4.5. Refreshing an Access Token
4.5.1. Threat: Eavesdropping Refresh Tokens from Authorization Server
An attacker may eavesdrop refresh tokens when they are transmitted
from the authorization server to the client.
Countermeasures:
o As per the core OAuth spec, the authorization servers must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS (see Section 5.1.1).
o If end-to-end confidentiality cannot be guaranteed, reducing scope
(see Section 5.1.5.1) and expiry time (see Section 5.1.5.3) for
issued access tokens can be used to reduce the damage in case of
leaks.
4.5.2. Threat: Obtaining Refresh Token from Authorization Server
Database
This threat is applicable if the authorization server stores refresh
tokens as handles in a database. An attacker may obtain refresh
tokens from the authorization server's database by gaining access to
the database or launching a SQL injection attack.
Impact: Disclosure of all refresh tokens.
Countermeasures:
o Enforce credential storage protection best practices
(Section 5.1.4.1).
o Bind token to client id, if the attacker cannot obtain the
required id and secret (Section 5.1.5.8).
4.5.3. Threat: Obtaining Refresh Token by Online Guessing
An attacker may try to guess valid refresh token values and send it
using the grant type "refresh_token" in order to obtain a valid
access token.
Impact: Exposure of a single refresh token and derivable access
tokens.
Countermeasures:
o For handle-based designs (Section 5.1.4.2.2).
o For assertion-based designs (Section 5.1.5.9).
o Bind token to client id, because the attacker would guess the
matching client id, too (see Section 5.1.5.8).
o Authenticate the client; this adds another element that the
attacker has to guess (see Section 5.2.3.4).
4.5.4. Threat: Refresh Token Phishing by Counterfeit Authorization
Server
An attacker could try to obtain valid refresh tokens by proxying
requests to the authorization server. Given the assumption that the
authorization server URL is well-known at development time or can at
least be obtained from a well-known resource server, the attacker
must utilize some kind of spoofing in order to succeed.
Countermeasures:
o Utilize server authentication (as described in Section 5.1.2).
4.6. Accessing Protected Resources
4.6.1. Threat: Eavesdropping Access Tokens on Transport
An attacker could try to obtain a valid access token on transport
between the client and resource server. As access tokens are shared
secrets between the authorization server and resource server, they
should be treated with the same care as other credentials (e.g., end-
user passwords).
Countermeasures:
o Access tokens sent as bearer tokens should not be sent in the
clear over an insecure channel. As per the core OAuth spec,
transmission of access tokens must be protected using transport-
layer mechanisms such as TLS (see Section 5.1.1).
o A short lifetime reduces impact in case tokens are compromised
(see Section 5.1.5.3).
o The access token can be bound to a client's identifier and require
the client to prove legitimate ownership of the token to the
resource server (see Section 5.4.2).
4.6.2. Threat: Replay of Authorized Resource Server Requests
An attacker could attempt to replay valid requests in order to obtain
or to modify/destroy user data.
Countermeasures:
o The resource server should utilize transport security measures
(e.g., TLS) in order to prevent such attacks (see Section 5.1.1).
This would prevent the attacker from capturing valid requests.
o Alternatively, the resource server could employ signed requests
(see Section 5.4.3) along with nonces and timestamps in order to
uniquely identify requests. The resource server should detect and
refuse every replayed request.
4.6.3. Threat: Guessing Access Tokens
Where the token is a handle, the attacker may attempt to guess the
access token values based on knowledge they have from other access
tokens.
Impact: Access to a single user's data.
Countermeasures:
o Handle tokens should have a reasonable level of entropy (see
Section 5.1.4.2.2) in order to make guessing a valid token value
infeasible.
o Assertion (or self-contained token) token contents should be
protected by a digital signature (see Section 5.1.5.9).
o Security can be further strengthened by using a short access token
duration (see Sections 5.1.5.2 and 5.1.5.3).
4.6.4. Threat: Access Token Phishing by Counterfeit Resource Server
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.
Countermeasures:
o Clients should not make authenticated requests with an access
token to unfamiliar resource servers, regardless of the presence
of a secure channel. If the resource server URL is well-known to
the client, it may authenticate the resource servers (see
Section 5.1.2).
o Associate the endpoint URL of the resource server the client
talked to with the access token (e.g., in an audience field) and
validate the association at a legitimate resource server. The
endpoint URL validation policy may be strict (exact match) or more
relaxed (e.g., same host). This would require telling the
authorization server about the resource server endpoint URL in the
authorization process.
o Associate an access token with a client and authenticate the
client with resource server requests (typically via a signature,
in order to not disclose a secret to a potential attacker). This
prevents the attack because the counterfeit server is assumed to
lack the capability to correctly authenticate on behalf of the
legitimate client to the resource server (Section 5.4.2).
o Restrict the token scope (see Section 5.1.5.1) and/or limit the
token to a certain resource server (Section 5.1.5.5).
