The [documentation](https://code.google.com/p/crypto-js/#The_Cipher_Input) says it's `OpenSSL`, not PBKDF2:

>When you use a CipherParams object in a string context, it's automatically converted to a string according to a format strategy. The default is an OpenSSL-compatible format

This can be verified in the [source code](https://code.google.com/p/crypto-js/source/browse/tags/3.1.2/src/cipher-core.js#781). The implementation of the OpenSSL KDF is in [evpkdf.js](https://code.google.com/p/crypto-js/source/browse/tags/3.1.2/src/evpkdf.js).

The key derivation function is usually referenced as `EVP_BytesToKey` which is the higher level function in the API that implements the KDF.