Diego Bentivoglio : 6 October 2025 12:31
WebSockets offer persistent two-way communication between client and server, essential for real-time applications like chat, gaming, dashboards, and notifications. However, this persistence introduces specific attack surfaces: if the channel or its rules are not adequately protected, data exfiltration, session hijacking, and vulnerabilities related to unfiltered input can occur. This article provides a practical explanation of the most significant risks and essential countermeasures for protecting this type of connection.
Their useful features include long connections, bidirectional traffic, and extremely low latency, which simultaneously create opportunities for attackers. A persistent connection means that a single breach can maintain access for a long time. Bidirectionality means that both the client and the server can send data, which means both sides must treat messages as untrusted. Dynamic endpoints, if built with user-controlled data, can trick the client into connecting to malicious servers. Finally, the lack of built-in handshaking control opens the door to potential injections or exploits from external sites.
The most significant types of attacks include traffic interception and modification, i.e., sniffing or man-in-the-middle attacks, when the unencrypted “ws://” protocol is used. Then there is connection injection, comparable to CSRF applied to WebSockets, where malicious pages trick the browser into establishing connections. Equally important is data exfiltration through redirects or messages sent to servers controlled by an attacker. Finally, vulnerabilities related to unvalidated input, capable of generating XSS, SQL injection, or unexpected commands.
The fundamental defense principles are clear. Encryption must always be mandatory, and the “wss://” protocol, i.e., WebSocket over TLS, must be used to prevent sniffing and man-in-the-middle attacks. Endpoints must be stable and uncontrollable by the user, defined by secure configurations, and never chained to external inputs. Handshakes must be authenticated and verified through mechanisms such as signed tokens or challenge-response, with the server checking the session state before accepting the connection. Server-side origin verification, by checking the “Origin” header against a whitelist, is an additional requirement.
All messages should be treated as untrusted, with rigorous validation through patterns, size limits, and constant sanitization, applying the “deny-by-default” principle. It’s best practice to limit privileges and capabilities, exposing only what’s strictly necessary and separating channels and permissions to reduce the impact of a compromise. Rate limiting mechanisms, message size limits, and inactivity timeouts are also required, as well as requiring periodic reconnections to renew credentials. Finally, logging and active monitoring allow for recording events such as rejected handshakes, expired tokens, or traffic anomalies, with alerts for suspicious patterns such as spikes in connections from the same IP.
The recommended defense patterns are based on conceptual examples. Authentication in the handshake requires a signed token to be verified server-side before establishing the channel. Origin whitelisting allows requests not originating from authorized domains to be rejected. Payload validation with formal patterns allows non-compliant messages to be rejected. Escaping content to be displayed in the UI is essential to prevent XSS. Channel segmentation ensures the separation of sensitive and non-sensitive traffic, reducing the impact of a compromise.
Indicators of a possible compromise include an abnormal increase in connections from unexpected sources, the presence of messages with external URLs or unusual payloads, repeated and rapid connections to different endpoints from the same client, and logs that highlight the leakage of sensitive data outside of normal application flows.
In conclusion, WebSockets enable powerful real-time experiences, but require clear rules to remain secure. With practices such as encryption, channel authentication, message validation, origin verification, and active monitoring, high performance can be maintained while drastically reducing the risk of abuse and data loss. The systematic application of these principles transforms a potentially dangerous channel into a more reliable and secure tool.
Diego Bentivoglio