4.6.5. Threat: Abuse of Token by Legitimate Resource Server or Client
A legitimate resource server could attempt to use an access token to
access another resource server. Similarly, a client could try to use
a token obtained for one server on another resource server.
Countermeasures:
o Tokens should be restricted to particular resource servers (see
Section 5.1.5.5).
4.6.6. Threat: Leak of Confidential Data in HTTP Proxies
An OAuth HTTP authentication scheme as discussed in [RFC6749] is
optional. However, [RFC2616] relies on the Authorization and
WWW-Authenticate headers to distinguish authenticated content so that
it can be protected. Proxies and caches, in particular, may fail to
adequately protect requests not using these headers. For example,
private authenticated content may be stored in (and thus be
retrievable from) publicly accessible caches.
Countermeasures:
o Clients and resource servers not using an OAuth HTTP
authentication scheme (see Section 5.4.1) should take care to use
Cache-Control headers to minimize the risk that authenticated
content is not protected. Such clients should send a
Cache-Control header containing the "no-store" option [RFC2616].
Resource server success (2XX status) responses to these requests
should contain a Cache-Control header with the "private" option
[RFC2616].
o Reducing scope (see Section 5.1.5.1) and expiry time
(Section 5.1.5.3) for access tokens can be used to reduce the
damage in case of leaks.
4.6.7. Threat: Token Leakage via Log Files and HTTP Referrers
If access tokens are sent via URI query parameters, such tokens may
leak to log files and the HTTP "referer".
Countermeasures:
o Use Authorization headers or POST parameters instead of URI
request parameters (see Section 5.4.1).
o Set logging configuration appropriately.
o Prevent unauthorized persons from access to system log files (see
Section 5.1.4.1.1).
o Abuse of leaked access tokens can be prevented by enforcing
authenticated requests (see Section 5.4.2).
o The impact of token leakage may be reduced by limiting scope (see
Section 5.1.5.1) and duration (see Section 5.1.5.3) and by
enforcing one-time token usage (see Section 5.1.5.4).
5. Security Considerations
This section describes the countermeasures as recommended to mitigate
the threats described in Section 4.
5.1. General
This section covers considerations that apply generally across all
OAuth components (client, resource server, token server, and user
agents).
5.1.1. Ensure Confidentiality of Requests
This is applicable to all requests sent from the client to the
authorization server or resource server. While OAuth provides a
mechanism for verifying the integrity of requests, it provides no
guarantee of request confidentiality. Unless further precautions are
taken, eavesdroppers will have full access to request content and may
be able to mount interception or replay attacks by using the contents
of requests, e.g., secrets or tokens.
Attacks can be mitigated by using transport-layer mechanisms such as
TLS [RFC5246]. A virtual private network (VPN), e.g., based on IPsec
VPNs [RFC4301], may be considered as well.
Note: This document assumes end-to-end TLS protected connections
between the respective protocol entities. Deployments deviating from
this assumption by offloading TLS in between (e.g., on the data
center edge) must refine this threat model in order to account for
the additional (mainly insider) threat this may cause.
This is a countermeasure against the following threats:
o Replay of access tokens obtained on the token's endpoint or the
resource server's endpoint
o Replay of refresh tokens obtained on the token's endpoint
o Replay of authorization "codes" obtained on the token's endpoint
(redirect?)
o Replay of user passwords and client secrets
5.1.2. Utilize Server Authentication
HTTPS server authentication or similar means can be used to
authenticate the identity of a server. The goal is to reliably bind
the fully qualified domain name of the server to the public key
presented by the server during connection establishment (see
[RFC2818]).
The client should validate the binding of the server to its domain
name. If the server fails to prove that binding, the communication
is considered a man-in-the-middle attack. This security measure
depends on the certification authorities the client trusts for that
purpose. Clients should carefully select those trusted CAs and
protect the storage for trusted CA certificates from modifications.
This is a countermeasure against the following threats:
o Spoofing
o Proxying
o Phishing by counterfeit servers
5.1.3. Always Keep the Resource Owner Informed
Transparency to the resource owner is a key element of the OAuth
protocol. The user should always be in control of the authorization
processes and get the necessary information to make informed
decisions. Moreover, user involvement is a further security
countermeasure. The user can probably recognize certain kinds of
attacks better than the authorization server. Information can be
presented/exchanged during the authorization process, after the
authorization process, and every time the user wishes to get informed
by using techniques such as:
o User consent forms.
o Notification messages (e.g., email, SMS, ...). Note that
notifications can be a phishing vector. Messages should be such
that look-alike phishing messages cannot be derived from them.
o Activity/event logs.
o User self-care applications or portals.
