Chhāndogya Upanishad verses 7.1.2 and 7.1.4 mention the Itihasa and Puranas as the 5th Veda:
- Nârada said: 'I know the Rig-veda, Sir, the Yagur-veda, the Sâma-veda, as the fourth the Âtharvana, as the fifth the
But the words itihasa and purana just mean "history", and so whenever this Upanishad was spoken, it was just referring to the history before that time.
However, certain characters appear in both the Vedas and Smriti, such as Krishna, son of Devaki mentioned in the Chhandogya Upanishad verse 3.17.6 and the Mahabharata.
The reason certain characters appear in both the Smriti and Shruti is because certain major events repeat in each cycle of creation. This is what the Brahma Sutras say in verses 1.3.27, 28, and 29.
Here is the commentary for 1.3.27:
"Indra and so on, again and again originate from the Vedic words. To
explain. Vedic words, such as Indra and so on, do not, like the word
Devadatta and the like, denote, on the basis of convention, one
particular individual only: they rather denote by their own power
particular species of beings, just as the word 'cow' denotes a
particular species of animals. When therefore a special individual of
the class called Indra has perished, the creator, apprehending from
the Vedic word 'Indra' which is present to his mind the class
characteristics of the beings denoted by that word, creates another
Indra possessing those very same characteristics; just as the potter
fashions a new jar, on the basis of the word 'jar' which is stirring
in his mind."
Here is Ramanujacharya's commentary for verse 1.3.28:
"As words such as Indra and Vasishtha, which denote gods and Rishis,
denote (not individuals only, but) classes, and as the creation of
those beings is preceded by their being suggested to the creative mind
through those words; for this reason the eternity of the Veda admits
of being reconciled with what scripture says about the mantras and
kândas (sections) of the sacred text having 'makers' and about Rishis
seeing the hymns. Such passages as 'He chooses the makers of
mantras'; 'Reverence to the Rishis who are the makers of mantras';
'That is Agni; this is a hymn of Visvâmitra.' For by means of these
very texts Pragâpati presents to his own mind the characteristics and
powers of the different Rishis who make the different sections, hymns,
and mantras, thereupon creates them endowed with those characteristics
and powers, and appoints them to remember the very same sections,
hymns, &c. The Rishis being thus gifted by Pragâpati with the
requisite powers, undergo suitable preparatory austerities and finally
see the mantras, and so on, proclaimed by the Vasishthas and other
Rishis of former ages of the world, perfect in all their sounds and
accents, without having learned them from the recitation of a teacher.
There is thus no conflict between the eternity of the Veda and the
fact that the Rishis are the makers of its sections, hymns, and so
And the commentary for verse 1.3.29:
"Having thereupon manifested the Vedas in exactly the same order and
arrangement they had had before, and having taught them to
Hiranyagarbha, he entrusts to him the new creation of the different
classes of beings, gods, and so on, just as it was before; and at the
same time abides himself within the world so created as its inner Self
What this means is that the creation of the universe is based on the words found inside the Veda, that Brahman recollects the words of the Veda and creates classes of beings based on those words, and that events found in the Vedas repeat every cycle.