By default Discourse adds rel="nofollow" to links posted by users.
There’s no rel="nofollow" on the user’s profile in the Top Links section.
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And, this is related, it’s also missing in the “Popular Links” section of a topic:
It’s now fixed 
https://github.com/discourse/discourse/commit/c117a48e93768a51b485264b6579380b90b2b3c6
The “Popular Links” section of a topic is harder since we need to test the trust level of the user that submitted each links… Not sure it’s worth the work.
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The easy way out would be to add it to every popular link. 
The link already exists somewhere within the topic and if the user has the right trust level, then it doesn’t have a nofollow attribute.
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