# CVE-2026-121763: WPMU DEV Hummingbird stored cross-site scripting

Source: https://junglewise.ai/threats/cve-2026-121763-improper-neutralization-of-input-during-web-page-generation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-09. Updated: 2026-11-09  
License: CC BY 4.0 with a link back

CVE-2026-121763 · Severity: medium · CVSS 6.5 · Published 2026-11-09

WPMU DEV Hummingbird is a WordPress plugin that optimizes site performance. A stored cross-site scripting vulnerability in versions up to 3.21.2 allows a subscriber-level attacker to inject malicious scripts that execute in visitors' browsers, potentially stealing data or hijacking accounts. Exploitation requires user interaction such as clicking a malicious link or visiting a crafted page.

Vendors: [WPMU DEV](https://junglewise.ai/threats/vendors/wpmu-dev).

## Quick answers

**Is CVE-2026-121763 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-121763 being exploited in the wild.

**How severe is CVE-2026-121763?** Medium severity, with a CVSS score of 6.5, published 9 November 2026.

**Which products and versions are affected?** WPMU DEV Hummingbird through 3.21.2.

**Is there a fix for CVE-2026-121763?** Yes: fixed in 4.0.0. Until then: Patchstack has issued a mitigation rule to block attack attempts until the plugin is updated. Sites unable to update immediately should apply the Patchstack protection or restrict subscriber-level access. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-121763 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-121763?** Check whether you run WPMU DEV Hummingbird at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 4.0.0

Patchstack has issued a mitigation rule to block attack attempts until the plugin is updated. Sites unable to update immediately should apply the Patchstack protection or restrict subscriber-level access.

## Technical details

A stored XSS vulnerability in Hummingbird up to version 3.21.2 allows subscribers to inject malicious JavaScript that persists and executes when other users view the affected content. The vulnerability requires a privileged attacker (subscriber role or higher) to craft and submit malicious input, with successful exploitation dependent on user interaction from the victim. The vulnerability was patched in version 4.0.0.

## Affected products

- WPMU DEV Hummingbird through 3.21.2

## Timeline

- 2026-10-26: disclosed: Reported by mroccon
- 2026-11-07: advisory
- 2026-11-09: other: Published on NVD

## References

- [https://wordpress.org/plugins](https://wordpress.org/plugins)
- [https://patchstack.com/database/wordpress/plugin/hummingbird-performance/vulnerability/wordpress-hummingbird-plugin-3-21-2-cross-site-scripting-xss-vulnerability?_s_id=cve](https://patchstack.com/database/wordpress/plugin/hummingbird-performance/vulnerability/wordpress-hummingbird-plugin-3-21-2-cross-site-scripting-xss-vulnerability?_s_id=cve)