CVE-2026-81624: Undertow WebSocket DOS Vulnerability

CVE-2026-81624 Overview

Undertow is a flexible, high-performance web server used in Red Hat JBoss Enterprise Application Platform (EAP) and WildFly. A flaw in how Undertow handles WebSocket connections allows remote attackers to exhaust server resources. Certain configuration limits, including message buffer sizes and session timeouts, cannot be adjusted and default to unlimited values. An unauthenticated attacker can send large volumes of data or hold WebSocket sessions open indefinitely. The result is memory or resource exhaustion, culminating in a denial-of-service (DoS) condition on the affected server. The issue is tracked as [CWE-770: Allocation of Resources Without Limits or Throttling].

Critical Impact
Remote, unauthenticated attackers can crash Undertow-based servers by exhausting memory and connection resources through unbounded WebSocket sessions.

Affected Products

Red Hat JBoss Enterprise Application Platform (EAP)
Red Hat WildFly application server
Applications embedding the Undertow web server for WebSocket functionality

Discovery Timeline

2026-08-31 - CVE-2026-81624 published to the National Vulnerability Database (NVD)
2026-09-01 - Last updated in NVD database

Technical Details for CVE-2026-81624

Vulnerability Analysis

The vulnerability resides in Undertow's WebSocket subsystem. WebSockets maintain long-lived, bidirectional TCP connections between clients and servers. Undertow does not expose configurable ceilings for critical WebSocket parameters, including incoming message buffer size and idle session timeout. Because these limits default to unlimited, the server accepts arbitrarily large frames and preserves idle connections without eviction. Attackers can weaponize this behavior to consume heap memory, file descriptors, and worker threads until the Java Virtual Machine (JVM) fails or the host becomes unresponsive.

Root Cause

The root cause is missing throttling controls on WebSocket resource consumption, classified under [CWE-770]. Administrators cannot cap message buffer sizes or force session timeouts through standard Undertow configuration. Applications relying on Undertow inherit these unbounded defaults, leaving no supported knob to enforce backpressure or reclaim idle connections.

Attack Vector

The attack requires network access to an exposed WebSocket endpoint. No authentication, user interaction, or elevated privileges are needed. An attacker completes a standard WebSocket handshake against a vulnerable endpoint, then either streams oversized frames to force continuous buffer allocation or opens many idle sessions and never disconnects. Both patterns drive the JVM toward OutOfMemoryError or thread starvation, producing a service outage. See the Red Hat CVE-2026-81624 Advisory and Red Hat Bug Report #2524868 for vendor-authored technical detail.

Detection Methods for CVE-2026-81624

Indicators of Compromise

Sustained growth in JVM heap usage on JBoss EAP or WildFly hosts without corresponding legitimate traffic increases.
Elevated counts of concurrent WebSocket sessions from a small number of source IP addresses.
Repeated OutOfMemoryError or GC overhead limit exceeded entries in server.log.
Sockets in ESTABLISHED state persisting for abnormally long durations against WebSocket endpoints.

Detection Strategies

Instrument application servers with metrics for active WebSocket session count, per-session lifetime, and inbound frame size distribution.
Alert on WebSocket sessions exceeding a defined duration or byte volume threshold appropriate for the application.
Correlate reverse-proxy access logs with backend session counts to identify clients opening many long-lived upgrades.

Monitoring Recommendations

Track JVM memory pressure, thread pool saturation, and garbage collection frequency on Undertow hosts.
Monitor network flow data for asymmetric WebSocket traffic patterns, such as high client-to-server byte counts with minimal replies.
Aggregate application server logs into a centralized analytics platform to baseline normal WebSocket behavior.

How to Mitigate CVE-2026-81624

Immediate Actions Required

Apply vendor updates from Red Hat for JBoss EAP and WildFly once the patched Undertow release is available. Consult the Red Hat CVE-2026-81624 Advisory for fixed versions.
Inventory all applications embedding Undertow and identify any endpoint exposing WebSocket handlers.
Restrict network exposure of WebSocket endpoints to trusted clients where feasible.

Patch Information

Red Hat has assigned CVE-2026-81624 and is tracking remediation through Red Hat Bug Report #2524868. Administrators should subscribe to Red Hat security notifications and deploy updated JBoss EAP, WildFly, and standalone Undertow packages as soon as fixes are released.

Workarounds

Place a reverse proxy such as HAProxy or NGINX in front of Undertow and enforce per-connection idle timeouts and maximum frame sizes at the proxy layer.
Apply per-source-IP rate limits and concurrent connection caps on WebSocket upgrade requests at the edge.
Reduce operating-system limits, such as ulimit -n and connection tracking thresholds, so that runaway session growth fails fast rather than exhausting the JVM.
Disable WebSocket support in deployed applications that do not require it until a vendor patch is applied.

bash

# NGINX example: enforce WebSocket timeouts and size limits in front of Undertow
location /ws/ {
    proxy_pass http://undertow_backend;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";

    # Cap idle time to reclaim stalled WebSocket sessions
    proxy_read_timeout 60s;
    proxy_send_timeout 60s;

    # Cap inbound frame/body size
    client_max_body_size 1m;

    # Limit concurrent connections per client IP
    limit_conn ws_per_ip 10;
}