The (1) Textimage 4.7.x before 4.7-1.2 and 5.x before 5.x-1.1 module for Drupal and the (2) Captcha 4.7.x before 4.7-1.2 and 5.x before 5.x-1.1 module for Drupal allow remote attackers to bypass the CAPTCHA test via an empty captcha element in $_SESSION.
https://exchange.xforce.ibmcloud.com/vulnerabilities/31994
https://exchange.xforce.ibmcloud.com/vulnerabilities/31984
http://www.vupen.com/english/advisories/2007/0431
http://www.securityfocus.com/bid/22329
http://secunia.com/advisories/23985
http://secunia.com/advisories/23983
http://cvs.drupal.org/viewcvs/drupal/contributions/modules/textimage/captcha.inc?r1=1.1&r2=1.1.2.1