In Zend Framework, Zend_Captcha_Word (v1) and Zend\Captcha\Word (v2) generate a "word" for a CAPTCHA challenge by selecting a sequence of random letters from a character set. Prior to this advisory, the selection was performed using PHP's internal array_rand() function. This function does not generate sufficient entropy due to its usage of rand() instead of more cryptographically secure methods such as openssl_pseudo_random_bytes(). This could potentially lead to information disclosure should an attacker be able to brute force the random number generation.

References

• zendframework/zend-captcha@43c276d
• zendframework/zend-captcha@5561ef8
• https://framework.zend.com/security/advisory/ZF2015-09
• https://github.com/FriendsOfPHP/security-advisories/blob/master/zendframework/zend-captcha/ZF2015-09.yaml