5.1.4. Credentials
This section describes countermeasures used to protect all kinds of
credentials from unauthorized access and abuse. Credentials are
long-term secrets, such as client secrets and user passwords as well
as all kinds of tokens (refresh and access tokens) or authorization
"codes".
5.1.4.1. Enforce Credential Storage Protection Best Practices
Administrators should undertake industry best practices to protect
the storage of credentials (for example, see [OWASP]). Such
practices may include but are not limited to the following
sub-sections.
5.1.4.1.1. Enforce Standard System Security Means
A server system may be locked down so that no attacker may get access
to sensitive configuration files and databases.
5.1.4.1.2. Enforce Standard SQL Injection Countermeasures
If a client identifier or other authentication component is queried
or compared against a SQL database, it may become possible for an
injection attack to occur if parameters received are not validated
before submission to the database.
o Ensure that server code is using the minimum database privileges
possible to reduce the "surface" of possible attacks.
o Avoid dynamic SQL using concatenated input. If possible, use
static SQL.
o When using dynamic SQL, parameterize queries using bind arguments.
Bind arguments eliminate the possibility of SQL injections.
o Filter and sanitize the input. For example, if an identifier has
a known format, ensure that the supplied value matches the
identifier syntax rules.
5.1.4.1.3. No Cleartext Storage of Credentials
The authorization server should not store credentials in clear text.
Typical approaches are to store hashes instead or to encrypt
credentials. If the credential lacks a reasonable entropy level
(because it is a user password), an additional salt will harden the
storage to make offline dictionary attacks more difficult.
Note: Some authentication protocols require the authorization server
to have access to the secret in the clear. Those protocols cannot be
implemented if the server only has access to hashes. Credentials
should be strongly encrypted in those cases.
5.1.4.1.4. Encryption of Credentials
For client applications, insecurely persisted client credentials are
easy targets for attackers to obtain. Store client credentials using
an encrypted persistence mechanism such as a keystore or database.
Note that compiling client credentials directly into client code
makes client applications vulnerable to scanning as well as difficult
to administer should client credentials change over time.
5.1.4.1.5. Use of Asymmetric Cryptography
Usage of asymmetric cryptography will free the authorization server
of the obligation to manage credentials.
5.1.4.2. Online Attacks on Secrets
5.1.4.2.1. Utilize Secure Password Policy
The authorization server may decide to enforce a complex user
password policy in order to increase the user passwords' entropy to
hinder online password attacks. Note that too much complexity can
increase the likelihood that users re-use passwords or write them
down, or otherwise store them insecurely.
5.1.4.2.2. Use High Entropy for Secrets
When creating secrets not intended for usage by human users (e.g.,
client secrets or token handles), the authorization server should
include a reasonable level of entropy in order to mitigate the risk
of guessing attacks. The token value should be >=128 bits long and
constructed from a cryptographically strong random or pseudo-random
number sequence (see [RFC4086] for best current practice) generated
by the authorization server.
5.1.4.2.3. Lock Accounts
Online attacks on passwords can be mitigated by locking the
respective accounts after a certain number of failed attempts.
Note: This measure can be abused to lock down legitimate service
users.
5.1.4.2.4. Use Tar Pit
The authorization server may react on failed attempts to authenticate
by username/password by temporarily locking the respective account
and delaying the response for a certain duration. This duration may
increase with the number of failed attempts. The objective is to
slow the attacker's attempts on a certain username down.
Note: This may require a more complex and stateful design of the
authorization server.
5.1.4.2.5. Use CAPTCHAs
The idea is to prevent programs from automatically checking a huge
number of passwords, by requiring human interaction.
Note: This has a negative impact on user experience.
5.1.5. Tokens (Access, Refresh, Code)
5.1.5.1. Limit Token Scope
The authorization server may decide to reduce or limit the scope
associated with a token. The basis of this decision is out of scope;
examples are:
o a client-specific policy, e.g., issue only less powerful tokens to
public clients,
o a service-specific policy, e.g., it is a very sensitive service,
o a resource-owner-specific setting, or
o combinations of such policies and preferences.
The authorization server may allow different scopes dependent on the
grant type. For example, end-user authorization via direct
interaction with the end user (authorization "code") might be
considered more reliable than direct authorization via grant type
"username"/"password". This means will reduce the impact of the
following threats:
o token leakage
o token issuance to malicious software
o unintended issuance of powerful tokens with resource owner
credentials flow
5.1.5.2. Determine Expiration Time
Tokens should generally expire after a reasonable duration. This
complements and strengthens other security measures (such as
signatures) and reduces the impact of all kinds of token leaks.
