Re: [OAUTH-WG] TAuth

[Takahiko Kawasaki <[e-mail]>]

IMHO,

The reason an OAuth 2.0 server does not authenticate _public_ clients is
because, by definition, a public client cannot keep its client secret
confidential and so client authentication is almost meaningless.

"Introducing TAuth: Why OAuth 2.0 is bad for banking APIs and how we're
fixing it" and the famous post "OAuth 2.0 and the Road to Hell" (by Mr.
Eran Hammer) blame OAuth 2.0, but they don't recognize that their
insistence relies on the assumption that a secret key embedded in a client
application can be kept confidential. On the other hand, OAuth 2.0
recognizes that the assumption is naive under some conditions and
explicitly distinguishes confidential clients from public clients (RFC
6749, 2.1. Client Types).

It sounds unconvincing to me that people who live in the 'confidential
client' world blame OAuth 2.0 which covers use cases for both confidential
clients and public clients.

My opinion with further details is written in my answer to the question
"How is OAuth 2 different from OAuth 1?" in StackOverflow.

http://stackoverflow.com/a/35775049/1174054


Best Regards,
Takahiko Kawasaki

2016-05-11 9:22 GMT+09:00 Dick Hardt <[e-mail]>:

> Let us know what you learn Justin!
>
> On Tue, May 10, 2016 at 5:05 PM, Justin Richer <[e-mail]> wrote:
>
>> I posted about this the other day. What I don't understand is that he's
>> saying people disable TLS checks so he's going to solve it with mutual TLS?
>>
>> I need to email this guy and learn more about what he's doing.
>>
>> --Justin
>>
>>  *Sent from my phone*
>>
>> -------- Original message --------
>> From: Brock Allen <[e-mail]>
>> Date: 5/10/16 6:44 PM (GMT-06:00)
>> To: Dick Hardt <[e-mail]>, Oauth Wrap Wg <[e-mail]>
>> Subject: Re: [OAUTH-WG] TAuth
>>
>> Doesn’t the recent PoP work address many of these concerns?
>>
>> Also, as for developers disabling SSL — does anyone still do this and
>> think it’s safe? Or are people just being lazy? Or are there certain
>> contexts that I’m unaware of where this is valid?
>>
>> -Brock
>>
>>
>> From: OAuth <[e-mail]> on behalf of Dick Hardt <
>> [e-mail]>
>> Date: Tuesday, May 10, 2016 at 7:24 PM
>> To: Oauth Wrap Wg <[e-mail]>
>> Subject: [OAUTH-WG] TAuth
>>
>> Does anyone think there are useful lessons here?
>>
>> Does adding TOKBIND resolve the issues?
>>
>> https://blog.teller.io/2016/04/26/tauth.html
>>
>> *Teller* <https://blog.teller.io/>
>>
>> Join waiting list <http://teller.io/#waitlist>
>>
>> Introducing TAuth: Why OAuth 2.0 is bad for banking APIs and how we're
>> fixing it
>>
>> This week we released our authorisation flow making it possible for you
>> to go from building apps that talk to your bank account, to building apps
>> that can talk to any bank account. This is huge. Check out this SMS bot
>> (how on trend) I hacked up yesterday morning. (and don't forget to join
>> the beta wait list <https://teller.io/>).
>>
>>
>> Getting to this point took longer than we expected. This is because there
>> wasn't a good story for delegating authorisation for sensitive APIs. The
>> most popular choice, OAuth 2.0 <http://oauth.net/2> - which has been
>> chosen by the Open Banking Working Group
>> <http://theodi.org/open-banking-standard>, BBVA
>> <https://www.bbvaapimarket.com/web/api_market/bbva/bbva-connect/documentation#3-legged-flow>,
>> RBS <https://bluebank.portal.azure-api.net/getting-started>, and Mondo
>> <https://getmondo.co.uk/docs/#authentication> - is also amongst the
>> worst from a security perspective.
>>
>> Teller provides an API for your bank account. The EU is forcing all
>> European banks to expose account APIs with PSD II
>> <http://ec.europa.eu/finance/payments/framework/index_en.htm> by end of
>> 2017. These banks are disconcertingly converging around OAuth 2.0* without
>> fully considering the impact on their customers, and something needs to be
>> done before it's too late.
>>
>> * One notable exception is the Open Bank Project
>> <http://www.openbankproject.com/>. It is sticking with OAuth 1.0a
>> <http://oauth.net/core/1.0a/> precisely because OAuth 1.0a doesn't share
>> the same security issues as OAuth 2.0.
>>
>> Man-in-the-middle
>>
>> One of the biggest problems with OAuth 2.0 is that it delegates all
>> security concerns to TLS but only the client authenticates the server (via
>> it's SSL certificate), the server does not authenticate the client. This
>> means the server has no way of knowing who is actually sending the request.
>> Is it a bona fide user, or is an attacker tampering with the request? When
>> an attacker is able to insinuate themselves between a legitimate user and
>> the server, it's called a man-in-the-middle (MITM) attack. It looks like
>> this:
>>
>>    - client attempts to connect to service
>>    - attacker successfully reroutes traffic to a host it controls
>>    - malicious host accepts connection from client
>>    - malicious host connects to service
>>    - service accepts connection from malicious host
>>    - client communicates with service proxied through malicious host,
>>    which can see and tamper with any data sent or received
>>
>> You're probably thinking "hang on, isn't this the point of SSL?" Yes it
>> is, but there are a number of ways to present a bogus certificate and a
>> client accept it. The most realistic threat is the client developer not
