Description

An unauthenticated remote peer can crash any NIOWebSocket-based server (including Vapor and Hummingbird) with a single 11-byte frame sent after a completed WebSocket handshake, dropping all active connections until the process restarts. This vulnerability is addressed in swift-nio version 2.101.0.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-08-20T19:16:52.353Z",
  "pubdate": "2026-08-20T19:16:52.353Z",
  "executiveSummary": "An unauthenticated remote denial of service vulnerability exists in NIOWebSocket-based servers, including those implemented using Vapor and Hummingbird frameworks. The flaw allows an unauthenticated remote attacker to cause a complete server crash by transmitting a specifically crafted 11-byte frame over an established WebSocket connection. Successful exploitation results in the immediate termination of the host process, thereby dropping all active client connections and causing a complete outage of services reliant on the affected component until the application is manually or automatically restarted. The attack vector requires network exposure to the WebSocket endpoint and a completed WebSocket handshake, but demands no prior authentication or elevated privileges. Given the low complexity of the payload and the catastrophic impact on service availability, the risk to unpatched deployments is high, necessitating immediate remediation to maintain system resilience.",
  "technicalDetails": "The vulnerability resides within the NIOWebSocket component of the Swift NIO ecosystem, specifically affecting swift-nio versions prior to 2.101.0. The root cause stems from improper handling or parsing of incoming WebSocket frames received after the completion of the standard WebSocket handshake protocol. Specifically, when the server processes a malformed or maliciously constructed 11-byte frame, it triggers an unhandled exception, fatal assertion failure, or memory corruption condition within the underlying Swift NIO event loop or WebSocket frame decoder.\nThe attack flow begins when an unauthenticated remote peer initiates and successfully completes a standard WebSocket handshake with the target server, such as a Vapor or Hummingbird application. Once the handshake is finalized and the connection transitions to the active framing state, the attacker transmits the malicious 11-byte frame payload. Upon receipt, the vulnerable NIOWebSocket parsing logic fails to safely validate or handle the frame structure. This triggers a runtime panic or fatal error within the Swift process execution context.\nBecause Swift runtime panics terminate the entire process to prevent undefined behavior, the crash immediately takes down the server instance. This results in a denial of service affecting all concurrent client connections, not merely the malicious session. Network exposure is strictly remote over standard TCP/IP networks via the exposed WebSocket port. The attacker requires no authentication credentials, session tokens, or specialized privileges, as the trigger occurs during standard post-handshake message processing. Remediation requires upgrading the dependent Swift NIO libraries to version 2.101.0 or later, where the frame parsing logic has been hardened to gracefully handle malformed inputs without causing fatal process termination."
}