Open Authentication Protocol T. Lodderstedt, Ed.
Internet-Draft YES Europe AG
Intended status: Best Current Practice J. Bradley
Expires: September 29, 2017 Ping Identity
A. Labunets
Facebook
March 30, 2017
OAuth Security Topics
draft-ietf-oauth-security-topics-02
Abstract
This draft gives a comprehensive overview on open OAuth security
topics. It is intended to serve as a working document for the OAuth
working group to systematically capture and discuss these security
topics and respective mitigations and eventually recommend best
current practice and also OAuth extensions needed to cope with the
respective security threats.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 29, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Recommended Best Practice . . . . . . . . . . . . . . . . . . 3
2.1. Protecting redirect-based flows . . . . . . . . . . . . . 4
2.2. TBD . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Recommended modifications and extensions to OAuth . . . . . . 4
4. OAuth Credentials Leakage . . . . . . . . . . . . . . . . . . 5
4.1. Insufficient redirect URI validation . . . . . . . . . . . 5
4.1.1. Attacks on Authorization Code Grant . . . . . . . . . 5
4.1.2. Attacks on Implicit Grant . . . . . . . . . . . . . . 6
4.1.3. Proposed Countermeasures . . . . . . . . . . . . . . . 7
4.2. Authorization code leakage via referrer headers . . . . . 9
4.2.1. Proposed Countermeasures . . . . . . . . . . . . . . . 9
4.3. Attacks in the Browser . . . . . . . . . . . . . . . . . . 9
4.3.1. Code in browser history (TBD) . . . . . . . . . . . . 9
4.3.2. Access token in browser history (TBD) . . . . . . . . 10
4.3.3. Javascript Code stealing Access Tokens (TBD) . . . . . 10
4.4. Dynamic OAuth Scenarios . . . . . . . . . . . . . . . . . 10
4.4.1. Access Token Phishing by Counterfeit Resource Server . 10
4.4.2. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . 11
5. OAuth Credentials Injection . . . . . . . . . . . . . . . . . 12
5.1. Code Injection . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Proposed Countermeasures . . . . . . . . . . . . . . . 14
5.1.2. Access Token Injection (TBD) . . . . . . . . . . . . . 15
5.1.3. XSRF (TBD) . . . . . . . . . . . . . . . . . . . . . . 16
6. Other Attacks . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Other Topics . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Document History . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
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It's been a while since OAuth has been published in RFC 6749
[RFC6749] and RFC 6750 [RFC6750]. Since publication, OAuth 2.0 has
gotten massive traction in the market and became the standard for API
protection and, as foundation of OpenID Connect, identity providing.
While OAuth was used in a variety of scenarios and different kinds of
deployments, the following challenges could be observed:
o OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (XSRF, referrer
header). Although most of these threats are discussed in RFC 6819
[RFC6819], continued exploitation demonstrates there may be a need
for more specific recommendations or that the existing mitigations
are too difficult to deploy.
o Technology has changed, e.g. the way browsers treat fragments in
some situations, which may change the implicit grant's underlying
security model.
o OAuth is used in much more dynamic setups than originally
anticipated, creating new challenges with respect to security.
Those challenges go beyond the original scope of RFC 6749
[RFC6749], RFC 6750 [RFC6749], and RFC 6819 [RFC6819].
The remainder of the document is organized as follows: The next
section gives a summary of the set of security mechanisms and
practices, the working group shall consider to recommend to OAuth
implementers. This is followed by a section proposing modifications
to OAuth intended to either simplify its usage and to strengten its
security.
The remainder of the draft gives a detailed analyses of the
weaknesses and implementation issues, which can be found in the wild
today along with a discussion of potential counter measures. First,
various scenarios how OAuth credentials (namely access tokens and
authorization codes) may be disclosed to attackers and proposes
countermeasures are discussed. Afterwards, the document discusses
attacks possible with captured credential and how they may be
prevented. The last sections discuss additional threats.
2. Recommended Best Practice
This section describes the set of security mechanisms the authors
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believe should be taken into consideration by the OAuth working group
to be recommended to OAuth implementers.