>> properly verifying the server certificate, i.e. was it ultimately signed by
>> a trusted certificate authority?
>>
>> *Follow* <https://twitter.com/tpope>
>>
>> <https://twitter.com/tpope>
>>
>> *Tim Pope* ‎@tpope <https://twitter.com/tpope>
>>
>> Pull requests to disable SSL certificate verification: more common than
>> you would think.
>>
>> 2:24 PM - 11 Feb 2013
>> <https://twitter.com/tpope/status/301094352434892800>
>>
>>    - 5 5 Retweets
>>    <https://twitter.com/intent/retweet?tweet_id=301094352434892800>5 5
>>    likes <https://twitter.com/intent/like?tweet_id=301094352434892800>
>>
>> Unfortunately a large <http://stackoverflow.com/a/7332983/223213> number
>> <http://stackoverflow.com/a/16298999/223213> of developers
>> <http://stackoverflow.com/a/36154521/223213> think that disabling SSL
>> peer verification is the correct fix to a SSL path validation error. There
>> are many more that will offer the same advice with the caveat that it
>> introduces a security issue <http://stackoverflow.com/a/12293898/223213>
>> that < 100% of readers will consider. As an API provider with a duty of
>> care to our users we can't simply hope developers on our platform don't do
>> this.
>>
>> Bearer tokens
>>
>> Once a user authorises a client application to access its account, the
>> application obtains a bearer token from the authorisation server. As the
>> name suggests if you have possession of the bearer token then you are
>> essentially the user. There is no cryptographic proof that the requesting
>> client is the intended developer and not an attacker. If an attacker is
>> able to successfully MITM a client it could have catastrophic implications
>> for the user, e.g. an empty bank account, loans opened in their name, etc.
>> OAuth 2.0 is simply a security car crash from a bank's perspective. They
>> have no way to prove that an API transaction is bona fide, exposing them to
>> unlimited liability.
>>
>> For more information on OAuth 2.0 shortcomings see OAuth Bearer Tokens
>> are a Terrible Idea
>> <https://hueniverse.com/2010/09/29/oauth-bearer-tokens-are-a-terrible-idea/>
>> and OAuth 2.0 and the Road to Hell
>> <https://hueniverse.com/2012/07/26/oauth-2-0-and-the-road-to-hell/> by Eran
>> Hammer <https://twitter.com/eranhammer> the original primary author of
>> OAuth 2.0 who formally removed his name from the standard, calling it "the
>> biggest professional disappointment of [his] career."
>>
>> Finding something better
>>
>> Sitting down to design the solution to this problem I had two high-level
>> goals:
>>
>>    - be the most secure solution for the user
>>    - not unnecessarily impede developer experience. Developers are users
>>    too and their needs deserve equal consideration.
>>
>> From a security perspective I wanted:
>>
>>    - cryptographic proof of client identity
>>    - to stop MITM attacks
>>    - to unimpeachably attribute a request to a given developer. In
>>    cryptography this is known as non-repudiation
>>    <https://en.wikipedia.org/wiki/Non-repudiation>.
>>
>> (N.B. Non-repudiation is a de facto requirement of PSD II. If a bank
>> can't prove an account owner authorised a transaction, they're liable for
>> any losses incurred by the user.)
>>
>> There are solutions to the bearer token problem like JWT tokens (RFC7523
>> <https://tools.ietf.org/html/rfc7523>) but in most cases these rely on a
>> shared secret which is used to computed a HMAC-based signature. Shared
>> secrets mean no non-repudiation. Public key cryptography can be used with
>> JWT tokens but they don't solve the problem of how the client will generate
>> key pairs, demonstrate proof of possession of the private key, and enrol
>> the public key with the API. Most importantly using JWT tokens make it
>> basically impossible for you to experiment with an API using cURL. A major
>> impediment to developer experience.
>>
>> In an ideal world we'd have cryptographic proof of the client's identity
>> without it having to leak through the application level (and stop us
>> cURLing the API!). As I thought about it it became the clear the answer was
>> hiding in plain sight: *SSL client certificates.*
>>
>> Client SSL Authentication
>>
>> A SSL handshake involving client certificates contains an extra message
>> at the end of the handshake, the *CertificateVerify*
>> <https://www.ietf.org/rfc/rfc2246.txt> message.
>>
>>   Client                            Server
>>
>>
>>   ClientHello        -------->
>>
>>                                     ServerHello
>>
>>                                     Certificate*
>>
>>                                     ServerKeyExchange*
>>
>>                                     CertificateRequest*
>>
>>                      <--------      ServerHelloDone
>>
>>   Certificate*
>>
>>   ClientKeyExchange
>>
>>   CertificateVerify*
>>
>>   [ChangeCipherSpec]
>>
>>   Finished           -------->
>>
>>                                  [ChangeCipherSpec]
>>
>>                      <--------             Finished
>>
>>   Application Data   <------->     Application Data
>>
>>
>>          Fig. 1 - Message flow for a full handshake
>>
>> The client collects all the handshake messages and signs them with it's
>> private key and sends the result to the server. The server then verifies
>> the signature using the public key of the client certificate. If the
