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The discovered flaw is a classic stored cross-site scripting (XSS) vulnerability residing within the `media` plugin of the TinyMCE rich text editor.
Prior to patched versions 5.11.1, 7.9.3, and 8.5.1, the `media` plugin fails to sufficiently neutralize certain user-controlled attributes. Specifically, attackers can inject arbitrary JavaScript into `data-mce-` attributes, a set of attributes used internally by TinyMCE to store metadata and configurations for media elements.
The attack workflow exploits the editor’s serialization and rendering process. When a user (often an attacker with low-privileged author access) inserts a media element, TinyMCE creates corresponding `data-mce-object` and other `data-mce-p-` attributes. Because these attributes are not properly sanitized before being stored, an attacker can craft a payload like `data-mce-anything=”javascript:alert(1)”` or `data-mce-src=”javascript:alert(‘XSS’)”` within the media element’s HTML.
This malicious input, alongside the benign content, is then stored persistently on the server. Any victim who subsequently views the page where this manipulated media element resides will have the attacker’s payload automatically executed by their web browser, as the content is rendered. The vulnerability is classified as CWE-79: Improper Neutralization of Input During Web Page Generation.
DailyCVE Form:
Platform: TinyMCE
Version: <5.11.1,<7.9.3,<8.5.1
Vulnerability: Stored XSS
Severity: HIGH (8.7 CVSS)
Date: 2026-05-28
Prediction: 2026-06-04
What Undercode Say:
This vulnerability targets the media plugin’s handling of data-mce- attributes. Here’s how to replicate the insecure configuration and verify if a system is vulnerable:
Clone a vulnerable version of TinyMCE
git clone --depth 1 --branch 5.11.0 https://github.com/tinymce/tinymce.git
cd tinymce
Run a simple HTTP server to test the vulnerable build
python3 -m http.server 8000
Example of vulnerable API call that might be present in a custom integration
curl -X POST https://target-site.com/api/content \
-H "Content-Type: application/json" \
-d '{"content": "<img data-mce-src=\"javascript:alert(\\\"XSS\\\")\" src=\"x\">"}'
Exploit:
A successful exploit leverages the unsanitized data-mce-src, data-mce-href, or `data-mce-style` attributes. An attacker can inject a payload like <img data-mce-src="javascript:alert('Stored XSS')" src="valid.jpg">. Once the content is rendered by a victim, the `alert()` function executes.
Protection:
Upgrade immediately to TinyMCE version 5.11.1, 7.9.3, or 8.5.1 or later. As a workaround, disable the `media` plugin via TinyMCE configuration or implement strict Content Security Policy (CSP) headers.
Impact:
Successful exploitation allows for session hijacking, credential theft, data exfiltration, and distribution of malware. The injected script has access to the same origin as the vulnerable site, enabling a wide range of malicious actions with the victim’s privileges.
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Sources:
Reported By: nvd.nist.gov
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