Depending on the risk associated with token leakage, tokens may
expire after a few minutes (e.g., for payment transactions) or stay
valid for hours (e.g., read access to contacts).
The expiration time is determined by several factors, including:
o risk associated with token leakage,
o duration of the underlying access grant,
o duration until the modification of an access grant should take
effect, and
o time required for an attacker to guess or produce a valid token.
5.1.5.3. Use Short Expiration Time
A short expiration time for tokens is a means of protection against
the following threats:
o replay
o token leak (a short expiration time will reduce impact)
o online guessing (a short expiration time will reduce the
likelihood of success)
Note: Short token duration requires more precise clock
synchronization between the authorization server and resource server.
Furthermore, shorter duration may require more token refreshes
(access token) or repeated end-user authorization processes
(authorization "code" and refresh token).
5.1.5.4. Limit Number of Usages or One-Time Usage
The authorization server may restrict the number of requests or
operations that can be performed with a certain token. This
mechanism can be used to mitigate the following threats:
o replay of tokens
o guessing
For example, if an authorization server observes more than one
attempt to redeem an authorization "code", the authorization server
may want to revoke all access tokens granted based on the
authorization "code" as well as reject the current request.
As with the authorization "code", access tokens may also have a
limited number of operations. This either forces client applications
to re-authenticate and use a refresh token to obtain a fresh access
token, or forces the client to re-authorize the access token by
involving the user.
5.1.5.5. Bind Tokens to a Particular Resource Server (Audience)
Authorization servers in multi-service environments may consider
issuing tokens with different content to different resource servers
and to explicitly indicate in the token the target server to which a
token is intended to be sent. SAML assertions (see
[OASIS.saml-core-2.0-os]) use the Audience element for this purpose.
This countermeasure can be used in the following situations:
o It reduces the impact of a successful replay attempt, since the
token is applicable to a single resource server only.
o It prevents abuse of a token by a rogue resource server or client,
since the token can only be used on that server. It is rejected
by other servers.
o It reduces the impact of leakage of a valid token to a counterfeit
resource server.
5.1.5.6. Use Endpoint Address as Token Audience
This may be used to indicate to a resource server which endpoint URL
has been used to obtain the token. This measure will allow the
detection of requests from a counterfeit resource server, since such
a token will contain the endpoint URL of that server.
5.1.5.7. Use Explicitly Defined Scopes for Audience and Tokens
Deployments may consider only using tokens with explicitly defined
scopes, where every scope is associated with a particular resource
server. This approach can be used to mitigate attacks where a
resource server or client uses a token for a different purpose than
the one intended.
5.1.5.8. Bind Token to Client id
An authorization server may bind a token to a certain client
identifier. This identifier should be validated for every request
with that token. This technique can be used to
o detect token leakage and
o prevent token abuse.
Note: Validating the client identifier may require the target server
to authenticate the client's identifier. This authentication can be
based on secrets managed independently of the token (e.g.,
pre-registered client id/secret on authorization server) or sent with
the token itself (e.g., as part of the encrypted token content).
5.1.5.9. Sign Self-Contained Tokens
Self-contained tokens should be signed in order to detect any attempt
to modify or produce faked tokens (e.g., Hash-based Message
Authentication Code or digital signatures).
5.1.5.10. Encrypt Token Content
Self-contained tokens may be encrypted for confidentiality reasons or
to protect system internal data. Depending on token format, keys
(e.g., symmetric keys) may have to be distributed between server
nodes. The method of distribution should be defined by the token and
the encryption used.
5.1.5.11. Adopt a Standard Assertion Format
For service providers intending to implement an assertion-based token
design, it is highly recommended to adopt a standard assertion format
(such as SAML [OASIS.saml-core-2.0-os] or the JavaScript Object
Notation Web Token (JWT) [OAuth-JWT]).
5.1.6. Access Tokens
The following measures should be used to protect access tokens:
o Keep them in transient memory (accessible by the client
application only).
o Pass tokens securely using secure transport (TLS).
o Ensure that client applications do not share tokens with 3rd
parties.
5.2. Authorization Server
This section describes considerations related to the OAuth
authorization server endpoint.
5.2.1. Authorization "codes"
5.2.1.1. Automatic Revocation of Derived Tokens If Abuse Is Detected
If an authorization server observes multiple attempts to redeem an
authorization grant (e.g., such as an authorization "code"), the
authorization server may want to revoke all tokens granted based on
the authorization grant.
5.2.2. Refresh Tokens
5.2.2.1. Restricted Issuance of Refresh Tokens
The authorization server may decide, based on an appropriate policy,
not to issue refresh tokens. Since refresh tokens are long-term
credentials, they may be subject to theft. For example, if the
authorization server does not trust a client to securely store such
tokens, it may refuse to issue such a client a refresh token.
