You have probably heard it by now, but US-CERT released a Vulnerability Note last week entitled "HTTP content scanning systems full-width/half-width Unicode encoding bypass." The short of it is that many HTTP content scanning systems (think IDS/IPS/WAFs) may not be able to properly decode data that is encoded using Unicode full-width/half-width encoding thus allowing a possible evasion issue for malicious traffic.
This is yet another Impedance Mismatch issue where one host may interpret data a certain way while another interprets in differently. In this case, security devices that use decoding functions may not properly decode the data and cannot therefore apply certain signatures. If this is the case, then you would have a false negative if the request was malicious and the destination host is able to decode the data and process it.
The $1,000,000 question here is does this issue affect ModSecurity? Yes. The %u syntax is a Microsoft-specific extension. While ModSecurity does not decode such encodings by default (meaning you have to explicitly address the issue in your rules if you need this feature) you can choose to decode them using the urlDecodeUni transformation function. In the current version of ModSecurity this transformation function cannot deal with the above-mentioned evasion technique. (On the positive side, the transformation function behaves exactly as documented in the reference manual.) Fortunately, it is quite easy to create a ModSecurity rule that can identify and block any use of this type of encoding. Here is the example rule that you can use:
# Disallow use of full-width unicode
SecRule REQUEST_FILENAME|ARGS|ARGS_NAMES|REQUEST_HEADERS|XML:/*|!REQUEST_HEADERS:Referer \
"\%u[fF]{2}[0-9a-fA-F]{2}" \
"t:none,deny,log,auditlog,status:400,msg:'Unicode Full/Half Width Abuse Attack Attempt',id:'950116',severity:'4'"
This rule is also included in the latest development release of the Core Rules.