bugtraq - Re: EEYE: Microsoft ASN.1 Library Length Overflow Heap Corruption

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Message-ID: <1076714051.6255.35.camel@alpha.isode.net>
Date: Fri, 13 Feb 2004 23:14:11 +0000
From: David Wilson <David.Wilson@...de.com>
To: tls@....tjls.com
Cc: bugtraq@...urityfocus.com
Subject: Re: EEYE: Microsoft ASN.1 Library Length Overflow Heap Corruption

> That's not actually correct.  Most network protocols use the
> "Distinguished Encoding Rules" (DER) not the "Basic Encoding Rules" 
> (BER).  BER is an abomination and should never, ever have been in
> the standard; the only protocol commonly used over IP that uses BER
> is LDAP, because it descends from DAP, which used BER.
> 
> So you can't reasonably assume that if it uses ASN.1, it uses
> BER.  That's presumably why Microsoft left certain ASN.1-using
> network services turned on.

DER is not a transfer encoding syntax. It's purpose is to give the same
sequence of octets for a given abstract value (as does CER - Canonical
Encoding Rules). LDAP, OSI, SNMP etc. all use BER, albeit with
restriction in some cases (e.g. no use of indefinite length encoding).
The cryptographic message syntax (RFC 3369) requires DER for some
elements, but specifically allows BER overall. [OSI can use PER - Packed
Encoding Rules - but that is a whole different ballgame]. DER is used
for signing, not for transfer. I.e. if you get an ASN.1 value in
protocol (whatever transfer syntax) you need to re-encode the value
(paying attention to the ASN.1 type) as DER to generate the hash.

In any case DER is a subset of BER (as is CER). The particular
vulnerability in question arises in the encoding of primitive elements,
where DER and BER are the basically the same (except that with DER you
are required to specify the length field in the minimum amount of space,
which is not the case for BER).

The bottom line is: given a PDU  or object encoded in DER, you would use
a BER decoder on it. I'm not sure what the issue is which causes BER to
be an "abomination" while DER is OK.

cheers

-- 
David Wilson <David.Wilson@...de.com>
Isode Limited