All there is is one private security company claiming that the source may have been within the Russian intelligence agency. Maybe. Why not.
but: a) whoever the source is, if the documents are authentified, unaltered and newsworthy, and if their revelation is of general interest (and the DNCLeaks def. were!) then anyone who gets them should publish them
b) Assange said repeatedly he had no way of knowing who a source was. WikiLeaks was actually built that way from day0.