Unicode IPS vulnerability: more details

Cisco has released security response acknowledging that the IPS software does not properly handle a rarely used Unicode encoding. Reading the security notice you might be left wondering what's going on. Here's the whole story.

Within an URI (web address), the ASCII characters can be encoded in one of three ways:

• Unless they are reserved, they can be included in the URI directly (for example, you can always use the letter a in an URI).
• You can always escape a character using its hexadecimal value. Letter a can thus be written as %61.
• Unicode character set includes full-width form of ASCII characters, where letter a can be encoded as a two-byte value 0xFF61 (or %ff%61 in an URI)

The IPS software (standalone or integrated in Cisco IOS) does not recognize the sequence %ff%61 as letter a. It's thus possible to evade some IPS triggers by replacing ASCII characters with their full-width Unicode encoding.

The vulnerability by itself does not open new attack routes, unless you use the IPS as the only means of protection of a vulnerable system (which is a bad practice anyway).