It is true that today the Ad Block Pro extension is safe. We know that it will not steal your data even though—as the other answers point out—it has the technical ability to do so. However Chrome extensions are silently and automatically updated

1) blocking a web page's requests to download ads from the servers that host them and
2) hiding any ads that can't be blocked. This also means that Ad Block Pro can see any forms you submit and your browsing history on that tab

Ad block Pro
Ads block Pro
Ads blocker
Ad Block
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Certainly this doesn't mean that blocking ads is illegal. But it does strongly suggest that the additional layer of technology employed by current-generation ad blockers to circumvent the technological defenses of ad block-detection scripts is illegal. Anti-circumvention law was enacted for a purpose