The CIA does not vouch for the accusation that the Russian government was responsible for the disclosures of email messages among high-level Democrats, which are thought to have eroded some of Hillary Clinton's support. It is merely a so-called leak from some anonymous person.
Even when the CIA vouches for information it often turns out not to be true. There were the altered aerial photographs of Auschwitz published in Life magazine in April 1979, and the altered satellite photographs of the Kuwaiti desert used to convince the Saudis that they were threatened by the Iraqi army in 1990.
In this particular matter there are also contradictions. The FBI found no evidence of Russian hacking. Furthermore Craig Murray, a close associate of Julian Assange, states:
"Neither of [the leaks] came from the Russians. The source had legal access to the information. The documents came from inside leaks, not hacks." (Daily Mail, 14 December 2016)
Not hacks by the Russians, but leaks from Bernie supporters, who disliked the way the Democratic nomination was handed to Hillary without a fair contest. This makes the Democrats look even worse.