Laundering tainted data

Question

When I do laundering tainted data with checking whether it has any bad characters are there unicode-properties which will filter the bad characters?

Answer 1

I think "no" is an understatement for an answer, but there you have it. No, Unicode does not have a concept of "bad" or "good" characters (let alone "ugly" ones).

Answer 2

User-Defined Character Properties in perlunicode

package Characters::Sid_com;

sub InBad {
return <<"BAD";
0000\t10FFFF
BAD
}

sub InEvil {
return <<"EVIL";
0488
0489
EVIL
}

sub InStupid {
return <<"STUPID";
E630\tE64F
F8D0\tF8FF
STUPID
}

⋮

die 'No.' if $tring =~ /
    (?: \p{Characters::Sid_com::InBad}
      | \p{Characters::Sid_com::InEvil}
      | \p{Characters::Sid_com::InStupid}
    )
/x;

Answer 3

XML (and thus XHTML) can only contains these chars:

\x09 \x0A \x0D
\x{0020}-\x{D7FF}
\x{E000}-\x{FFFD}
\x{10000}-\x{10FFFF}

Of the above, the following should be avoided:

\x7F-\x84
\x86-\x9F
\x{FDD0}-\x{FDEF}
\x{1FFFE}-\x{1FFFF}
\x{2FFFE}-\x{2FFFF}
\x{3FFFE}-\x{3FFFF}
\x{4FFFE}-\x{4FFFF}
\x{5FFFE}-\x{5FFFF}
\x{6FFFE}-\x{6FFFF}
\x{7FFFE}-\x{7FFFF}
\x{8FFFE}-\x{8FFFF}
\x{9FFFE}-\x{9FFFF}
\x{AFFFE}-\x{AFFFF}
\x{BFFFE}-\x{BFFFF}
\x{CFFFE}-\x{CFFFF}
\x{DFFFE}-\x{DFFFF}
\x{EFFFE}-\x{EFFFF}
\x{FFFFE}-\x{FFFFF}
\x{10FFFE}-\x{10FFFF}

If you are generating XHTML, you need to escape the following:

• & ⇒ &
• < ⇒ <
• > ⇒ > (optional)
• " ⇒ " (optional except in attribute values delimited with ")
• ' ⇒ ' (optional except in attribute values delimited with ')

HTML should have the same if not looser requirements, so if you stick to this, you should be safe.