I’ve looked at the markup being returned and it looks clean to me. The .htaccess file looks fine. A git status command shows that none of the files on the server have been modified.
Can somebody identify what is causing Google to be concerned?
According to Google Chrome:
Diagnostic page for feedvalidator.org
What is the current listing status for feedvalidator.org?
Site is listed as suspicious - visiting this web site may harm your computer.
Part of this site was listed for suspicious activity 1 time(s) over the past 90 days.
What happened when Google visited this site?
Of the 7 pages we tested on the site over the past 90 days, 5 page(s) resulted in malicious software being downloaded and installed without user consent. The last time Google visited this site was on 2012-12-18, and the last time suspicious content was found on this site was on 2012-12-18.
Malicious software includes 3 trojan(s), 2 exploit(s). Successful infection resulted in an average of 3 new process(es) on the target machine.
Malicious software is hosted on 4 domain(s), including checklistearpiercing.net/, guchpaygoogles.net/, guchpaygoogles.info/.
This site was hosted on 1 network(s) including AS32244 (LIQUID).
Has this site acted as an intermediary resulting in further distribution of malware?
Over the past 90 days, feedvalidator.org did not appear to function as an intermediary for the infection of any sites.
Has this site hosted malware?
Yes, this site has hosted malicious software over the past 90 days. It infected 2 domain(s), including inyourneighbourhood.wordpress.com/, googlemapsapi.blogspot.com/.
How did this happen?
In some cases, third parties can add malicious code to legitimate sites, which would cause us to show the warning message.
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If you are the owner of this web site, you can request a review of your site using Google Webmaster Tools. More information about the review process is available in Google’s Webmaster Help Center.