2.1. Protecting redirect-based flows
Authorization servers shall utilize exact matching of client redirect
URIs against pre-registered URIs. This measure contributes to the
prevention of leakage of authorization codes and access tokens
(depending on the grant type). It also helps to detect mix up
attacks.
Clients shall avoid any redirects or forwards, which can be
parameterized by URI query parameters, in order to provide a further
layer of defence against token leakage. If there is a need for this
kind of redirects, clients are advised to implement appropriate
counter measures against open redirection, e.g. as described by the
OWASP [owasp].
Clients shall ensure to only process redirect responses of the OAuth
authorization server they send the respective request to and in the
same user agent this request was initiated in. In particular,
clients shall implement appropriate XSRF prevention by utilizing one-
time use XSRF tokens carried in the STATE parameter, which are
securely bound to the user agent. Moreover, the client shall store
the authorization server's identity it send an authorization request
to in a transaction-specific manner, which is also bound to the
particular user agent. Furthermore, clients should use AS-specific
redirect URIs as a means to identify the AS a particular response
came from. Matching this with the before mentioned information
regarding the AS the client sent the request to helps to detect mix-
up attacks.
Note: [I-D.bradley-oauth-jwt-encoded-state] gives advice on how to
implement XSRF prevention and AS matching using signed JWTs in the
STATE parameter.
Clients shall use PKCE [RFC7636] in order to (with the help of the
authorization server) detect attempts to inject authorization codes
into the authorization response. The PKCE challenges must be
transaction-specific and securely bound to the user agent, in which
the transaction was started.
Note: although PKCE so far was recommended as mechanism to protect
native apps, this advice applies to all kinds of OAuth clients,
including web applications.
2.2. TBD
3. Recommended modifications and extensions to OAuth
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This section describes the set of modifications and extensions the
authors believe should be taken into consideration by the OAuth
working group change and extend OAuth in order to strengthen its
security and make it simpler to implement. It also recommends some
changes to the OAuth set of specs.
Remove requirement to check actual redirect URI at token endpoint -
seems to be complicated to implement properly and could be
compromised
4. OAuth Credentials Leakage
This section describes a couple of different ways how OAuth
credentials, namely authorization codes and access tokens, can be
exposed to attackers.
4.1. Insufficient redirect URI validation
Some authorization servers allow clients to register redirect URI
patterns instead of complete redirect URIs. In those cases, the
authorization server, at runtime, matches the actual redirect URI
parameter value at the authorization endpoint against this pattern.
This approach allows clients to encode transaction state into
additional redirect URI parameters or to register just a single
pattern for multiple redirect URIs. As a downside, it turned out to
be more complex to implement and error prone to manage than exact
redirect URI matching. Several successful attacks have been observed
in the wild, which utilized flaws in the pattern matching
implementation or concrete configurations. Such a flaw effectively
breaks client identification or authentication (depending on grant
and client type) and allows the attacker to obtain an authorization
code or access token, either:
o by directly sending the user agent to a URI under the attackers
control or
o by exposing the OAuth credentials to an attacker by utilizing an
open redirector at the client in conjunction with the way user
agents handle URL fragments.
4.1.1. Attacks on Authorization Code Grant
For a public client using the grant type code, an attack would look
as follows:
Let's assume the redirect URL pattern "https://*.example.com/*" had
been registered for the client "s6BhdRkqt3". This pattern allows
redirect URIs from any host residing in the domain example.com. So
if an attacker manager to establish a host or subdomain in
"example.com" he can impersonate the legitimate client. Assume the
attacker sets up the host "evil.example.com".
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(1 )The attacker needs to trick the user into opening a tampered URL
in his browser, which launches a page under the attacker's
control, say "https://www.evil.com".
