Possible Duplicate:
How does stackoverflow handle spam?

I am on the site almost every day and have been since early on, yet I don't recall ever seeing any SPAM comments or questions, not even once.

I was just curious as to whether the reputation/spam filter is just really good at filtering them or are they just getting squashed almost instantaneously by the community?

I'm never gonna get that Citizen patrol badge!

  • Argh! Wasn't aware it was a dupe when I composed my answer... Aug 6, 2009 at 3:52
  • Doesn't make it any less valid as a duplicate, should really be closed. Aug 6, 2009 at 4:00
  • @Barack Obama - I don't think its actually a duplicate; related question, but not duplicate. One is asking what the SO engine does to prevent spam, this one is asking how and why the system seems so effective to the OP. Aug 6, 2009 at 4:03
  • @Mr. President: OK, I closed it. @JohnFx: I think that the answers here should be added to the parent thread, so I'll probably merge the two questions sometime in a few hours (just heads up) Aug 6, 2009 at 4:04
  • @yshuditelu: I think that the distinction between "the SO engine" and "the system" are pretty much interchangeable, at least when it comes to the answers to both questions. Though I could reopen this, I think that it's better to consolidate the answers to both questions into one thread. Aug 6, 2009 at 4:06
  • I'll concede to the will of the community on this one, but I don't really think it is a dupe. I was more interested in which particular SPAM fighting mechanism is most responsible for the great success at keeping it in check here, not so much what the actual mechanism is.
    – JohnFx
    Aug 6, 2009 at 14:20
  • @JohnFx: To answer your question, both as a moderator and a member of the community, I've personally eliminated very little spam, so I suspect that low-value target + custom software + post throttling does most of the work. Aug 6, 2009 at 16:08
  • With all the Google juice it has, I'd think it is a relatively high value target for comment spam.
    – JohnFx
    Aug 6, 2009 at 18:12

2 Answers 2


There are several mechanisms Stack Overflow uses to keep spam down:

First, most spam is designed for specific software - Wordpress, phpBB, etc. Since Stack Overflow has a custom codebase from the ground up, that would mean tailoring a spam engine to these specific sites, which I doubt is worth spammers time, since programmers and computer enthusiasts are able to instantly recognize it for what it is.

Second, the software itself has several anti-flood features (see the blog for details) that prevent a user from any one IP from posting too fast. There are also capchas on the sites that try to force the user to prove that they are human if they exhibit robot-like usage patterns.

Third, the software has tools for users of a certain rep to recognize and remove spam. If six users vote a post as spam and/or offensive, it's deleted from the system. Once it hits three votes its banished from the front page, meaning most people visiting won't even see it.

Finally, if spam somehow gets through all these checks, there are moderators that watch the various sites that can instantly remove spam and suspend/delete abusive accounts. Although moderators try to be as vigilant as we can, we also rely on the community to notify us of spam and other forms of abuse through the moderator flag.


It's really a combination of both the reputation/spam filter and the community self-policing. The limitations caused by low reputation help by making it harder and more frustrating to spam the system. And beyond that, because user's of a certain amount of rep gain moderator privileges, there are many, active people around, able to quickly snuff out any spam that does get through. Additionally, I believe the SO team puts IP blocks and things of that sort in place to add an additional layer of protection from people/places that have repeatedly attempted to spam the system.

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