Any fix has to happen on the outside. Look to see if the shingles beneath the ridge vent appear sound. If this is an old aluminium ridge vent look at the ends, if I recall correctly they use to come with a plastic end piece that fit into the end of the vent, see if it is still there; also they had aluminum strips that fit over the pieces to seal one piece to another. Also look to see if it appears to remain sealed to the roof shingles. With any type of ridge vent just look for the obvious, something that doesn't look right. A photo would be useful for others to make comment on. Also, does this leak with every rain or just with a strong wind from a particular direction? If there is a pattern to when it leaks it will likely assist you in finding the cause. good luck.
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