Rationale (summary)
Questions that are migrated into this site with highly-upvoted, poor-quality answers reflect poorly on our community. When the voting on answers cannot reasonably reflect the Sec.SE community's opinion (as opposed to that of the originating site), I propose to reject the migration altogether. Point the asker to existing duplicates on Sec.SE, or suggest that they re-ask on Sec.SE.
Rationale (rant-sized)
Security is a topic where it is often difficult to verify an answer: you can't work out the math for yourself, or check that the program performs as advertised, or look up the citation in the primary source. I think that by and large voting works well on this site to pick good answers from bad. Not always, but as often as on other Stack Exchange sites with more directly verifiable answers.
There is however a case where this vetting by voting breaks up completely: questions that have attracted a lot of traffic from other Stack Exchange sites. Most of these are the result of incoming migrations. My informal impression is that almost every time a migration comes in with a large number of votes on answers, these votes favor poor answers.
Example: yesterday's specimen, To salt, or not to salt? This is a topic that's come up before: Why is using salt more secure? Why would salt not have prevented LinkedIn passwords from getting cracked? (Conflict of interest disclaimer: I answered one of these questions.) I don't think that any of the answers that came with the migration improves the site, compared with the answers on the previous questions.
If this question was asked on the site, it would be a fine, on-topic question. And I think it would attract good answers. But it came in with poor answers, and when the top answer has 30 upvotes coming in from the source sites, there's no way we, the Sec.SE community, can make any disagreement with this answer visible.
Even if the question is eventually closed as a duplicate, it will remain on the site, with high visibility due to its rather high score. To the casual observer, these highly-upvoted answers have the backing of the Sec.SE community.
I think the best solution would be to reset all votes on migrated threads. This simple feature request has been sitting there for over a year, so I won't hold my breath. I think we should take the matter into our own hands.
I don't think we can selectively delete incoming answers or edit them beyond recognition, which is all we can do since we don't have the downvoting power to refute answers with 30 upvotes: most of these answers are not actively harmful, they're just misguided or incomplete. But in a case like yesterday's, the thread as a whole does not improve the site. What would most benefit the asker is a pointer to the existing threads on the topic.
This leaves the alternative of deleting the question as a whole. Closure doesn't help, because it leaves the poor answers in place. Furthermore, there isn't always an applicable close reason — typically these questions are good, on-topic questions, and they are not necessarily duplicates. Deleting the question rejects the migration; the question becomes closed and unlocked on the originating site, with the answers undeleted. This had best happen quickly, before the Sec.SE community has contributed answers on its own. A meta thread would take too long for this, so I propose that moderators act directly.
This isn't a new concern — Downsides of migrating questions from high-volume sites Another debateable migration — so let's do something about it.
Proposed policy
When a question is migrated from another site with highly-upvoted existing answers, Sec.SE moderators review the answers, and decide whether to keep the migration. If, in their best judgement, the quality of the answers reflect poorly on the site, then at their own discretion, moderators may delete the question. Moderators' decisions are final. It is recommended that the deleting moderator leaves a comment on the original question on the source site, either citing existing Sec.SE questions on the question's topic or suggesting to repost the question on [sec.se].