(2 )This URL initiates an authorization request with the client id of
a legitimate client to the authorization endpoint. This is the
example authorization request (line breaks are for display
purposes only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fevil.client.example.com%2Fcb HTTP/1.1
Host: server.example.com
(4 )The authorization validates the redirect URI in order to identify
the client. Since the pattern allows arbitrary domains host names
in "example.com", the authorization request is processed under the
legitimate client's identity. This includes the way the request
for user consent is presented to the user. If auto-approval is
allowed (which is not recommended for public clients according to
RFC 6749), the attack can be performed even easier.
(5 )If the user does not recognize the attack, the code is issued and
directly sent to the attacker's client.
(6 )Since the attacker impersonated a public client, it can directly
exchange the code for tokens at the respective token endpoint.
Note: This attack will not directly work for confidential clients,
since the code exchange requires authentication with the legitimate
client's secret. The attacker will need to utilize the legitimate
client to redeem the code (e.g. by mounting a code injection
attack). This and other kinds of injections are covered in Section
OAuth Credentials Injection.
4.1.2. Attacks on Implicit Grant
The attack described above works for the implicit grant as well. If
the attacker is able to send the authorization response to a URI
under his control, he will directly get access to the fragment
carrying the access token.
Additionally, implicit clients can be subject to a further kind of
attacks. It utilizes the fact that user agents re-attach fragments
to the destination URL of a redirect if the location header does not
contain a fragment (see [RFC7231], section 9.5). The attack described
here combines this behavior with the client as an open redirector in
order to get access to access tokens. This allows circumvention even
of strict redirect URI patterns (but not strict URL matching!).
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Assume the pattern for client "s6BhdRkqt3" is "https://
client.example.com/cb?*", i.e. any parameter is allowed for
redirects to "https://client.example.com/cb". Unfortunately, the
client exposes an open redirector. This endpoint supports a
parameter "redirect_to", which takes a target URL and will send the
browser to this URL using a HTTP 302.
(1 )Same as above, the attacker needs to trick the user into opening
a tampered URL in his browser, which launches a page under the
attacker's control, say "https://www.evil.com".
(2 )The URL initiates an authorization request, which is very similar
to the attack on the code flow. As differences, it utilizes the
open redirector by encoding "redirect_to=https://client.evil.com"
into the redirect URI and it uses the response type "token" (line
breaks are for display purposes only):
GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb%26redirect_to
%253Dhttps%253A%252F%252Fclient.evil.com%252Fcb HTTP/1.1
Host: server.example.com
(5 )Since the redirect URI matches the registered pattern, the
authorization server allows the request and sends the resulting
access token with a 302 redirect (some response parameters are
omitted for better readability)
HTTP/1.1 302 Found
Location: https://client.example.com/cb?
redirect_to%3Dhttps%3A%2F%2Fclient.evil.com%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&...
(6 )At the example.com, the request arrives at the open redirector.
It will read the redirect parameter and will issue a HTTP 302 to
the URL "https://evil.example.com/cb".
HTTP/1.1 302 Found
Location: https://client.evil.com/cb
(7 )Since the redirector at example.com does not include a fragment
in the Location header, the user agent will re-attach the original
fragment
"#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and will
navigate to the following URL:
https://client.evil.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...
(8 )The attacker's page at client.evil.com can access the fragment
and obtain the access token.
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4.1.3. Proposed Countermeasures
The complexitity of implementing and managing pattern matching
correctly obviously causes security issues. This document therefore
proposes to simplify the required logic and configuration by using
exact redirect URI matching only. This means the authorization
server shall compare the two URIs using simple string comparison as
defined in [RFC3986], Section 6.2.1..
This would cause the following impacts:
o This change will require all OAuth clients to maintain the
transaction state (and XSRF tokens) in the "state" parameter.
This is a normative change to RFC 6749 since section 3.1.2.2
allows for dynamic URI query parameters in the redirect URI. In
order to assess the practical impact, the working group needs to
collect data on whether this feature is realy used in deployments
today.
o The working group may also consider this change as a step towards
improved interoperability for OAuth implementations since RFC 6749
is somewhat vague on redirect URI validation. Notably there are
no rules for pattern matching. One may therefore assume all
clients utilizing pattern matching will do so in a deployment
specific way. On the other hand, RFC 6749 already recommends
exact matching if the full URL had been registered.
o Clients with multiple redirect URIs need to register all of them
explicitly.
