| OAuth Working Group | N. Sakimura, Ed. |
| Internet-Draft | Nomura Research Institute |
| Intended status: Standards Track | J. Bradley |
| Expires: January 9, 2016 | Ping Identity |
| N. Agarwal | |
| July 8, 2015 |
Proof Key for Code Exchange by OAuth Public Clients
draft-ietf-oauth-spop-15
OAuth 2.0 public clients utilizing the Authorization Code Grant are susceptible to the authorization code interception attack. This specification describes the attack as well as a technique to mitigate against the threat through the use of Proof Key for Code Exchange (PKCE, pronounced "pixy").
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OAuth 2.0 [RFC6749] public clients are susceptible to the authorization code interception attack.
The attacker thereby intercepts the authorization code returned from the authorization endpoint within communication path not protected by TLS, such as inter-app communication within the operating system of the client.
Once the attacker has gained access to the authorization code it can use it to obtain the access token.
Figure 1 shows the attack graphically. In step (1) the native app running on the end device, such as a smart phone, issues an OAuth 2.0 Authorization Request via the browser/operating system. The Redirection Endpoint URI in this case typically uses a custom URI scheme. Step (1) happens through a secure API that cannot be intercepted, though it may potentially be observed in advanced attack scenarios. The request then gets forwarded to the OAuth 2.0 authorization server in step (2). Because OAuth requires the use of TLS, this communication is protected by TLS, and also cannot be intercepted. The authorization server returns the authorization code in step (3). In step (4), the Authorization Code is returned to the requester via the Redirection Endpoint URI that was provided in step (1).
A malicious app that has been designed to attack this native app has previously registered itself as a handler for the custom URI scheme is now able to intercept the Authorization Code in step (4). This allows the attacker to request and obtain an access token in steps (5) and (6), respectively.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| End Device (e.g., Smart Phone) |
| |
| +-------------+ +----------+ | (6) Access Token +----------+
| |Legitimate | | Malicious|<--------------------| |
| |OAuth 2.0 App| | App |-------------------->| |
| +-------------+ +----------+ | (5) Authorization | |
| | ^ ^ | Grant | |
| | \ | | | |
| | \ (4) | | | |
| (1) | \ Authz| | | |
| Authz| \ Code | | | Authz |
| Request| \ | | | Server |
| | \ | | | |
| | \ | | | |
| v \ | | | |
| +----------------------------+ | | |
| | | | (3) Authz Code | |
| | Operating System/ |<--------------------| |
| | Browser |-------------------->| |
| | | | (2) Authz Request | |
| +----------------------------+ | +----------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Figure 1: Authorization Code Interception Attack.
A number of pre-conditions need to hold in order for this attack to work:
While this is a long list of pre-conditions the described attack has been observed in the wild and has to be considered in OAuth 2.0 deployments.
While the OAuth 2.0 Threat Model Section 4.4.1 [RFC6819] describes mitigation techniques they are, unfortunately, not applicable since they rely on a per-client instance secret or aper client instance redirect URI.
To mitigate this attack, this extension utilizes a dynamically created cryptographically random key called "code verifier". A unique code verifier is created for every authorization request and its transformed value, called "code challenge", is sent to the authorization server to obtain the authorization code. The authorization code obtained is then sent to the token endpoint with the "code verifier" and the server compares it with the previously received request code so that it can perform the proof of possession of the "code verifier" by the client. This works as the mitigation since the attacker would not know this one-time key, since it is sent over TLS and cannot be intercepted.
+-------------------+
| Authz Server |
+--------+ | +---------------+ |
| |--(A)- Authorization Request ---->| | |
| | + t(code_verifier), t_m | | Authorization | |
| | | | Endpoint | |
| |<-(B)---- Authorization Code -----| | |
| | | +---------------+ |
| Client | | |
| | | +---------------+ |
| |--(C)-- Access Token Request ---->| | |
| | + code_verifier | | Token | |
| | | | Endpoint | |
| |<-(D)------ Access Token ---------| | |
+--------+ | +---------------+ |
+-------------------+
Figure 2: Abstract Protocol Flow
This specification adds additional parameters to the OAuth 2.0 Authorization and Access Token Requests, shown in abstract form in Figure 1.
