CVE-2026-42437: OpenClaw WebSocket DoS Vulnerability

CVE-2026-42437 Overview

CVE-2026-42437 is a denial of service vulnerability in OpenClaw versions 2026.4.9 before 2026.4.10. The flaw exists in the voice-call realtime WebSocket path, which accepts oversized frames without proper validation. Remote attackers can transmit oversized WebSocket frames to exhaust server resources and render the service unavailable. The vulnerability is classified under CWE-770: Allocation of Resources Without Limits or Throttling. Any deployment that exposes the webhook path to untrusted networks is at risk. No authentication or user interaction is required to trigger the condition.

Critical Impact

Unauthenticated remote attackers can disrupt OpenClaw voice-call services by sending oversized WebSocket frames, causing service-wide unavailability.

Affected Products

• OpenClaw 2026.4.9
• OpenClaw versions prior to 2026.4.10 exposing the voice-call realtime webhook path
• Deployments using extensions/voice-call/src/webhook/realtime-handler.ts

Discovery Timeline

• 2026-05-05 - CVE-2026-42437 published to NVD
• 2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-42437

Vulnerability Analysis

The vulnerability resides in the voice-call extension's realtime WebSocket handler. The handler reads incoming frames from connected clients without enforcing a maximum message size. An attacker can open a WebSocket connection to the webhook path and send arbitrarily large frames. The server attempts to buffer the entire frame, consuming memory and CPU resources. Sustained or sufficiently large frames cause resource exhaustion, terminating availability for legitimate users.

The vulnerability falls under the Denial of Service category, specifically Resource Exhaustion. The attack requires only network reachability to the WebSocket endpoint. Because the realtime path is part of the voice-call extension, deployments that expose this functionality to the internet are directly exploitable.

Root Cause

The root cause is missing input validation on WebSocket frame size in extensions/voice-call/src/webhook/realtime-handler.ts. Prior to the patch, no upper bound existed on inbound realtime messages. The fix introduces a MAX_REALTIME_MESSAGE_BYTES constant set to 256 KiB and rejects frames exceeding this limit.

Attack Vector

An unauthenticated remote attacker connects to the voice-call realtime WebSocket endpoint and transmits frames larger than the server can safely buffer. Repeated oversized frames degrade or terminate service. The attack is network-based and does not require credentials or user interaction.

// Source: https://github.com/openclaw/openclaw/commit/afadb7dae6738819ad9c7d2597ace0516957d20e
// Patch in extensions/voice-call/src/webhook/realtime-handler.ts

 const STREAM_TOKEN_TTL_MS = 30_000;
 const DEFAULT_HOST = "localhost:8443";
+const MAX_REALTIME_MESSAGE_BYTES = 256 * 1024;
 
 function normalizePath(pathname: string): string {
   const trimmed = pathname.trim();

The patch defines a 256 KiB ceiling for inbound realtime WebSocket messages. Frames exceeding this limit are rejected before they consume server resources.

Detection Methods for CVE-2026-42437

Indicators of Compromise

• WebSocket connections to the voice-call realtime path transmitting frames larger than 256 KiB
• Spikes in memory or CPU consumption on OpenClaw nodes correlating with inbound WebSocket traffic
• Repeated WebSocket connection resets or service crashes on the voice-call extension process
• Unusual sustained inbound traffic volume to the webhook path from a single source IP

Detection Strategies

• Inspect WebSocket frames at reverse proxies or load balancers and log frames exceeding 256 KiB to the realtime path
• Monitor application logs from realtime-handler.ts for oversized message errors after upgrading
• Correlate process restarts of the voice-call extension with inbound network telemetry

Monitoring Recommendations

• Enable rate limiting and per-connection byte counters on the WebSocket gateway fronting OpenClaw
• Alert on memory pressure events on hosts running the voice-call extension
• Track outlier client IPs by total bytes sent to the realtime WebSocket endpoint

How to Mitigate CVE-2026-42437

Immediate Actions Required

• Upgrade OpenClaw to version 2026.4.10 or later, which enforces the MAX_REALTIME_MESSAGE_BYTES limit
• Restrict network exposure of the voice-call realtime WebSocket path to trusted networks where feasible
• Place a reverse proxy in front of OpenClaw and configure a WebSocket frame size limit of 256 KiB

Patch Information

The fix is committed in openclaw/openclaw commit afadb7d and described in the GitHub Security Advisory GHSA-vw3h-q6xq-jjm5. Additional details are available in the VulnCheck Denial of Service Advisory. Upgrading is the recommended remediation path.

Workarounds

• Enforce a 256 KiB maximum frame size on upstream proxies such as NGINX or HAProxy fronting the WebSocket endpoint
• Apply network-level rate limiting on connections to the voice-call realtime path
• Disable the voice-call extension if the realtime WebSocket functionality is not required

# NGINX configuration example to limit WebSocket frame size upstream of OpenClaw
http {
    client_max_body_size 256k;

    server {
        listen 8443 ssl;

        location /voice-call/realtime {
            proxy_pass http://openclaw_backend;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            proxy_read_timeout 60s;
            proxy_buffer_size 256k;
            proxy_buffers 4 256k;
        }
    }
}