Note: clients with just a single redirect URI would not even need to
send a redirect URI with the authorization request. Does it make
sense to emphasize this option? Would that further simplify use of
the protocol and foster security?
o Exact redirect matching does not work for native apps utilizing a
local web server due to dynamic port numbers - at least wild cards
for port numbers are required.
Question: Does redirect uri validation solve any problem for native
apps? Effective against impersonation when used in conjunction with
claimed HTTPS redirect URIs only.
For Windows token broker exact redirect URI matching is impotant as
the redirect URI encodes the app identity. For custom scheme
redirects there is a question however it is probably a usfull part of
defense in depth.
Additional recommendations:
o Servers on which callbacks are hosted must not expose open
redirectors (see respective section).
o Clients may drop fragments via intermediary URLs with "fix
fragments" (e.g. https://developers.facebook.com/blog/post/552/)
to prevent the user agent from appending any unintended fragments.
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Alternatives to exact redirect URI matching:
o authenticate client using digital signatures (JAR? https://
tools.ietf.org/html/draft-ietf-oauth-jwsreq-09)
4.2. Authorization code leakage via referrer headers
It is possible authorization codes are unintentionally disclosed to
attackers, if a OAuth client renders a page containing links to other
pages (ads, faq, ...) as result of a successful authorization
request.
If the user clicks onto one of those links and the target is under
the control of an attacker, it can get access to the response URL in
the referrer header.
It is also possible that an attacker injects cross-domain content
somehow into the page, such as ![]()
(f.e. if this is blog web site
etc.): the implication is obviously the same - loading this content
by browser results in leaking referrer with a code.
4.2.1. Proposed Countermeasures
There are some means to prevent leakage as described above:
o Use of the HTML link attribute rel="noreferrer" (Chrome
52.0.2743.116, FF 49.0.1, Edge 38.14393.0.0, IE/Win10)
o Use of the "referrer" meta link attribute (possible values e.g.
noreferrer, origin, ...) (cf. https://w3c.github.io/webappsec-
referrer-policy/ - work in progress (seems Google, Chrome and Edge
support it))
o Redirect to intermediate page (sanitize history) before sending
user agent to other pages
Note: double check redirect/referrer header behavior
o Use form post mode instead of redirect for authorization response
(don't transport credentials via URL parameters and GET)
Note: There shouldn't be a referer header when loading HTTP content
from a HTTPS -loaded page (e.g. help/faq pages)
Note: This kind of attack is not applicable to the implicit grant
since fragments are not be included in referrer headers (cf. https:/
/tools.ietf.org/html/rfc7231#section-5.5.2)
4.3. Attacks in the Browser
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4.3.1. Code in browser history (TBD)
When browser navigates to "client.com/redirection_endpoint?code=abcd"
as a result of a redirect from a provider's authorization endpoint.
Proposed countermeasures: code is one time use, has limited duration,
is bound to client id/secret (confidential clients only)
4.3.2. Access token in browser history (TBD)
When a client or just a web site which already has a token
deliberately navigates to a page like provider.com/
get_user_profile?access_token=abcdef.. Actually RFC6750 discourages
this practice and asks to transfer tokens via a header, but in
practice web sites often just pass access token in query
When browser navigates to client.com/
redirection_endpoint#access_token=abcef as a result of a redirect
from a provider's authorization endpoint.
Proposal: replace implicit flow with postmessage communication
4.3.3. Javascript Code stealing Access Tokens (TBD)
sandboxing using service workers
4.4. Dynamic OAuth Scenarios
OAuth initially assumed a static relationship between client,
authorization server and resource servers. The URLs of AS and RS
were know to the client at deployment time and built an anchor for
the trust relationsship among those parties. The validation whether
the client talks to a legitimate server is based on TLS server
authentication (see [RFC6819], Section 4.5.4).
