Trustwave SpiderLabs Security Advisory TWSL2021-004:
Stored Authenticated XSS in WordPress Plugin Virtual Robots.txt

Published: 03/29/2021
Version: 1.0

Vendor: https://infolific.com/technology/
Product: WordPress Plugin Virtual Robots.txt
Version affected:  <= 1.9

Product description:
The plugin provides a non-filesystem implementation of responses to requests for
robots.txt

Finding 1: WordPress Plugin Virtual Robots.txt <= 1.9 - Stored Authenticated XSS
*****Credit: Martin Vierula of Trustwave
CVE: CVE-2021-28121
CWE: CWE-79

The Settings page of Virtual Robots.txt is susceptible to Cross-Site
Scripting (XSS) attacks.

A user-input line like:

Disallow: /wp-register.php

Can be modified to:

Disallow: /wp-register.php</textarea></td></tr><script>alert(1);</script>

The vulnerability does require admin+ privileges to exploit.  More precisely,
the action requires the manage_options capability, which by default only admin+
users have.

Remediation Steps:
Upgrade Virtual Robots.txt plugin to version 1.10 or the latest stable version.

Revision History:
03/03/2021 - Vulnerability disclosed to vendor
03/03/2021 - Patch released by vendor
03/29/2021 - Advisory published

References
1. https://wordpress.org/plugins/pc-robotstxt/#developers

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About Trustwave SpiderLabs:
SpiderLabs(R) is the advanced security team at Trustwave focused on
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security and security research. The team has performed over a thousand
incident investigations, thousands of penetration tests and hundreds of
application security tests globally. In addition, the SpiderLabs Research
team provides intelligence through bleeding-edge research and proof of
concept tool development to enhance Trustwave's products and services.
https://www.trustwave.com/spiderlabs

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