5.2.2.2. Binding of Refresh Token to "client_id"
The authorization server should match every refresh token to the
identifier of the client to whom it was issued. The authorization
server should check that the same "client_id" is present for every
request to refresh the access token. If possible (e.g., confidential
clients), the authorization server should authenticate the respective
client.
This is a countermeasure against refresh token theft or leakage.
Note: This binding should be protected from unauthorized
modifications.
5.2.2.3. Refresh Token Rotation
Refresh token rotation is intended to automatically detect and
prevent attempts to use the same refresh token in parallel from
different apps/devices. This happens if a token gets stolen from the
client and is subsequently used by both the attacker and the
legitimate client. The basic idea is to change the refresh token
value with every refresh request in order to detect attempts to
obtain access tokens using old refresh tokens. Since the
authorization server cannot determine whether the attacker or the
legitimate client is trying to access, in case of such an access
attempt the valid refresh token and the access authorization
associated with it are both revoked.
The OAuth specification supports this measure in that the token's
response allows the authorization server to return a new refresh
token even for requests with grant type "refresh_token".
Note: This measure may cause problems in clustered environments,
since usage of the currently valid refresh token must be ensured. In
such an environment, other measures might be more appropriate.
5.2.2.4. Revocation of Refresh Tokens
The authorization server may allow clients or end users to explicitly
request the invalidation of refresh tokens. A mechanism to revoke
tokens is specified in [OAuth-REVOCATION].
This is a countermeasure against:
o device theft,
o impersonation of a resource owner, or
o suspected compromised client applications.
5.2.2.5. Device Identification
The authorization server may require the binding of authentication
credentials to a device identifier. The International Mobile Station
Equipment Identity [IMEI] is one example of such an identifier; there
are also operating system-specific identifiers. The authorization
server could include such an identifier when authenticating user
credentials in order to detect token theft from a particular device.
Note: Any implementation should consider potential privacy
implications of using device identifiers.
5.2.2.6. X-FRAME-OPTIONS Header
For newer browsers, avoidance of iFrames can be enforced on the
server side by using the X-FRAME-OPTIONS header (see
[X-Frame-Options]). This header can have two values, "DENY" and
"SAMEORIGIN", which will block any framing or any framing by sites
with a different origin, respectively. The value "ALLOW-FROM"
specifies a list of trusted origins that iFrames may originate from.
This is a countermeasure against the following threat:
o Clickjacking attacks
5.2.3. Client Authentication and Authorization
As described in Section 3 (Security Features), clients are
identified, authenticated, and authorized for several purposes, such
as to:
o Collate requests to the same client,
o Indicate to the user that the client is recognized by the
authorization server,
o Authorize access of clients to certain features on the
authorization server or resource server, and
o Log a client identifier to log files for analysis or statistics.
Due to the different capabilities and characteristics of the
different client types, there are different ways to support these
objectives, which will be described in this section. Authorization
server providers should be aware of the security policy and
deployment of a particular client and adapt its treatment
accordingly. For example, one approach could be to treat all clients
as less trustworthy and unsecure. On the other extreme, a service
provider could activate every client installation individually by an
administrator and in that way gain confidence in the identity of the
software package and the security of the environment in which the
client is installed. There are several approaches in between.
5.2.3.1. Don't Issue Secrets to Clients with Inappropriate Security
Policy
Authorization servers should not issue secrets to clients that cannot
protect secrets ("public" clients). This reduces the probability of
the server treating the client as strongly authenticated.
For example, it is of limited benefit to create a single client id
and secret that are shared by all installations of a native
application. Such a scenario requires that this secret must be
transmitted from the developer via the respective distribution
channel, e.g., an application market, to all installations of the
application on end-user devices. A secret, burned into the source
code of the application or an associated resource bundle, is not
protected from reverse engineering. Secondly, such secrets cannot be
revoked, since this would immediately put all installations out of
work. Moreover, since the authorization server cannot really trust
the client's identifier, it would be dangerous to indicate to end
users the trustworthiness of the client.
There are other ways to achieve a reasonable security level, as
described in the following sections.
5.2.3.2. Require User Consent for Public Clients without Secret
Authorization servers should not allow automatic authorization for
public clients. The authorization server may issue an individual
client id but should require that all authorizations are approved by
the end user. For clients without secrets, this is a countermeasure
against the following threat:
o Impersonation of public client applications.
5.2.3.3. Issue a "client_id" Only in Combination with "redirect_uri"
The authorization server may issue a "client_id" and bind the
"client_id" to a certain pre-configured "redirect_uri". Any
authorization request with another redirect URI is refused
automatically. Alternatively, the authorization server should not
accept any dynamic redirect URI for such a "client_id" and instead
should always redirect to the well-known pre-configured redirect URI.
