Quark messed with the security system in the airlock.
Is that stated in the script?
You tell me if this is sufficiently explicit for you.
In the teaser, Odo hears something coming from one of the airlocks. When he opens the airlock to check it out, he finds Quark sitting there quietly. When they leave the airlock, the camera ominously zooms in on a device that appears not to belong there and happens to be in a spot to which Odo and O’Brien had their backs turned. (Why didn’t Odo search the airlock, when he clearly didn’t for a second believe Quark’s explanation for what he was doing there? I have no answer for that, and neither does the script.)
Later O’Brien tells Sisko that the intruders had somehow defeated the security system in the airlock. Why Sisko asks how, O’Brien says he and Odo found Quark in that airlock without a believable explanation.
In another scene, we see Quark trying to sell liquidated chains to Yeto, who Quark thought had “come all this way, braved the storm” just to make that purchase, so Quark was obviously expecting him.
When Kira confronts him and accuses him outright of helping the intruders defeat the security system and bring weapons onto the station, he just looks guilty, and makes no serious effort to plead his innocence. (When she first makes the accusation, he perfunctorily says he doesn’t know what she’s talking about; when she makes it clear she doesn’t believe his denial, he stops trying.)
I think that’s enough to condemn him beyond a reasonable doubt.