - Assert response status
101,Upgrade: websocket, aConnectionheader containingUpgrade, and a validSec-WebSocket-Accept. - If the client offered subprotocols, verify that the server-selected
Sec-WebSocket-Protocolwas among them. Likewise, any selected extension should have been requested by the client. - For a non-
101response, preserve and assert the HTTP status and response body. It is a failed upgrade, not an established socket.
Exercise negative and boundary cases as well as the happy path: wrong path, unsupported subprotocol, missing or invalid credentials, expired session, rejected origin, and server refusal. Record the actual response, selected protocol, error, and close outcome. The expected error status and payload depend on the service contract; there is no universal failure status for all these cases.
How to test authentication and authorization
WebSocket does not supply application-level authentication or authorization rules. RFC 6455 permits authentication behavior during the handshake, but the service defines its mechanism and permissions; OWASP recommends testing both the connection and subsequent message handling. See the OWASP WebSocket testing guidance.
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Check credentials at connection time
Use the mechanism the API actually supports, such as cookies, HTTP authentication, or TLS authentication. Test valid, absent, malformed, expired, and revoked credentials. Assert the service’s documented response for each rather than assuming one status code applies to all implementations.
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After connecting, test protected message types or actions with an authorized identity, an authenticated but unauthorized identity, and an unauthenticated or expired identity where applicable. Change identifiers in messages to check that one user cannot read or alter another user’s data. A connection that was authenticated successfully does not automatically authorize every later message.
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How to test origin, transport, and message safety
- Browser origin: Test accepted and rejected
Originvalues against the deployed policy. RFC 6455 allows a server to use Origin when deciding whether to accept a connection and to reject a request with an HTTP error. Do not assume every non-browser client sends an Origin header. - TLS: For sensitive traffic, verify use of
wss://, certificate validity, and the TLS configuration required by the deployment. - Payload validation: Send invalid schema values, unexpected message types, boundary-size payloads, and encoded or otherwise untrusted content. Assert safe rejection or handling and check for unintended side effects.
- Authorized testing: Run replay and fuzz tests only against systems you are permitted to test.
How to test disconnects and reconnection
Test normal closure separately from transport loss or abnormal closure. RFC 6455 recommends a randomized delay before the first retry and increasingly longer delays after repeated failures. It describes a random initial delay in the 0–5 second range as reasonable guidance, not a mandatory setting for every API. The RFC’s guidance is in Section 7.2.3.
- Establish a connection and record the normal close behavior expected by the service.
- Cause a transient transport failure and observe when the client makes its first reconnect attempt.
- Cause repeated failures and verify that delay grows according to the configured policy and respects any configured cap.
- Restore service and verify the client reconnects without many clients retrying in lockstep and creating a synchronized retry storm.
- Check application recovery separately: assert whether the client restores subscriptions, resumes from a cursor, requests a snapshot, or follows another documented rule.
The protocol does not determine whether an application replays missed events or restores subscriptions. Make that behavior an explicit contract assertion, including what happens to events missed during the outage.
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Repeatable scenarios to put in a test suite
These cases cover connection, message, event, close, and recovery behavior. Adapt expected responses and close outcomes to the API’s contract.
- Connect: Complete the handshake and verify the negotiated subprotocol.
- Valid message: Send a valid request and assert the expected response or event within the defined timing window.
- Invalid or unauthorized message: Send malformed and unauthorized messages; assert rejection and no unintended state change.
- Server-pushed event: Verify that the expected event arrives without a client poll or request.
- Normal close: Close the connection and assert the documented close handling.
- Transport loss and recovery: Drop the connection, observe retry behavior, reconnect, and assert the application’s state-recovery rule.
- Concurrency or sustained sessions: Run multiple clients or sustained connections and track handshake success, message latency, errors, and close behavior.
Keep each case repeatable by fixing its initial state, inputs, timing window, and expected result. Capture connection status, handshake request and response data, sent and received messages, timestamps, and close code or reason when available. A test that checks only whether the socket opened misses the API’s authorization and message semantics.
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Choosing a tool for the test
| Approach | Useful for | What the cited documentation establishes |
|---|---|---|
| Postman | Interactive exploration of a raw WebSocket request | Its documentation describes creating a raw WebSocket request, entering a ws:// or wss:// URL, connecting, and disconnecting. It is useful for manual inspection; the cited page does not establish a complete automated assertion workflow. Postman WebSocket requests. |
| Grafana k6 | Scripted scenarios and WebSocket load testing | Its documentation covers event handlers and checking handshake status 101. It recommends k6/websockets for new tests and says the experimental WebSocket module is deprecated; consult the current module documentation because APIs can change. k6 WebSockets. |
| OWASP ZAP | Security-oriented message inspection and replay | The archived OWASP Testing Guide v4 suggests using ZAP’s WebSocket tab to intercept, replay, and fuzz messages. Treat it as orientation, not definitive current OWASP procedure. OWASP Testing Guide v4: WebSockets. |
These options serve different jobs: interactive exploration, scripted functional or load scenarios, and security replay or fuzzing. Choose based on the assertions and reporting your workflow needs; the cited documentation does not establish a complete feature-by-feature comparison.
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