This is a countermeasure for clients without secrets against the
following threats:
o Cross-site scripting attacks
o Impersonation of public client applications
5.2.3.4. Issue Installation-Specific Client Secrets
An authorization server may issue separate client identifiers and
corresponding secrets to the different installations of a particular
client (i.e., software package). The effect of such an approach
would be to turn otherwise "public" clients back into "confidential"
clients.
For web applications, this could mean creating one "client_id" and
"client_secret" for each web site on which a software package is
installed. So, the provider of that particular site could request a
client id and secret from the authorization server during the setup
of the web site. This would also allow the validation of some of the
properties of that web site, such as redirect URI, web site URL, and
whatever else proves useful. The web site provider has to ensure the
security of the client secret on the site.
For native applications, things are more complicated because every
copy of a particular application on any device is a different
installation. Installation-specific secrets in this scenario will
require obtaining a "client_id" and "client_secret" either
1. during the download process from the application market, or
2. during installation on the device.
Either approach will require an automated mechanism for issuing
client ids and secrets, which is currently not defined by OAuth.
The first approach would allow the achievement of a certain level of
trust in the authenticity of the application, whereas the second
option only allows the authentication of the installation but not the
validation of properties of the client. But this would at least help
to prevent several replay attacks. Moreover, installation-specific
"client_ids" and secrets allow the selective revocation of all
refresh tokens of a specific installation at once.
5.2.3.5. Validate Pre-Registered "redirect_uri"
An authorization server should require all clients to register their
"redirect_uri", and the "redirect_uri" should be the full URI as
defined in [RFC6749]. The way that this registration is performed is
out of scope of this document. As per the core spec, every actual
redirect URI sent with the respective "client_id" to the end-user
authorization endpoint must match the registered redirect URI. Where
it does not match, the authorization server should assume that the
inbound GET request has been sent by an attacker and refuse it.
Note: The authorization server should not redirect the user agent
back to the redirect URI of such an authorization request.
Validating the pre-registered "redirect_uri" is a countermeasure
against the following threats:
o Authorization "code" leakage through counterfeit web site: allows
authorization servers to detect attack attempts after the first
redirect to an end-user authorization endpoint (Section 4.4.1.7).
o Open redirector attack via a client redirection endpoint
(Section 4.1.5).
o Open redirector phishing attack via an authorization server
redirection endpoint (Section 4.2.4).
The underlying assumption of this measure is that an attacker will
need to use another redirect URI in order to get access to the
authorization "code". Deployments might consider the possibility of
an attacker using spoofing attacks to a victim's device to circumvent
this security measure.
Note: Pre-registering clients might not scale in some deployments
(manual process) or require dynamic client registration (not
specified yet). With the lack of dynamic client registration, a
pre-registered "redirect_uri" only works for clients bound to certain
deployments at development/configuration time. As soon as dynamic
resource server discovery is required, the pre-registered
"redirect_uri" may no longer be feasible.
5.2.3.6. Revoke Client Secrets
An authorization server may revoke a client's secret in order to
prevent abuse of a revealed secret.
Note: This measure will immediately invalidate any authorization
"code" or refresh token issued to the respective client. This might
unintentionally impact client identifiers and secrets used across
multiple deployments of a particular native or web application.
This a countermeasure against:
o Abuse of revealed client secrets for private clients
5.2.3.7. Use Strong Client Authentication (e.g., client_assertion/
client_token)
By using an alternative form of authentication such as client
assertion [OAuth-ASSERTIONS], the need to distribute a
"client_secret" is eliminated. This may require the use of a secure
private key store or other supplemental authentication system as
specified by the client assertion issuer in its authentication
process.
5.2.4. End-User Authorization
This section includes considerations for authorization flows
involving the end user.
5.2.4.1. Automatic Processing of Repeated Authorizations Requires
Client Validation
Authorization servers should NOT automatically process repeat
authorizations where the client is not authenticated through a client
secret or some other authentication mechanism such as a signed
authentication assertion certificate (Section 5.2.3.7) or validation
of a pre-registered redirect URI (Section 5.2.3.5).
5.2.4.2. Informed Decisions Based on Transparency
The authorization server should clearly explain to the end user what
happens in the authorization process and what the consequences are.
For example, the user should understand what access he is about to
grant to which client for what duration. It should also be obvious
to the user whether the server is able to reliably certify certain
client properties (web site URL, security policy).
