Multiple cross-site scripting (XSS) vulnerabilities in captcha\captcha.php in the Captcha! 2.5d and earlier plugin for WordPress allow remote attackers to inject arbitrary web script or HTML via the (1) captcha_ttffolder, (2) captcha_numchars, (3) captcha_ttfrange, or (4) captcha_secret parameter.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2008-0218
http://www.securityfocus.com/archive/1/485786/100/0/threaded
http://websecurity.com.ua/1588/
http://securityreason.com/securityalert/3539
http://lists.grok.org.uk/pipermail/full-disclosure/2008-January/059439.html