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Re: [Full-Disclosure] [anti-XSS]about CERT/CC:malicious_code_mitigation

Woops, O'Brian is pissed again ;)

Real solution is to have "per input" input validation which will always let some "potentially bad" things through, but help mitigate exposures and then do things "right". Right of course means always binding data to sql statements and properly performing context sensitive output encoding.

dd

Valdis.Kletnieks@xxxxxx wrote:
On Sat, 07 Aug 2004 06:25:00 -0000, bitlance winter said:


#! The first function takes the negative approach.
#! Use a list of bad characters to filter the data
sub FilterNeg {
   local( $fd ) = @_;
   $fd =~ s/[\<\>\"\'\%\;\)\(\&\+]//g;
   return( $fd ) ;
}


*BZZT!!* Wrong. Don't do this in production code, because...


I have understood that bad characters are
< > " ' % ; ) ( & +


If it turns out that * (asterisk) is a "bad character", you're screwed.

If it turns out that *any other* character is "bad", you're screwed.

The *proper* way to do the filtering is to *remove* *all* characters
not known to be good.  Something like:

$fd =~ s/[^-_ a-zA-Z0-9]//g;

Only pass alphabetic, numeric, space, hyphen, and underscore.  Add other
characters *only* if you can show they are *not* a problem.


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