Not even venial.
From the Catechism of the Catholic Church (1997), 2477-2478:
"Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury. He becomes guilty:
- of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
- of detraction who, without objectively valid reason, discloses another's faults and failings to persons who did not know them;
- of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.
To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor's thoughts, words, and deeds in a favorable way:
Every good Christian ought to be more ready to give a favorable interpretation to another's statement than to condemn it. But if he cannot do so, let him ask how the other understands it. And if the latter understands it badly, let the former correct him with love. If that does not suffice, let the Christian try all suitable ways to bring the other to a correct interpretation so that he may be saved."