Otto42 of OttoDestruct, a key WordPress developer and supporter, reports that there is an “attack” on older versions of WordPress right now. The number of sites hit by this is growing every hour. Protect your WordPress blog now: UPDATE NOW!!!Update your WordPress blog before you continue reading this post. That’s how critical this issue is.
There are two clues that your WordPress site has been attacked.
There are strange additions to the pretty permalinks, such as
example.com/category/post-title/%&(%7B$%7Beval(base64_decode($_SERVER%5BHTTP_REFERER%5D))%7D%7D|.+)&%/. The keywords are “eval” and “base64_decode.”
The second clue is that a “back door” was created by a “hidden” Administrator. Check your site users for “Administrator (2)” or a name you do not recognize. You will probably be unable to access that account, but Journey Etc. has a possible solution.
This only affects self hosted Wordpress sites not Wordpress.com blogs. If you have already been hacked, they also have some suggestions for fixing it. Again, upgrade now.