⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 8.8

How to Fix CVE-2026-33943: Code Injection RCE in happy-dom

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.8 - High
Actively exploited?Not currently listed in CISA KEV
Affected>= 15.10.0, < 20.8.8
Fixed inSee vendor advisory
Type (CWE)CWE-94: Improper Control of Generation of Code ('Code Injection')

What is CVE-2026-33943?

CVE-2026-33943 is a code injection flaw in happy-dom. Attacker-controlled input is evaluated as code by the application runtime, giving the attacker arbitrary execution inside the process. Vendor description: Happy DOM is a JavaScript implementation of a web browser without its graphical user interface. In versions 15.10.0 through 20.8.7, a code injection vulnerability in ECMAScriptModuleCompiler allows an attacker to achieve Remote Code Execution (RCE) by injecting arbitrary JavaScript expressions inside export { } declarations in ES module scripts processed by happy-dom.

Why this CVE matters

Code injection against an application server is a direct path to remote code execution. The attacker executes inside the application runtime, which means database credentials, integration keys, and any secrets the process has loaded in memory are all exposed.

For deployments of happy-dom that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open happy-dom's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-33943

  1. Read the vendor advisory in full: https://github.com/capricorn86/happy-dom/security/advisories/GHSA-6q6h-j7hj-3r64
  2. Upgrade happy-dom to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Rotate any credentials, API keys, or session tokens that the vulnerable service touched. An unauthenticated RCE-class flaw means anything the process could see should be treated as exposed.
  5. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  6. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

<!-- enrich-agent-8 -->

Update the npm package happy-dom to 20.8.8

Vendor advisory: https://github.com/capricorn86/happy-dom/security/advisories/GHSA-6q6h-j7hj-3r64


# Patch in-place inside an existing project.
npm install happy-dom@20.8.8
npm audit fix

# Confirm the patched version landed.
npm list happy-dom

# Lock-file enforcement (CI / production).
npm ci


# Same workflow from a Windows admin workstation.
npm install happy-dom@20.8.8
npm audit fix
npm list happy-dom

Verify the fix landed


# Vendor advisory: https://github.com/capricorn86/happy-dom/security/advisories/GHSA-6q6h-j7hj-3r64
# 1. Confirm the running version matches the fixed-in version listed above.

# 2. Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
#    The scanner should no longer flag this CVE on the patched target.

# 3. Inspect recent service / kernel logs for crash-loops or rollback events.
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

<!-- enrich-agent-8 -->

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for unexpected administrator accounts in happy-dom, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-33943 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-33943?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

Do I need to assume compromise if my happy-dom was internet-facing and unpatched?

For an unauthenticated RCE-class flaw exposed to the public internet during the known exploitation window, yes. Review logs, rotate credentials the process could access, and look for unexpected accounts, scheduled tasks, or outbound connections.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*