5.2.4.3. Validation of Client Properties by End User
In the authorization process, the user is typically asked to approve
a client's request for authorization. This is an important security
mechanism by itself because the end user can be involved in the
validation of client properties, such as whether the client name
known to the authorization server fits the name of the web site or
the application the end user is using. This measure is especially
helpful in situations where the authorization server is unable to
authenticate the client. It is a countermeasure against:
o A malicious application
o A client application masquerading as another client
5.2.4.4. Binding of Authorization "code" to "client_id"
The authorization server should bind every authorization "code" to
the id of the respective client that initiated the end-user
authorization process. This measure is a countermeasure against:
o Replay of authorization "codes" with different client credentials,
since an attacker cannot use another "client_id" to exchange an
authorization "code" into a token
o Online guessing of authorization "codes"
Note: This binding should be protected from unauthorized
modifications (e.g., using protected memory and/or a secure
database).
5.2.4.5. Binding of Authorization "code" to "redirect_uri"
The authorization server should be able to bind every authorization
"code" to the actual redirect URI used as the redirect target of the
client in the end-user authorization process. This binding should be
validated when the client attempts to exchange the respective
authorization "code" for an access token. This measure is a
countermeasure against authorization "code" leakage through
counterfeit web sites, since an attacker cannot use another redirect
URI to exchange an authorization "code" into a token.
5.3. Client App Security
This section deals with considerations for client applications.
5.3.1. Don't Store Credentials in Code or Resources Bundled with
Software Packages
Because of the number of copies of client software, there is limited
benefit in creating a single client id and secret that is shared by
all installations of an application. Such an application by itself
would be considered a "public" client, as it cannot be presumed to be
able to keep client secrets. A secret, burned into the source code
of the application or an associated resource bundle, cannot be
protected from reverse engineering. Secondly, such secrets cannot be
revoked, since this would immediately put all installations out of
work. Moreover, since the authorization server cannot really trust
the client's identifier, it would be dangerous to indicate to end
users the trustworthiness of the client.
5.3.2. Use Standard Web Server Protection Measures (for Config Files
and Databases)
Use standard web server protection and configuration measures to
protect the integrity of the server, databases, configuration files,
and other operational components of the server.
5.3.3. Store Secrets in Secure Storage
There are different ways to store secrets of all kinds (tokens,
client secrets) securely on a device or server.
Most multi-user operating systems segregate the personal storage of
different system users. Moreover, most modern smartphone operating
systems even support the storage of application-specific data in
separate areas of file systems and protect the data from access by
other applications. Additionally, applications can implement
confidential data by using a user-supplied secret, such as a PIN or
password.
Another option is to swap refresh token storage to a trusted backend
server. This option in turn requires a resilient authentication
mechanism between the client and backend server. Note: Applications
should ensure that confidential data is kept confidential even after
reading from secure storage, which typically means keeping this data
in the local memory of the application.
5.3.4. Utilize Device Lock to Prevent Unauthorized Device Access
On a typical modern phone, there are many "device lock" options that
can be utilized to provide additional protection when a device is
stolen or misplaced. These include PINs, passwords, and other
biometric features such as "face recognition". These are not equal
in the level of security they provide.
5.3.5. Link the "state" Parameter to User Agent Session
The "state" parameter is used to link client requests and prevent
CSRF attacks, for example, attacks against the redirect URI. An
attacker could inject their own authorization "code" or access token,
which can result in the client using an access token associated with
the attacker's protected resources rather than the victim's (e.g.,
save the victim's bank account information to a protected resource
controlled by the attacker).
The client should utilize the "state" request parameter to send the
authorization server a value that binds the request to the user
agent's authenticated state (e.g., a hash of the session cookie used
to authenticate the user agent) when making an authorization request.
Once authorization has been obtained from the end user, the
authorization server redirects the end-user's user agent back to the
client with the required binding value contained in the "state"
parameter.
The binding value enables the client to verify the validity of the
request by matching the binding value to the user agent's
authenticated state.
5.4. Resource Servers
The following section details security considerations for resource
servers.
5.4.1. Authorization Headers
Authorization headers are recognized and specially treated by HTTP
proxies and servers. Thus, the usage of such headers for sending
access tokens to resource servers reduces the likelihood of leakage
or unintended storage of authenticated requests in general, and
especially Authorization headers.
5.4.2. Authenticated Requests
An authorization server may bind tokens to a certain client
identifier and enable resource servers to validate that association
on resource access. This will require the resource server to
authenticate the originator of a request as the legitimate owner of a
particular token. There are several options to implement this
countermeasure:
o The authorization server may associate the client identifier with
the token (either internally or in the payload of a self-contained
token). The client then uses client certificate-based HTTP
authentication on the resource server's endpoint to authenticate
its identity, and the resource server validates the name with the
name referenced by the token.
o Same as the option above, but the client uses his private key to
sign the request to the resource server (the public key is either
contained in the token or sent along with the request).
o Alternatively, the authorization server may issue a token-bound
key, which the client uses in a Holder-of-Key proof to
authenticate the client's use of the token. The resource server
obtains the secret directly from the authorization server, or the
secret is contained in an encrypted section of the token. In that
way, the resource server does not "know" the client but is able to
validate whether the authorization server issued the token to that
client.