>> signature can be verified with the public key, the server knows the client
>> is in possession of the private key, and is therefore a bona fide user.
>>
>> Let's look at this in the context of our original attack:
>>
>>    - client attempts to connect to service
>>    - attacker successfully reroutes traffic to a host it controls
>>    - malicious host accepts connection from client accepting the
>>    client's certificate
>>    - malicious host connects to service
>>    - the malicious host will fail to SSL handshake because the host
>>    doesn’t have the private key for the client’s certificate. The attacker
>>    therefore cannot compute the correct *CertificateVerify* handshake
>>    message. The *CertificateVerify* message from the first handshake
>>    cannot be used because the handshake sequences diverge (different server
>>    certificates presented by the host).
>>
>> Introducing TAuth
>>
>> TAuth is Client SSL Authentication + User Tokens + Great Tooling.
>>
>> Client SSL authentication is often overlooked because of the poor UX of
>> using client certificates in the browser and that generating certificates
>> is a painful multistep process involving arcane OpenSSL CLI incantations.
>> However as we're talking about API clients the browser UX point is
>> irrelevant. As far as certificate generation goes, we can write better
>> tools. These days it's possible to generate a key pair, a PKCS#10
>> certificate request, and sign it all in the browser. Thanks to WebCrypto
>> <http://www.w3.org/TR/WebCryptoAPI/> the whole process is reduced to one
>> click.
>>
>> This is how Teller does it:
>>
>> A private key and a SSL certificate signed by Teller generated in one
>> click.
>>
>> And this is what a request looks like with client certificates:
>>
>> Let's recap what we've achieved here:
>>
>>    - Cryptographic proof of the client identity
>>    - The cURLability of the API is preserved
>>    - The client developer has generated a private key known only to them
>>    and no one else, meaning the bank can actually say "you did that
>>    transaction" (non-repudiation)
>>
>> Token security
>>
>> Notice in the above example. The Teller API accepts connections from
>> clients without a client certificate. We do this because we provide
>> developers with read-only personal access tokens for their own accounts if
>> they want to quickly hack something up and not bother with provisioning
>> certs. Now notice how the API does not accept the token presented, but
>> accepts it when used with the client SSL certificate. TAuth bearer tokens
>> are bound on the server side to a private key through an application. This
>> means they are useless without the private key (which only the developer
>> ever has) and therefore not sensitive. As matter of fact, here is one for
>> my bank account:
>>
>>
>> You'll need the private key it's bound to for it to be of any use, and
>> that has never left my laptop.
>>
>> TAuth tokens do not expire (but can be revoked). OAuth 2.0 introduced the
>> concept of time-limited tokens. Large internet companies found it useful
>> for scaling purposes to issue self-encoded, encrypted tokens. The drawbacks
>> are developers have to pay the complexity cost of refreshing tokens and
>> most importantly tokens cannot be revoked, they're good until they expire.
>> For bank account APIs this is an undesirable property, a token should be
>> void as soon as the account owner wishes. TAuth checks the token revocation
>> status at each request.
>>
>> Given that we have no need for self-encoded tokens and that tokens are
>> useless to anyone without the private key, we can consider them public and
>> directly return them in the callback further simplifying things for the
>> developer compared to OAuth without compromising the user's security.
>>
>> Bonus: DDOS mitigation
>>
>> TAuth can help mitigate layer 7 based DDOS attacks
>> <https://en.wikipedia.org/wiki/Application_layer_DDoS_attack> too. If
>> the client does not present a valid client certificate the server can just
>> choose to bounce the connection.
>>
>> Conclusion
>>
>> OAuth 2.0 is simply not fit for use with sensitive APIs and all the
>> pieces needed to build something that is exist today. Aside from unbound
>> bearer tokens, OAuth is too open-ended and complicated to get right for
>> most developers. It's so bad it even has it's own threat model as a separate
>> RFC <https://tools.ietf.org/html/rfc6819> to offer mitigations for the
>> endless problems that can happen.
>>
>> *TAuth stands for Trusted Authentication*, and it provides the best
>> security for users while maintaining the highest possible quality developer
>> experience. Less can go wrong when everything is simpler. *If your bank
>> has OAuth 2.0 in production you must ask yourself, do they really know what
>> they're doing?*
>>
>> TAuth is available in production today for our existing beta users and
>> we've already begun the work to make it an open standard we hope the
>> industry adopts. If you're at a bank and want to offer your customers the
>> security they deserve email me - *sg at teller.io <http://teller.io>*
>>
>> Sign up for the beta waiting list at Teller.io <http://teller.io/>.
>> _______________________________________________ OAuth mailing list
>> [e-mail] https://www.ietf.org/mailman/listinfo/oauth
>>
>
>
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