"May have existed"?
Molotov himself inadvertently implied their existence numerous times in his conversations with Chuev (Molotov Remembers; Molotov-Chuev interviews between 1970-1986, published 1993).
Khrushchev acknowledged their existence in his memoirs, which nobody disputes the provenance of, and which were published in the West in the 1970s.
The CPSU itself acknowledged their existence at the end of 1989, and published their copy of the text in their official journal.
The literal Soviet documents, held by the General Department of the CPSU Central Committee, were located after the archives were declassified in 1992, and have since been published widely.
No party seems to dispute the existence of the secret protocols. If you have a single credible source that does, feel free to cite them.