CVE-2025-53620: Qwik City DoS Vulnerability

CVE-2025-53620 Overview

CVE-2025-53620 is a critical denial of service vulnerability affecting @builder.io/qwik-city, the meta-framework for Qwik. The vulnerability exists in how the server handles Qwik Server Action QRLs (Qwik Resource Locators). When a Server Action QRL is executed, the server dynamically loads the file containing the symbol. If an invalid qfunc is sent to the server, the resulting error is not properly handled, causing the Node.js process to exit unexpectedly.

Critical Impact

An unauthenticated remote attacker can crash the entire Node.js server process by sending a malformed Server Action request, resulting in complete service disruption.

Affected Products

• @builder.io/qwik-city versions prior to 1.13.0
• Applications built using the Qwik meta-framework with Server Actions enabled
• Node.js deployments running vulnerable Qwik City applications

Discovery Timeline

• 2025-07-09 - CVE-2025-53620 published to NVD
• 2025-07-10 - Last updated in NVD database

Technical Details for CVE-2025-53620

Vulnerability Analysis

This vulnerability is classified under CWE-248 (Uncaught Exception), which occurs when an exception is thrown but not caught, leading to abnormal program termination. In the context of Qwik City, the server-side request handler for Server Actions fails to implement proper error handling when processing dynamically loaded QRL symbols.

When Qwik Server Actions are invoked, the framework dynamically resolves and loads the corresponding symbol from the specified file path. The vulnerability manifests when a malformed or invalid qfunc parameter is submitted to the server. Instead of gracefully handling the error and returning an appropriate HTTP error response, the unhandled exception propagates up the call stack and terminates the Node.js process.

This architectural flaw allows any unauthenticated network attacker to repeatedly crash the server with minimal effort, as no authentication or special privileges are required to invoke the vulnerable code path.

Root Cause

The root cause is the absence of try-catch error handling or equivalent exception management around the dynamic module loading and symbol resolution logic in the Server Action request handler. When the framework attempts to load a non-existent or malformed symbol, the resulting error is not caught, causing Node.js to terminate with an uncaught exception.

Attack Vector

The attack can be executed remotely over the network without any authentication requirements. An attacker simply needs to send a crafted HTTP request to a Qwik City application endpoint that processes Server Actions, including an invalid or malformed qfunc parameter. The server will attempt to resolve the invalid QRL, fail to handle the resulting error, and crash.

The attack is trivial to execute and can be automated to repeatedly crash servers as they restart, creating an effective denial of service condition. See the GitHub Security Advisory for additional technical details on the vulnerability mechanism.

Detection Methods for CVE-2025-53620

Indicators of Compromise

• Repeated unexpected Node.js process crashes or restarts in production environments
• Application logs showing uncaught exception errors related to QRL resolution or dynamic module loading
• Unusual HTTP requests to Server Action endpoints containing malformed or invalid qfunc parameters
• Process monitoring alerts indicating rapid service restarts

Detection Strategies

• Monitor Node.js process stability and implement alerting for unexpected process terminations
• Implement web application firewall (WAF) rules to detect and block malformed Server Action requests
• Review HTTP access logs for suspicious patterns targeting Server Action endpoints
• Deploy application performance monitoring (APM) to track unhandled exceptions in the Qwik City request handler

Monitoring Recommendations

• Configure process managers (PM2, systemd, Docker) to alert on repeated crash-restart cycles
• Set up log aggregation to correlate uncaught exception events across application instances
• Implement rate limiting on Server Action endpoints to reduce the impact of automated attacks
• Monitor for anomalous traffic patterns targeting Qwik application endpoints

How to Mitigate CVE-2025-53620

Immediate Actions Required

• Upgrade @builder.io/qwik-city to version 1.13.0 or later immediately
• Review production deployment logs for signs of exploitation attempts
• Implement rate limiting on Server Action endpoints as a temporary protective measure
• Ensure process managers are configured to restart crashed services while monitoring for attack patterns

Patch Information

The vulnerability has been fixed in @builder.io/qwik-city version 1.13.0. The patch implements proper error handling around the dynamic QRL loading mechanism, ensuring that invalid qfunc parameters result in graceful error responses rather than process termination. Refer to the GitHub Security Advisory for complete patch details.

Workarounds

• Deploy a reverse proxy or WAF to filter and validate incoming Server Action requests before they reach the application
• Implement input validation at the network edge to reject requests with malformed QRL parameters
• Use container orchestration with automatic restart policies to minimize downtime during attacks while patching is in progress
• Consider temporarily disabling Server Actions if they are not critical to application functionality

# Upgrade to patched version
npm update @builder.io/qwik-city@1.13.0

# Or install the specific patched version
npm install @builder.io/qwik-city@1.13.0

# Verify installed version
npm list @builder.io/qwik-city