GD::SecurityImage versions through 1.75 for Perl use rand to generate secrets.
The random method creates the challenge text used for the CAPTCHA by sampling characters from an array using Perls built-in rand function, and generates a (by default) six-character string.
The built-in rand function is unsuitable for security applications because it is predictable and reversible.
Weakness
The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG’s algorithm is not cryptographically strong.
Extended Description
When a non-cryptographic PRNG is used in a cryptographic context, it can expose the cryptography to certain types of attacks. Often a pseudo-random number generator (PRNG) is not designed for cryptography. Sometimes a mediocre source of randomness is sufficient or preferable for algorithms that use random numbers. Weak generators generally take less processing power and/or do not use the precious, finite, entropy sources on a system. While such PRNGs might have very useful features, these same features could be used to break the cryptography.