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ModSecurity 2: Explicit Normalisation Options

One of the things I realy dislike in ModSecurity 1.x is that its anti-evasion features are implicit. A series of transformations is always performed on input data and always in the same order. This is somewhat convenient because it saves you from having to think about the evasion issues. This approach - implicit normalisation - is not foolproof (no surprises there). First, there are occassions where you need some other (sometimes peculiar) transformation to take place before you look at data. Second, the context in which input data is used *is* important. It is not always appropriate to perform a particular transformation - you might even be helping the attackers avoid detection (or prevention).

That's why, when I set to design ModSecurity 2.x, I came up with a flexible solution that allows one to configure normalisation features correctly in every possible sitation. The new capabilities do not come for free: ModSecurity 2.x is a better tool but it is also more difficult to use. Enough about that, let's discuss the improvements.

There are 19 normalisation functions documented in the ModSecurity 2.x reference manual. They are:

  1. lowercase
  2. replaceNulls
  3. removeNulls
  4. compressWhitespace
  5. removeWhitespace
  6. replaceComments
  7. urlDecode
  8. urlEncode
  9. urlDecodeUni
  10. base64Encode
  11. base64Decode
  12. md5
  13. sha1
  14. hexDecode
  15. hexEncode
  16. htmlEntityDecode
  17. escapeSeqDecode
  18. normalisePath
  19. normalisePathWin

The names of most are self-explanatory. (For the others refer to the manual.) By default ModSecurity 2.x will perform lowercase, replaceNulls and compressWhitespace on input data. If you need something else you will have to reconfigure this setting using the new action "t". As before you can use SecDefaultAction to set the defaults for all rules that follow:

 SecDefaultAction log,auditlog,deny,status:403,phase:2,t:lowercase,t:replaceNulls,t:compressWhitespace

The above is an example of a default configuration. You can also have a per-rule setting, either by changing the normalisation options completely, or by adding or removing from the default configuration. Here's an example where "compressWhitespace" is removed and "replaceComments" added.

 SecRule ARGS keyword t:-compressWhitespace,t:replaceComments

To completely replace the configured normalisation functions simly use the special name "none".

 SecRule ARGS keyword t:none,t:normalisePathWin

And if the built-in normalisation functions are not enough for you there is good news - ModSecurity 2.x has an API that allows you to add a new normalisation function without having to touch its source code. (There are examples of this in the distribution.)