Bare byte decoding and French characters

It seems we found the real root cause for this issue. It's related to IE11, which introduced a more granular option for UTF8 encoding (for more details check this website). There you can see that IE11 differentiate URL encoding for the path and for the query-string (the latter one is disabled by default). When enable this option and restart the browser, French characters in the query-part of the URL will be correctly UTF8 encoded and the violation will not be triggered anymore.

According to Western European as an alternative language code for the ASM policy, I don't know if this is really a good option, because then it's the other way round. The ASM expects Western European encoding, but if a browser sends it as UTF8 it will fail again to convert the character.

Here is a list of differences between Western European and UTF8, as you can see everything after %7F will different.

And as UTF8 is some kind of standard in the meanwhile, I would always recommend to use it as ASM policy language and correct the issue on the browser (which might be complicated or nearly impossible for public websites, where you don't know which clients will access it). Alternatively you could write an iRule, which checks the query-part of the URI for such "high" ASCII-characters and respond with an error message describing how to change this setting in IE11 (or other browsers which might be affected from this behavior as well).

Ciao Stefan :)