An attacker who intercepts the Authorization Grant at (B) is unable to redeem it for an Access Token, as they are not in possession of the code_verifier secret.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in Key words for use in RFCs to Indicate Requirement Levels [RFC2119]. If these words are used without being spelled in uppercase then they are to be interpreted with their normal natural language meanings.
This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].
STRING denotes a sequence of zero or more ASCII [RFC0020] characters.
OCTETS denotes a sequence of zero or more octets.
ASCII(STRING) denotes the octets of the ASCII [RFC0020] representation of STRING where STRING is a sequence of zero or more ASCII characters.
BASE64URL-ENCODE(OCTETS) denotes the base64url encoding of OCTETS, per Section 3 producing a STRING.
BASE64URL-DECODE(STRING) denotes the base64url decoding of STRING, per Section 3, producing a sequence of octets.
SHA256(OCTETS) denotes a SHA2 256bit hash [RFC6234] of OCTETS.
In addition to the terms defined in OAuth 2.0 [RFC6749], this specification defines the following terms:
The client first creates a code verifier, code_verifier, for each OAuth 2.0 [RFC6749] Authorization Request, in the following manner:
code_verifier = high entropy cryptographic random STRING using the Unreserved Characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~" from Sec 2.3 of [RFC3986], with a minimum length of 43 characters and a maximum length of 128 characters.
ABNF for code_verifier is as follows.
code-verifier = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39
NOTE: code verifier SHOULD have enough entropy to make it impractical to guess the value. It is RECOMMENDED that the output of a suitable random number generator be used to create a 32-octet sequence. The Octet sequence is then base64url encoded to produce a 43-octet URL safe string to use as the code verifier.
The client then creates a code challenge derived from the code verifier by using one of the following transformations on the code verifier:
If the client is capable of using S256, it MUST use S256, as S256 is Mandatory To Implement (MTI) on the server. Clients are permitted to use plain only if they cannot support S256 for some technical reason and know via out of band configuration that the server supports plain.
The plain transformation is for compatibility with existing deployments and for constrained environments that can't use the S256 transformation.
ABNF for code_challenge is as follows.
code-challenge = 43*128unreserved unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" ALPHA = %x41-5A / %x61-7A DIGIT = %x30-39
The client sends the code challenge as part of the OAuth 2.0 Authorization Request (Section 4.1.1 of [RFC6749].) using the following additional parameters:
When the server issues the authorization code in the authorization response, it MUST associate the code_challenge and code_challenge_method values with the authorization code so it can be verified later.
Typically, the code_challenge and code_challenge_method values are stored in encrypted form in the code itself, but could alternatively be stored on the server, associated with the code. The server MUST NOT include the code_challenge value in client requests in a form that other entities can extract.
The exact method that the server uses to associate the code_challenge with the issued code is out of scope for this specification.
If the server requires Proof Key for Code Exchange (PKCE) by OAuth Public Clients, and the client does not send the code_challenge in the request, the authorization endpoint MUST return the authorization error response with error value set to invalid_request. The error_description or the response of error_uri SHOULD explain the nature of error, e.g., code challenge required.
If the server supporting PKCE does not support the requested transform, the authorization endpoint MUST return the authorization error response with error value set to invalid_request. The error_description or the response of error_uri SHOULD explain the nature of error, e.g., transform algorithm not supported.
Upon receipt of the Authorization Code, the client sends the Access Token Request to the token endpoint. In addition to the parameters defined in the OAuth 2.0 Access Token Request (Section 4.1.3 of [RFC6749]), it sends the following parameter:
The code_challenge_method is bound to the Authorization Code when the Authorization Code is issued. That is the method that the token endpoint MUST use to verify the code_verifier.