Authenticated requests are a countermeasure against abuse of tokens
by counterfeit resource servers.
5.4.3. Signed Requests
A resource server may decide to accept signed requests only, either
to replace transport-level security measures or to complement such
measures. Every signed request should be uniquely identifiable and
should not be processed twice by the resource server. This
countermeasure helps to mitigate:
o modifications of the message and
o replay attempts
5.5. A Word on User Interaction and User-Installed Apps
OAuth, as a security protocol, is distinctive in that its flow
usually involves significant user interaction, making the end user a
part of the security model. This creates some important difficulties
in defending against some of the threats discussed above. Some of
these points have already been made, but it's worth repeating and
highlighting them here.
o End users must understand what they are being asked to approve
(see Section 5.2.4.2). Users often do not have the expertise to
understand the ramifications of saying "yes" to an authorization
request and are likely not to be able to see subtle differences in
the wording of requests. Malicious software can confuse the user,
tricking the user into approving almost anything.
o End-user devices are prone to software compromise. This has been
a long-standing problem, with frequent attacks on web browsers and
other parts of the user's system. But with the increasing
popularity of user-installed "apps", the threat posed by
compromised or malicious end-user software is very strong and is
one that is very difficult to mitigate.
o Be aware that users will demand to install and run such apps, and
that compromised or malicious ones can steal credentials at many
points in the data flow. They can intercept the very user login
credentials that OAuth is designed to protect. They can request
authorization far beyond what they have led the user to understand
and approve. They can automate a response on behalf of the user,
hiding the whole process. No solution is offered here, because
none is known; this remains in the space between better security
and better usability.
o Addressing these issues by restricting the use of user-installed
software may be practical in some limited environments and can be
used as a countermeasure in those cases. Such restrictions are
not practical in the general case, and mechanisms for after-the-
fact recovery should be in place.
o While end users are mostly incapable of properly vetting
applications they load onto their devices, those who deploy
authorization servers might have tools at their disposal to
mitigate malicious clients. For example, a well-run authorization
server must only assert client properties to the end user it is
effectively capable of validating, explicitly point out which
properties it cannot validate, and indicate to the end user the
risk associated with granting access to the particular client.
[OASIS.sstc-sec-analysis-response-01]
Linn, J., Ed., and P. Mishra, Ed., "SSTC Response to
"Security Analysis of the SAML Single Sign-on Browser/
Artifact Profile"", January 2005,
<http://www.oasis-open.org/committees/download.php/
11191/sstc-gross-sec-analysis-response-01.pdf>.
[OAuth-ASSERTIONS]
Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0", Work in Progress,
December 2012.
[OAuth-HTTP-MAC]
Richer, J., Ed., Mills, W., Ed., and H. Tschofenig, Ed.,
"OAuth 2.0 Message Authentication Code (MAC) Tokens", Work
in Progress, November 2012.
[OAuth-JWT]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", Work in Progress, December 2012.
[OAuth-REVOCATION]
Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "Token
Revocation", Work in Progress, November 2012.
[OPENID] "OpenID Foundation Home Page", <http://openid.net/>.
[OWASP] "Open Web Application Security Project Home Page",
<https://www.owasp.org/>.
[Portable-Contacts]
Smarr, J., "Portable Contacts 1.0 Draft C", August 2008,
<http://portablecontacts.net/>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[SSL-Latency]
Sissel, J., Ed., "SSL handshake latency and HTTPS
optimizations", June 2010.
[Sec-Analysis]
Gross, T., "Security Analysis of the SAML Single Sign-on
Browser/Artifact Profile", 19th Annual Computer Security
Applications Conference, Las Vegas, December 2003.
[X-Frame-Options]
Ross, D. and T. Gondrom, "HTTP Header X-Frame-Options",
Work in Progress, October 2012.
[iFrame] World Wide Web Consortium, "Frames in HTML documents",
W3C HTML 4.01, December 1999,
<http://www.w3.org/TR/html4/present/frames.html#h-16.5>.
Authors' Addresses
Torsten Lodderstedt (editor)
Deutsche Telekom AG
EMail: torsten@lodderstedt.net
Mark McGloin
IBM
EMail: mark.mcgloin@ie.ibm.com
Phil Hunt
Oracle Corporation
EMail: phil.hunt@yahoo.com