With the increasing adoption of OAuth, this simple model dissolved
and, in several scenarios, was replaced by a dynamic establishment of
the relationship between clients on one side and the authorization
and resource servers of a particular deployment on the other side.
This way the same client can be used to access services of different
providers (in case of standard APIs, such as e-Mail or OpenID
Connect) or serves as a frontend to a particular tenant in a multi-
tenancy.
Extensions of OAuth, such as [RFC7591] and [I-D.ietf-oauth-discovery]
were developed in order to support the usage of OAuth in dynamic
scenarios.
As a challenge to the community, such usage scenarios open up new
attack angles, which are discussed in this section.
4.4.1. Access Token Phishing by Counterfeit Resource Server
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An attacker may pretend to be a particular resource server and to
accept tokens from a particular authorization server. If the client
sends a valid access token to this counterfeit resource server, the
server in turn may use that token to access other services on behalf
of the resource owner.
Potential mitigation strategies:
o AS may publish information about its legitimate resource servers,
clients must only send access tokens to this servers
o Clients indicate resource server they intend to use the access
token for at AS, AS may refuse to issue tokens for ressource
servers it does not know
o AS indicates resource servers a particular access token is good
for to client - client enforced audience restriction - prevents
disclosure (e.g. OAuth Response Metadata (https://tools.ietf.org/
html/draft-sakimura-oauth-meta-07)
o Access tokens are audience restricted - prevents replay if the
audience is a URL determined by the client, reduces impact in case
of legitimate resource server uses token at other resource server
(e.g. https://tools.ietf.org/html/draft-campbell-oauth-resource-
indicators-01)
o Access Token is sender restricted - sender is cryptographically
verified
* https://tools.ietf.org/html/draft-ietf-oauth-pop-
architecture-08
* https://tools.ietf.org/html/draft-jones-oauth-token-binding-00
* https://datatracker.ietf.org/doc/draft-campbell-oauth-mtls
* https://datatracker.ietf.org/doc/html/draft-sakimura-oauth-jpop
4.4.2. Mix-Up
Mix-up is another kind of attack on more dynamic OAuth scenarios (or
at least scenarios where a OAuth client interacts with multiple
authorization servers). The goal of the attack is to obtain an
authorization code or an access token by tricking the client into
sending those credentials to the attacker (which acts as MITM between
client and authorization server)
A detailed description of the attack and potential countermeasures is
given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-
mitigation-01.
Potential mitigations:
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o AS returns client_id and its iss in the response. Client compares
this data to AS it believed it sent the user agent to.
o ID token carries client id and issuer (requires OpenID Connect)
o Clients use AS-specific redirect URIs, for every authorization
request store intended AS and compare intention with actual
redirect URI where the response was received (no change to OAuth
required)
5. OAuth Credentials Injection
Credential injection means an attacker somehow obtained a valid OAuth
credential (code or token) and is able to utilize this to impersonate
the legitimate resource owner or to cause a victim to access
resources under the attacker's control (XSRF).
5.1. Code Injection
In such an attack, the adversary attempts to inject a stolen
authorization code into a legitimate client on a device under his
control. In the simplest case, the attacker would want to use the
code in his own client. But there are situations where this might
not be possible or intended. Example are:
o The code is bound to a particular confidential client and the
attacker is unable to obtain the required client credentials to
redeem the code himself and/or
o The attacker wants to access certain functions in this particular
client. As an example, the attacker potentially wants to
impersonate his victim in a certain app.
o Another example could be that access to the authorization and
resource servers is some how limited to networks, the attackers is
unable to access directly.
How does an attack look like?
(1 )The attacker obtains an authorization code by executing any of
the attacks described above (OAuth Credentials Leakage).
(2 )It performs an OAuth authorization process with the legitimate
client on his device.
(3 )The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client.
(4 )The client sends the code to the authorization server's token
endpoint, along with client id, client secret and actual
redirect_uri.
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(5 )The authorization server checks the client secret, whether the
code was issued to the particular client and whether the actual
redirect URI matches the redirect_uri parameter.
