After a couple of panicked days trying to find and remove the malware from this blog and my other one, I finally contacted support at my web hosting company. They scanned through my account and told me exactly which files had been affected and how.
Basically, what happened was that someone hacked my FTP password and added a base64-encoded script to a couple of files on each of my blogs. The encoded script looked like this:
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PC9pZnJhbWU+PC9kaXY+JzsNCn0=
only without the line feeds I have added for readability. Out of curiosity, I copied the code and took it to a site that can decode base64-encoding.
I’m not going to re-post the decoded script here. If you’re that curious, you can copy the encoded version above, remove the line breaks, and take it to the URL mentioned above.
Essentially, when this was tacked on to my index.php file, it was saying, “if this page is being loaded from one of these IP addresses, or a browser identified as one of these, then display the page normally. For everyone else, add this iframe, positioned off-screen and very small, that downloads malicious software”.
This is a shining example of why you should keep your passwords long, random, and frequently changed.