Upon receipt of the request at the Access Token endpoint, the server verifies it by calculating the code challenge from received code_verifier and comparing it with the previously associated code_challenge, after first transforming it according to the code_challenge_method method specified by the client.
If the code_challenge_method from Section 4.2 was S256, the received code_verifier is hashed by SHA-256, then base64url encoded, and then compared to the code_challenge. i.e.,
BASE64URL-ENCODE(SHA256(ASCII(code_verifier ))) == code_challenge
If the code_challenge_method from Section 4.2 was plain, they are compared directly. i.e.,
code_verifier == code_challenge.
If the values are equal, the Access Token endpoint MUST continue processing as normal (as defined by OAuth 2.0 [RFC6749]). If the values are not equal, an error response indicating invalid_grant as described in section 5.2 of [RFC6749] MUST be returned.
Server implementations of this specification MAY accept OAuth2.0 Clients that do not implement this extension. If the code_verifier is not received from the client in the Authorization Request, servers supporting backwards compatibility revert to a normal OAuth 2.0 [RFC6749] protocol.
As the OAuth 2.0 [RFC6749] server responses are unchanged by this specification, client implementations of this specification do not need to know if the server has implemented this specification or not, and SHOULD send the additional parameters as defined in Section 3. to all servers.
This specification makes a registration request as follows:
This specification registers the following parameters in the IANA OAuth Parameters registry defined in OAuth 2.0 [RFC6749].
This specification establishes the PKCE Code Challenge Method registry. The new registry should be a sub-registry of OAuth Parameters registry.
Additional code_challenge_method types for use with the authorization endpoint are registered using the Specification Required policy [RFC5226], which includes review of the request by one or more Designated Experts. The DEs will ensure there is at least a two-week review of the request on the oauth-ext-review@ietf.org mailing list, and that any discussion on that list converges before they respond to the request. To allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that an acceptable specification will be published.
Registration requests and discussion on the oauth-ext-review@ietf.org mailing list should use an appropriate subject, such as "Request for PKCE code_challenge_method: example").
The Designated Expert(s) should consider the discussion on the mailing list, as well as the overall security properties of the challenge Method when evaluating registration requests. New methods should not disclose the value of the code_verifier in the request to the Authorization endpoint. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.
This specification registers the Code Challenge Method Parameter names defined in Section 4.2 in this registry.
The security model relies on the fact that the code verifier is not learned or guessed by the attacker. It is vitally important to adhere to this principle. As such, the code verifier has to be created in such a manner that it is cryptographically random and has high entropy that it is not practical for the attacker to guess.
The client SHOULD create a code_verifier with a minimum of 256bits of entropy. This can be done by having a suitable random number generator create a 32-octet sequence. The Octet sequence can then be base64url encoded to produce a 43-octet URL safe string to use as a code_challenge that has the required entropy.
Clients MUST NOT downgrade to plain after trying S256 method. Servers that support PKCE are required to support S256, and servers that do not support PKCE will simply ignore the unknown code_verifier OAuth 2.0 (see Section 3.2 of [RFC6749]. Because of that, an error when S256 is presented can only mean that the server is faulty or that a MITM attacker is trying a downgrade attack.
S256 method protects against eavesdroppers observing or intercepting the code_challenge, because the challenge cannot be used without the verifier. With the plain method, there is a chance that code_challenge will be observed by the attacker on the device, or in the http request. Since the code challenge is the same as the code verifier in this case, plain method does not protect against the eavesdropping of the initial request.
The use of S256 protects against disclosure of code_verifier value to an attacker.
Because of this, plain SHOULD NOT be used, and exists only for compatibility with deployed implementations where the request path is already protected. The plain method SHOULD NOT be used in new implementations, unless they cannot support S256 for some technical reason.
The S256 code_challenge_method or other cryptographically secure code_challenge_method extension SHOULD be used. The plain code_challenge_method relies on the operating system and transport security not to disclose the request to an attacker.
