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I'm working with a site that was hacked a couple of months ago and was sending out a ton of spam. The problem seemed to have started with a dormant WordPress blog that was part of the site (so the site is not WordPress itself, the blog was). Since that was no longer being used, I removed the blog, its database, and found infected files in the images folder and in a javascript file on the main site. That dramatically decreased 404 errors. Then they started to increase again. I tried using my .htacess file to block referrers who were supposedly linking this site from theirs with the viagra links. Again, crawl errors decreased dramatically and have now started up again. I can't figure out what is happening here. I've removed any weird code (looked mainly for base decode64) and blocked referrals from spam sites, so shouldn't the crawl errors being going down and staying down, at least from those spam sites?

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The 404 errors are because of other sites linking to you? –  w3d Dec 2 '13 at 14:59
    
Yes. If I click on the "Linked From" tab in Google Webmasters after selecting a particular 404 error, it shows a list of all these sites that supposedly have on link on their site to a page or directory on my site. These pages and directories all have some sort of viagra reference but don't exist. Some of the worst site offenders have domains like: burundichina, dendenboy, hdjxzh, kyawsint, etc. I know some of this stuff just takes time to clear, but I go from like 300+ crawl errors to 30 and then back up to a few hundred. I'm worried there's still some vulnerability I haven't caught. –  user1491394 Dec 2 '13 at 17:07
    
I don't know why removing the infected content would necessarily decrease the 404 errors - that looks like coincidence to me. How did you blocked referers? What HTTP status code did you return? Presumably you would have seen this type of error increase in GWT? It might be a never ending battle to block these sites, but a 404 would be the correct response IMO. It's the requests from these sites that don't result in a 404 that are the problem. In a hacked site these requests could result in a 30x redirect, sending the unsuspecting visitor to the malicious site. –  w3d Dec 3 '13 at 23:31

1 Answer 1

Blocking by referrer won't help: Googlebot doesn't send a referrer header when it crawls, so it will never see your block.

Here is what Google's John Mueller (who works on Webmaster Tools and Sitemaps) has to say about 404 errors that appear in Webmaster tools:

HELP! MY SITE HAS 939 CRAWL ERRORS!!1

I see this kind of question several times a week; you’re not alone - many websites have crawl errors.

  1. 404 errors on invalid URLs do not harm your site’s indexing or ranking in any way. It doesn’t matter if there are 100 or 10 million, they won’t harm your site’s ranking. http://googlewebmastercentral.blogspot.ch/2011/05/do-404s-hurt-my-site.html
  2. In some cases, crawl errors may come from a legitimate structural issue within your website or CMS. How you tell? Double-check the origin of the crawl error. If there's a broken link on your site, in your page's static HTML, then that's always worth fixing. (thanks +Martino Mosna)
  3. What about the funky URLs that are “clearly broken?” When our algorithms like your site, they may try to find more great content on it, for example by trying to discover new URLs in JavaScript. If we try those “URLs” and find a 404, that’s great and expected. We just don’t want to miss anything important (insert overly-attached Googlebot meme here). http://support.google.com/webmasters/bin/answer.py?answer=1154698
  4. You don’t need to fix crawl errors in Webmaster Tools. The “mark as fixed” feature is only to help you, if you want to keep track of your progress there; it does not change anything in our web-search pipeline, so feel free to ignore it if you don’t need it. http://support.google.com/webmasters/bin/answer.py?answer=2467403
  5. We list crawl errors in Webmaster Tools by priority, which is based on several factors. If the first page of crawl errors is clearly irrelevant, you probably won’t find important crawl errors on further pages. http://googlewebmastercentral.blogspot.ch/2012/03/crawl-errors-next-generation.html
  6. There’s no need to “fix” crawl errors on your website. Finding 404’s is normal and expected of a healthy, well-configured website. If you have an equivalent new URL, then redirecting to it is a good practice. Otherwise, you should not create fake content, you should not redirect to your homepage, you shouldn’t robots.txt disallow those URLs -- all of these things make it harder for us to recognize your site’s structure and process it properly. We call these “soft 404” errors. http://support.google.com/webmasters/bin/answer.py?answer=181708
  7. Obviously - if these crawl errors are showing up for URLs that you care about, perhaps URLs in your Sitemap file, then that’s something you should take action on immediately. If Googlebot can’t crawl your important URLs, then they may get dropped from our search results, and users might not be able to access them either.

The 404 errors that Google reports are for your benefit. If they are not actually problems that need to be corrected, you don't need to do anything about them.

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That's kinda what I thought (+1). I did just wonder, however, if the high number of spam-keyword-like backlinks from hacked sites could possibly have a negative SEO effect? Or, as I suspect, the fact that these links return a 404 counteracts that? –  w3d Dec 4 '13 at 22:02
    
My big concern right now actually isn't even the SEO, I get nervous seeing hundreds of 404 errors in this way because this same thing preceded the spam attack. I'm sure I removed any infected code from my site, so I shouldn't be sending out any spam links showing up as 404s. Will I just have to wait this out until these spammers (link farms, whatever they are) move on somewhere else? –  user1491394 Dec 5 '13 at 0:07
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@w3d spam keywords linking to 404 pages shouldn't hurt. But you might not want to "claim" those links by redirecting them to real pages. –  Stephen Ostermiller Dec 5 '13 at 2:08

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