[secdir] secdir review of draft-ietf-simple-msrp-sessmatch

["Richard L. Barnes" <[e-mail]>]

I have reviewed this document as part of the security directorate's  
ongoing effort to review all IETF documents being processed by the  
IESG.  These comments were written primarily for the benefit of the  
security area directors.  Document editors and WG chairs should treat  
these comments just like any other last call comments.

This document changes the URI matching algorithm used in MSRP.  MSRP  
sessions are typically initiated using SDP bodies in SIP.  These SDP  
bodies contain MSRP URIs that the peers use to contact each other.   
When one peer receives a request to initiate a session, he verifies  
that the URI being requested is one that he initiated in SDP, thereby  
using the URI as a shared secret to authenticate that the originator  
of the session actually received the SDP body in question.

According to the current SDP specification, this comparison is  
performed over the whole URI; this document restricts the comparison  
to the "session-id" component, omitting the host, port, and transport  
components.  The goal of the document is to facilitate a certain class  
of man-in-the-middle attack, namely to allow a signaling intermediary  
to insert a media intermediary.  The restriction on the URI comparison  
is needed in order for the media intermediary not to have to modify  
URIs in MSRP packets to reflect the modifications to URIs in SDP  
bodies performed to redirect traffic through the media intermediary.

I have a few significant reservations about this document:

This extension makes it more difficult for MSRP entities to secure  
their communications against attackers in the signaling path.  The  
current model provides a basic integrity protection, in that signaling  
intermediaries cannot redirect traffic to an arbitrary third party;  
they must at least advise the third party about how to modify MSRP  
packets.  The proposed modification would remove even this cost.   
Moreover, it raises the cost of providing integrity protection to  
messages, since Alice must now employ both integrity and  
confidentiality protections on an end-to-end basis; if her messages  
are only integrity-protected, then a proxy can remove the integrity  
protection and redirect traffic without it being observable to Alice.

The document needs to clarify what the impacts are for authentication  
in secure modes of MSRP.  In particular:
-- The distinction between "self-signed" and "public" certificates is  
inappropriate.  The proper distinction is between the name-based  
authentication in Section 14.2 of RFC 4975 and the fingerprint-based  
authentication in Section 14.4.  
-- In either case, changing the host name need not result in an  
authentication failure, since the media intermediary can simply  
authenticate as itself to both endpoints, having changed the  
respective MSRP URIs appropriately.
-- There is currently no requirement that a endpoint identity in the  
To-Path URI matches the endpoint identity authenticated at the TLS  
layer, because these two are required to be the same.  This document  
changes that assumption, and should note that these two identities can  
differ. 

The document also precludes any name-based multiplexing, where a  
single MSRP process (single IP address and port) directs requests to  
different virtual recipients based on the domain name in the To-Path  
header.  (In analogy to Host-based multiplexing in HTTP, which is very  
widely deployed.)  Since with this extension, the domain in the To- 
Path is completely unpredictable from the recipient's perspective, it  
is useless to the recipient.

The document has no backward-compatibility.  MSRP implementations that  
do not support this extension will not be able to receive MSRP  
sessions from implementations that do.   In that regard,  this  
document seems more like an new version of MSRP rather than an update.