Nimbus JOSE+JWT before 4.39 proceeds improperly after detection of an invalid HMAC in authenticated AES-CBC decryption, which allows attackers to conduct a padding oracle attack.

In Nimbus JOSE+JWT before 4.39, there is no integer-overflow check when converting length values from bytes to bits, which allows attackers to conduct HMAC bypass attacks by shifting Additional Authenticated Data (AAD) and ciphertext so that different plaintext is obtained for the same HMAC.