Re: [TLS] Malformed Finished handling

Hubert Kario <hkario@redhat.com> Mon, 09 July 2018 12:04 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Eric Rescorla <ekr@rtfm.com>
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Date: Mon, 09 Jul 2018 13:29:28 +0200
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Subject: Re: [TLS] Malformed Finished handling
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On Wednesday, 4 July 2018 18:54:10 CEST Eric Rescorla wrote:
> On Wed, Jul 4, 2018 at 6:36 AM, Hubert Kario <hkario@redhat.com>; wrote:
> > On Wednesday, 4 July 2018 15:00:18 CEST Eric Rescorla wrote:
> > > I think it's a close call, because the length is sort of external to the
> > > language.
> > 
> > which language? the decode_error alert description literally says "length
> > of
> > the message was incorrect."
> 
> The protocol definition language.

Sorry, I really don't see it. There are multiple references to "length" in the 
draft, and as far as I can tell, all are quite consistent.
 
> -Ekr
> 
> > > That's why, for instance, NSS sends "illegal_parameter". So,
> > > absent specific text about this value, I think this is something we can
> > > leave to the implementations.
> > 
> > but the text is explicit, if a message continues past message boundary,
> > the
> > correct response is decode_error. Decode_error is also consistent with the
> > way
> > above-expected length Client Hello MUST be handled.
> > 
> > Just because the description of the message uses a single opaque array
> > doesn't
> > make this message syntactically any different from other Handshake
> > protocol
> > message.
> > 
> > if the interpretation of "I know this _message_ _length_ is wrong because
> > of
> > some other values I negotiated before, so I'll send illegal_parameter" was
> > correct, then overflow_error, decrypt_error and probably few others would
> > also
> > need to be replaced with illegal_parameter...
> > 
> > > -Ekr
> > > 
> > > On Wed, Jul 4, 2018 at 2:54 AM, Hubert Kario <hkario@redhat.com>; wrote:
> > > > Despite this, is it correct to terminate a connection with
> > > > "illegal_parameter"
> > > > upon receiving a Finished handshake message with a 100 byte payload?
> > 
> > or a
> > 
> > > > 20
> > > > byte payload? My opinion is that it is not, "decode_error" is more
> > > > specific so
> > > > it should be used instead.
> > > > 
> > > > 
> > > > Specification says the following on the matter:
> > > > 
> > > > The draft 28 specifies the Finished message as having following
> > 
> > structure:
> > > >       struct {
> > > >       
> > > >           opaque verify_data[Hash.length];
> > > >       
> > > >       } Finished;
> > > > 
> > > > At multiple places it also talks about handling messages that have
> > 
> > sizes
> > 
> > > > that
> > > > don't match their structure as requiring termination of connection
> > > > with
> > > > "decode_error".
> > > > 
> > > > The generic situation in Section 6:
> > > >    Peers which receive a message which
> > > >    cannot be parsed according to the syntax (e.g., have a length
> > > >    extending beyond the message boundary or contain an out-of-range
> > > >    length) MUST terminate the connection with a "decode_error" alert.
> > > > 
> > > > as description of the alert in Section 6.2:
> > > >    decode_error  A message could not be decoded because some field was
> > > >    
> > > >       out of the specified range or the length of the message was
> > > >       incorrect.  This alert is used for errors where the message does
> > > >       not conform to the formal protocol syntax.
> > > > 
> > > > In specific about Client Hello, in Section 4.1.2:
> > > >    If negotiating a version of TLS prior to 1.3, a server MUST check
> > > >    that the message either contains no data after
> > > >    legacy_compression_methods or that it contains a valid extensions
> > > >    block with no data following.  If not, then it MUST abort the
> > > >    handshake with a "decode_error" alert.
> > > > 
> > > > And specific handling of Certificate from server in Section 4.4.2.4:
> > > >    If the server supplies an empty Certificate message, the client
> > > >    MUST
> > > >    abort the handshake with a "decode_error" alert.
> > > > 
> > > > Description of "illegal_parameter" in Section 6:
> > > >    Peers which receive a message which is syntactically correct but
> > > >    semantically invalid (e.g., a DHE share of p - 1, or an invalid
> > 
> > enum)
> > 
> > > >    MUST terminate the connection with an "illegal_parameter" alert.
> > > > 
> > > > Alert description in Section 6.2:
> > > >    illegal_parameter  A field in the handshake was incorrect or
> > > >    
> > > >       inconsistent with other fields.  This alert is used for errors
> > > >       which conform to the formal protocol syntax but are otherwise
> > > >       incorrect.
> > > > 
> > > > (it's also mentioned in over a dozen places in the draft)
> > > > --
> > > > Regards,
> > > > Hubert Kario
> > > > Senior Quality Engineer, QE BaseOS Security team
> > > > Web: www.cz.redhat.com
> > > > Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic
> > > > _______________________________________________
> > > > TLS mailing list
> > > > TLS@ietf.org
> > > > https://www.ietf.org/mailman/listinfo/tls
> > 
> > --
> > Regards,
> > Hubert Kario
> > Senior Quality Engineer, QE BaseOS Security team
> > Web: www.cz.redhat.com
> > Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic


-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic