The encryption and decryption process were vulnerable against the Bleichenbacher's attack, which is a padding oracle vulnerability disclosed in the 98'.
The issue was about the wrong padding utilized, which allowed to retrieve the encrypted content.
The OPENSSL_PKCS1_PADDING version, aka PKCS v1.5 was vulnerable (is the one set by default when using openssl_* methods), while the PKCS v2.0 isn't anymore (it's also called OAEP).

A fix for this vulnerability was merged at Cosmicist/AsymmetriCrypt@a0318cf.

References

• Cosmicist/AsymmetriCrypt#4
• Cosmicist/AsymmetriCrypt#5
• https://github.com/FriendsOfPHP/security-advisories/blob/master/asymmetricrypt/asymmetricrypt/2017-11-20.yaml