Crypt::CBC versions between 1.21 and 3.04 for Perl may use the rand() function as the default source of entropy, which is not cryptographically secure, for cryptographic functions.
This issue affects operating systems where /dev/urandom is unavailable. In that case, Crypt::CBC will fallback to use the insecure rand() function.
The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG’s algorithm is not cryptographically strong.
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.