(6 )If all checks succeed, the authorization server issues access and
other tokens to the client.
(7 )The attacker just impersonated the victim.
Obviously, the check in step (5) will fail, if the code was issued to
another client id, e.g. a client set up by the attacker.
An attempt to inject a code obtained via a malware pretending to be
the legitimate client should also be detected, if the authorization
server stored the complete redirect URI used in the authorization
request and compares it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the AS to ... "ensure that the
"redirect_uri" parameter is present if the "redirect_uri" parameter
was included in the initial authorization request as described in
Section 4.1.1, and if included ensure that their values are
identical." In the attack scenario described above, the legitimate
client would use the correct redirect URI it always uses for
authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not
land at the attackers page). So the authorization server would detect
the attack and refuse to exchange the code.
Note: this check could also detect attempt to inject a code, which
had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled:
o the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
o the client has bound this data to this particular instance
But this approach conflicts with the idea to enforce exact redirect
URI matching at the authorization endpoint. Moreover, it has been
observed that providers very often ignore the redirect_uri check
requirement at this stage, maybe, because it doesn't seem to be
security-critical from reading the spec.
Other providers just pattern match the redirect_uri parameter against
the registered redirect URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the
respective authorization code for every transaction. But this kind
of check obviously does not fulfill the intent of the spec, since the
tampered redirect URI is not considered. So any attempt to inject a
code obtained using the client_id of a legitimate client or by
utilizing the legitimate client on another device won't be detected
in the respective deployments.
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It is also assumed that the requirements defined in [RFC6749],
Section 4.1.3, increase client implementation complexity as clients
need to memorize or re-construct the correct redirect URI for the
call to the tokens endpoint.
The authors therefore propose to the working group to drop this
feature in favor of more effective and (hopefully) simpler approaches
to code injection prevention as described in the following section.
5.1.1. Proposed Countermeasures
The general proposal is to bind every particular authorization code
to a certain client on a certain device (or in a certain user agent)
in the context of a certain transaction. There are multiple
technical solutions to achieve this goal:
Nonce OpenID Connect's existing "nonce" parameter is used for this
purpose. The nonce value is one time use and created by the
client. The client is supposed to bind it to the user agent
session and sends it with the initial request to the OpenId
Provider (OP). The OP associates the nonce to the
authorization code and attests this binding in the ID token,
which is issued as part of the code exchange at the token
endpoint. If an attacker injected an authorization code in
the authorization response, the nonce value in the client
session and the nonce value in the ID token will not match
and the attack is detected. assumption: attacker cannot get
hold of the user agent state on the victims device, where he
has stolen the respective authorization code.
pro:
- existing feature, used in the wild
con:
- OAuth does not have an ID Token - would need to push that
down the stack
Code-bound State It has been discussed in the security workshop in
December to use the OAuth state value much similar in the way
as described above. In the case of the state value, the idea
is to add a further parameter state to the code exchange
request. The authorization server then compares the state
value it associated with the code and the state value in the
parameter. If those values do not match, it is considered an
attack and the request fails. Note: a variant of this
solution would be send a hash of the state (in order to
prevent bulky requests and DoS).
pro:
- use existing concept
con:
- state needs to fulfil certain requirements (one time use,
complexity)
- new parameter means normative spec change
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PKCE Basically, the PKCE challenge/verifier could be used in the
same way as Nonce or State. In contrast to its original
intention, the verifier check would fail although the client
uses its correct verifier but the code is associated with a
challenge, which does not match.
pro:
- existing and deployed OAuth feature
con:
- currently used and recommended for native apps, not web
apps
Token Binding Code must be bind to UA-AS and UA-Client legs -
requires further data (extension to response) to manifest
binding id for particular code.
Note: token binding could be used in conjunction with PKCE as
an option (https://tools.ietf.org/html/draft-campbell-oauth-
tbpkce).
pro:
- highly secure
con:
- highly sophisticated, requires browser support, will it
work for native apps?
per instance client id/secret ...
