Learn Spanish (Latin America): Se sabe que un colega lo filtró.

In impersonal structures like se sabe que (it is known that), the verb after que is in the indicative when declaring a fact or certainty. Since we are stating what is known as a reality, we use the indicative filtró. If the phrase expressed doubt, like no se sabe que, it would trigger the subjunctive.

I thought filtrar meant to filter. Does it also mean to leak?

Yes! Just like in English, where liquid passes through a filter or a crack, filtrar can mean to filter physically, but it is also the standard verb for leaking information, secrets, or documents to the press.

Why is lo placed before filtró rather than attached to it?

In Spanish, object pronouns like lo must go immediately before a conjugated verb. You can only attach them to the end of infinitives, gerunds (the -ndo form), or affirmative commands. Since filtró is conjugated in the past tense, lo has to go in front.

Is se sabe an impersonal se or a passive se here?

It functions as an impersonal se, meaning one knows or people know. When se plus a verb is followed by a que clause rather than a specific noun subject, it is considered impersonal. It translates naturally to the English phrase it is known, but grammatically in Spanish, it is generalizing the subject to an unspecified group of people.