@Monal I have a question, cannot find it in FAQ/mans. Is monal *only* supporting SASL SCRAM-SHA-*-PLUS now? I'm offering SCRAM-SHA-256 but it closes the stream and shows "weak auth" error. a bit odd and there's no setting to turn it off
@ruff it supports SCRAM with SASL2 and PLAIN with SASL1
@ruff (SCRAM with and without channel binding, e.g. with and without -PLUS)
@Monal yup got it so i need to use /authentication:urn:xmpp:sasl:2 in features instead of /mechanisms:urn:ietf:params:xml:ns:xmpp-sasl for it to accept scram.
@ruff exactly. You can check the wiki at https://monal-im.org for best practices and server configuration advice's, too :)
@Monal Ok, added 0388 implementation, not advertising -PLUS, and getting now this:
_incomplete XEP-0388 support, XEP-0440 MUST be implemented and this mandates that servers MUST at least implement tls-server-end-point__
Ok, fair enough, if server sent PLUS, but it didn't
@ruff you'll have to implement xep-0474 to sign your mechanism list for monal to accept it. Otherwise an attacker could trivially strip the *-PLUS mechanisms and XEP-0440 channel bindings list to downgrade you to no channel binding.
@Monal Ok so only way to use SCRAM is to implement 388 and 474, while PLAIN could be used without all the hassles? like really? :)
@ruff regarding PLAIN, well, that's the common denominator supported almost everywhere. But it doesn't give any security guarantees at all.
@Monal ok so the idea behind such scrutinity for SCRAM is to avoid user having false perception of being secure by using more secure mech? But if client choses plain - he's on his own and accepts all the risks.
@ruff did you read xep 0474? It contains a whole block of text going into detail why sending "y" isn't as simple as it may seem.
Anyways, we can discuss this further on xmpp, if you want: xmpp:thilo.molitor@juforum.de
@Monal yes, I was part of similar discussion some time ago on xmpp maillist but while i supported the idea it's better to send n when you don't have mechanism to agree on binding type and you suspect that type you support is unlikely to be supported by tge server it's better to send n, but if server is not sending any -PLUS - y for me is a must have. Perhaps I need to undust my xmpp maillist and start it over there.
@ruff it took me several hours to figure out every possible combination of PLUS method (non)advertising, SSDP (non)support and gss header, each with and without mitm attacker. But now I found a way to support your usecase without introducing any new security holes.
Here it is: https://github.com/monal-im/Monal/commit/e19cba5d2917c1cac1a75617fd6bdada15ebcf8f
This will be part of the next beta and stable releases and is already part of the latest alpha release :)
@Monal yup that is exactly what I meant, thanks for improving. I'm still working with my test cases to check ssdp by using sasl optional extensions. first indication is positive though, so once finished with draft implementation will send proposal to the list.
@ruff what proposal exactly do you mean? Ssdp already uses an optional attribute (h is optional, only m would be mandatory).
@Monal this attribute must be sent by server, you cannot force arbitrary sasl backend to send custom attribute. You can send it from the client though as optional extension and any compliant backend should ignore it but include into authmessage (hence signature).
@ruff why should servers not be allowed to send optional attributes? Nothing in the rfc says "optional attributes are only allowed if the client sends them". So: sure, the server is allowed to send optional attributes. And clients are of course allowed to abort authentication if the are not satisfied with the optional attributes they get from the server.
I really don't see the rfc saying otherwise.
@Monal it's not about being "allowed" it's about having possibility. As mentioned early I'm working with dovecot backend. I have no means to inject custom attribute into its server-first. Similar is with other sasl libs i worked with.
@ruff yeah, so you won't be able to implement ssdp, but you would not be able to implement it if the client sent the hash, too.
You need the other side to check the hash. So in principle both directions are possible. But the client needs to know, if the server checks the ssdp hash and that can only be communicated in a scram attribute as well, because it could be manipulated by an attacker otherwise.
@ruff
--> the server always needs to send an optional attribute, either the hash itself, or a flag indicating ssdp support.
@Monal I think you don't understand what I'm saying. SASL SCRAM is not xmpp feature. It is industry standard protocol. It is used with variety of protocols like SMTP, IMAP, LDAP to name a few. And you now want to mandate all libs to support xmpp XEP? like really?
@Monal Do i understand it correctly that current design goal is to make it mandatory to support for server but optional for client? Eg server will always send it but if client is not supporting that it will just transparently pass it over (eg. without actually verifying the signature hash against sdp section)?
@Monal might be the better solution would be to merge ssdp into sasl2 - if client tries to use it server knows client must be sending h=xxx back and if it is missed or modified then it's indication of mitm.
btw recent dovecot backend (v1.3+) claims to support binding, although I don't see backend protocol options which would allow using it - eg a way to send cb-data to be included into c= attribute. So likely will be limited to native connections.
@Monal Ok then to make it mandatory to implement for sasl2 - i.e. could be used for rfc sasl optionally but for sasl2 mandatory hence client must always send h= when using sasl2. having sasl auth section protected is good regardles of the mech used. Protecting it without server supporting plus could also have merits as a generic downgrade protection.
But yes optional use for rfc with client sending it invalidates the protection as it is sufficient to strip it to emulate client lack of support.
@ruff ssdp can be used with sasl1 (rfc 6120), too. Why limiting it to sasl2? Only the handling of non-support might be different: sasl1 doesn't mandate xep-0440 support, so no lowest common denominator for channel-bindings (tls-server-end-point as lowest common denominator allows a client to abort authentication if PLUS was advertised, but the cb types announced by the server did not include anything the client supported, because at least tls-server-end-point should have been possible)