robots.txt (revaluation)Many of my websites contain robots.txt files that appear to be used to expose compromised websites (when you open it, it notifies the attacker). These files include a reference to a sitemap that actually points to an exploited file (index.php). If Googlebot or another search-bot fetches that sitemap, it could automatically reveal the infected website to the attacker. The attacker have put search bots in work for him (smart, I must say).
Every index.php file referenced by these robots.txt files appears to be infected at the top. Below the infection lies your original code (but double check it!!!).
Note that simply deleting the robots.txt files is not enough! You also must carefully inspect and clean every affected index.php file. Make sure to thoroughly check each robots.txt file, as the infection may vary between them and you might end up losing the infected index.php file.
An infected index.php file is still useful to the attacker! The robots.txt is just a complement.
SSH command to search all robots.txt files:
find /home -type f -name "robots.txt"