If the code_challenge_method is plain, and the code_challenge is to be returned inside authorization code to achieve a stateless server, it MUST be encrypted in such a manner that only the server can decrypt and extract it.
In order to reduce implementation complexity Salting is not used in the production of the code_challenge, as the code_verifier contains sufficient entropy to prevent brute force attacks. Concatenating a publicly known value to a code_verifier (containing 256 bits of entropy) and then hashing it with SHA256 to produce a code_challenge would not increase the number of attempts necessary to brute force a valid value for code_verifier.
While the S256 transformation is like hashing a password there are important differences. Passwords tend to be relatively low entropy words that can be hashed offline and the hash looked up in a dictionary. By concatenating a unique though public value to each password prior to hashing, the dictionary space that an attacker needs to search is greatly expanded.
Modern graphics processors now allow attackers to calculate hashes in real time faster than they could be looked up from a disk. This eliminates the value of the salt in increasing the complexity of a brute force attack for even low entropy passwords.
All the OAuth security analysis presented in [RFC6819] applies so readers SHOULD carefully follow it.
Curent security considerations can be found in Recommendations for Secure Use of TLS and DTLS [BCP195]. This supersedes the TLS version recommendations in OAuth 2.0 [RFC6749].
The initial draft of this specification was created by the OpenID AB/Connect Working Group of the OpenID Foundation.
This specification is the work of the OAuth Working Group, which includes dozens of active and dedicated participants. In particular, the following individuals contributed ideas, feedback, and wording that shaped and formed the final specification:
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| [RFC6819] | Lodderstedt, T., McGloin, M. and P. Hunt, "OAuth 2.0 Threat Model and Security Considerations", RFC 6819, January 2013. |
This appendix describes how to implement a base64url encoding function without padding based upon standard base64 encoding function that uses padding.
To be concrete, example C# code implementing these functions is shown below. Similar code could be used in other languages.
static string base64urlencode(byte [] arg)
{
string s = Convert.ToBase64String(arg); // Regular base64 encoder
s = s.Split('=')[0]; // Remove any trailing '='s
s = s.Replace('+', '-'); // 62nd char of encoding
s = s.Replace('/', '_'); // 63rd char of encoding
return s;
}
An example correspondence between unencoded and encoded values follows. The octet sequence below encodes into the string below, which when decoded, reproduces the octet sequence.
3 236 255 224 193
A-z_4ME
The client uses output of a suitable random number generator to create a 32-octet sequence. The octets representing the value in this example (using JSON array notation) are:"
[116, 24, 223, 180, 151, 153, 224, 37, 79, 250, 96, 125, 216, 173, 187, 186, 22, 212, 37, 77, 105, 214, 191, 240, 91, 88, 5, 88, 83, 132, 141, 121]
Encoding this octet sequence as a Base64url provides the value of the code_verifier:
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
The code_verifier is then hashed via the SHA256 hash function to produce:
[19, 211, 30, 150, 26, 26, 216, 236, 47, 22, 177, 12, 76, 152, 46, 8, 118, 168, 120, 173, 109, 241, 68, 86, 110, 225, 137, 74, 203, 112, 249, 195]
Encoding this octet sequence as a base64url provides the value of the code_challenge:
E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
The authorization request includes:
code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
&code_challange_method=S256
The Authorization server then records the code_challenge and code_challenge_method along with the code that is granted to the client.
in the request to the token_endpoint the client includes the code received in the authorization response as well as the additional paramater:
code_verifier=dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
The Authorization server retrieves the information for the code grant. Based on the recorded code_challange_method being S256, it then hashes and base64url encodes the value of code_verifier. BASE64URL-ENCODE(SHA256(ASCII(code_verifier )))
The calculated value is then compared with the value of code_challenge:
BASE64URL-ENCODE(SHA256(ASCII(code_verifier ))) == code_challenge
If the two values are equal then the Authorization server can provide the tokens as long as there are no other errors in the request. If the values are not equal then the request must be rejected, and an error returned.