Note on pre-warmed secrets: An attacker can circumvent the
countermeasures described above if he is able to create or capture
the respective secret or code_challenge on a device under his
control, which is then used in the victim's authorization request.
Exact redirect URI matching of authorization requests can prevent the
attacker from using the pre-warmed secret in the faked authorization
transaction on the victim's device.
Unfortunately it does not work for all kinds of OAuth clients. It is
effective for web and JS apps and for native apps with claimed URLs.
What about other native apps? Treat nonce or PKCE challenge as
replay detection tokens (needs to ensure cluster-wide one-time use)?
5.1.2. Access Token Injection (TBD)
Note: An attacker in possession of an access token can access any
resources the access token gives him the permission to. This kind of
attacks simply illustrates the fact that bearer tokens utilized by
OAuth are reusable similar to passwords unless they are protected by
further means.
(where do we treat access token replay/use at the resource server?
https://tools.ietf.org/html/rfc6819#section-4.6.4 has some text about
it but is it sufficient?)
The attack described in this section is about injecting a stolen
access token into a legitimate client on a device under the
adversaries control. The attacker wants to impersonate a victim and
cannot use his own client, since he wants to access certain functions
in this particular client.
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Proposal: token binding, hybrid flow+nonce(OIDC), other
cryptographical binding between access token and user agent instance
5.1.3. XSRF (TBD)
injection of code or access token on a victim's device (e.g. to
cause client to access resources under the attacker's control)
mitigation: XSRF tokens (one time use) w/ user agent binding (cf.
https://www.owasp.org/index.php/
CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)
6. Other Attacks
Using the AS as Open Redirector - error handling AS (redirects)
(draft-ietf-oauth-closing-redirectors-00)
Using the Client as Open Redirector
redirect via status code 307 - use 302
7. Other Topics
why to rotate refresh tokens
why audience restriction
how to support multi AS per RS
...
differentiate native, JS and web clients
federated login to apps (code flow to own AS in browser and federated
login to 3rd party IDP in browser)
do not put sensitive data in URL/GET parameters (Jim Manico)
8. Acknowledgements
We would like to thank Jim Manico and Phil Hunt for their valuable
feedback.
9. IANA Considerations
This draft includes no request to IANA.
10. Security Considerations
All relevant security considerations have been given in the
functional specification.
11. References
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11.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, DOI 10.17487/RFC3986, January 2005, .
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012, .
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/
RFC6750, October 2012, .
[RFC6819] Lodderstedt, T., Ed., McGloin, M. and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819, DOI
10.17487/RFC6819, January 2013, .
[RFC7231] Fielding, R.Ed., and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
10.17487/RFC7231, June 2014, .
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M. and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015, .
11.2. Informative References
[I-D.bradley-oauth-jwt-encoded-state]
Bradley, J., Lodderstedt, T. and H. Zandbelt, "Encoding
claims in the OAuth 2 state parameter using a JWT",
Internet-Draft draft-bradley-oauth-jwt-encoded-state-07,
March 2017.
[I-D.ietf-oauth-discovery]
Jones, M., Sakimura, N. and J. Bradley, "OAuth 2.0
Authorization Server Metadata", Internet-Draft draft-ietf-
oauth-discovery-04, August 2016.
[RFC7636] Sakimura, N., Ed., Bradley, J. and N. Agarwal, "Proof Key
for Code Exchange by OAuth Public Clients", RFC 7636, DOI
10.17487/RFC7636, September 2015, .
[owasp] "Open Web Application Security Project Home Page", ,
.
Appendix A. Document History
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[[ To be removed from the final specification ]]
-01
o Added references to mitigation methods for token leakage
o Added reference to Token Binding for Authorization Code
o incorporated feedback of Phil Hunt
o fixed numbering issue in attack descriptions in section 2
-00 (WG document)
o turned the ID into a WG document and a BCP
o Added federated app login as topic in Other Topics
Authors' Addresses
Torsten Lodderstedt, editor
YES Europe AG
Email: torsten@lodderstedt.net
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
Andrey Labunets
Facebook
Email: isciurus@fb.com
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