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full-disclosure-uk: [Full-disclosure] noise about full-width enc

[Full-disclosure] noise about full-width encoding bypass?

From: Brian Eaton <eaton.lists_at_nospam>
Date: Mon May 21 2007 - 14:22:21 GMT
To: "Web Security" <websecurity@webappsec.org>, Full-Disclosure <full-disclosure@lists.grok.org.uk>


Has anyone had a look at the full-width unicode encoding trick discussed here?

http://www.kb.cert.org/vuls/id/739224

AFAICT, this technique could be useful for a homograph attack. I don't think it's useful for much else. However, a few vendors have reacted already, so I may be missing something important.

Here's why I think the attack is mostly harmless:

Let's say an attacker wants to use this technique to hide a SQL injection attack. They decide to use a full-width encoding for single quote, 0xff 0x07. They successfully bypass the IDS, because the IDS is only scanning for normal single quotes. (You can see the encodings and their graphical representation here: http://www.unicode.org/charts/PDF/UFF00.pdf)

If the SQL engine is processing queries in Unicode, then 0xff 0x07 will be treated as a normal unicode character, not a single quote. The sequence 0xff 0x07 is not equivalent to 0x27, the real single quote value. No SQL injection occurs.

If the SQL engine is processing queries in UTF-8, then 0xff 0x07 will be converted from Unicode to UTF-8: 0xef 0xbc 0x87. Again, the engine does not recognize 0xef 0xbc 0x87 as equivalent to 0x27.

If the SQL engine is processing queries in ASCII or ISO-8859-1, the conversion from unicode to the code page used by the engine will fail.  Either the engine will give up on the query, or it might substitute a question mark (?) for the unconvertible character.

To summarize: I think half-width and full-width unicode characters are characters that happen to have the same graphical representation as other characters, but don't carry any special significance outside of that graphical representation. The graphical representation can be important in homograph attacks, but otherwise I don't see this technique as particularly useful to an attacker.

Any comments on what I may have missed?

Regards,
Brian



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