| OAuth Working Group | N. Sakimura, Ed. |
| Internet-Draft | Nomura Research Institute |
| Intended status: Standards Track | J. Bradley |
| Expires: August 4, 2015 | Ping Identity |
| N. Agarwal | |
| January 31, 2015 |
Proof Key for Code Exchange by OAuth Public Clients
draft-ietf-oauth-spop-07
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.
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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 authorization request via the browser/operating system, which then gets forwarded to the OAuth 2.0 authorization server in step (2). The authorization server returns the authorization code in step (3). The malicious app is able to observe the authorization code in step (4) since it is registered to the custom URI scheme used by the legitimate app. This allows the attacker to reguest and obtain an access token in step (5) and step (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 Section 4.4.1 of [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.
+-------------------+
| Authz Server |
+--------+ | +---------------+ |
| |--(A)- Authorization Request ---->| | |
| | + t(code_verifier), t | | 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(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 url and filename safe Alphabet [A-Z] / [a-z] / [0-9] / "-" / "_" from Sec 5 of RFC 4648 [RFC4648], with length less than 128 characters.
ABNF for code_verifier is as follows.
code-verifier = 42*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 42-octet URL safe string to use as the code verifier.
The client then creates a code challenge, code_challenge, derived from the code_verifier by using one of the following transformations on the code_verifier:
It is RECOMMENDED to use the S256 transformation when possible.
ABNF for code_challenge is as follows.
code-challenge = 42*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 [RFC6749] Authorization Request (Section 4.1.1.) using the following additional parameters:
When the server issues the code in the Authorization Response, it MUST associate the code_challenge and code_challenge_method values with the 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 PKCE, 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.
If the client is capable of using S256, it MUST use S256, as S256 is MTI on the server. Clients MAY use plain only if they cannot support S256 for some technical reason and knows that the server supports plain.
Upon receipt of the code, the client sends the Access Token Request to the token endpoint. In addition to the parameters defined in OAuth 2.0 [RFC6749] Access Token Request (Section 4.1.3.), it sends the following parameter:
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 first hashed with SHA-256 then compared to the base64url decoded code_challenge. i.e.,
SHA256(ASCII(code_verifier )) == BASE64URL-DECODE(code_challenge).
If the code_challenge_method from Section 4.2 was plain, they are compared directly. i.e.,
code_challenge == code_verifier.
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 OAuth 2.0 [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 SHOULD 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.
Additional code_challenge_method types for use with the authorization endpoint are registered with a Specification Required ([RFC5226]) after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published.
Registration requests must be sent to the oauth-ext-review@ietf.org mailing list for review and comment, with an appropriate subject (e.g., "Request for PKCE code_challenge_method: example").
Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.
IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.
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. It is RECOMMENDED that the output of a suitable random number generator be used to create a 32-octet sequence.
Clients MUST NOT try down grading the algorithm after trying S256 method. If the server is PKCE compliant, then S256 method works. If the server does not support PKCE, it does not generate error. Only the time that the server returns that it does not support S256 is there is a MITM trying the algorithm downgrade attack.
S256 method protects against eavesdroppers observing or intercepting the code_challenge. If the plain method is used, there is a chance that it will be observed by the attacker on the device. The use of S256 protects against it.
If code_challenge is to be returned inside authorization code to achieve a stateless server, it has to be encrypted in such a manner that only the server can decrypt and extract it.
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 42-octet URL safe string to use as a code_challenge that has the required entropy.
Salting is not used in the production of the code_verifier, as the code_chalange contains sufficient entropy to prevent brute force attacks. Concatenating a publicly known value to a code_challenge (with 256 bits of entropy) and then hashing it with SHA256 would actually reduce the entropy in the resulting code_verifier making it easier for an attacker to brute force.
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.
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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| [RFC0020] | Cerf, V., "ASCII format for network interchange", RFC 20, October 1969. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC4648] | Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. |
| [RFC6234] | Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011. |
| [RFC6749] | Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, October 2012. |
| [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 base64url encoding and decoding functions without padding based upon standard base64 encoding and decoding functions that do use 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;
}
static byte [] base64urldecode(string arg)
{
string s = arg;
s = s.Replace('-', '+'); // 62nd char of encoding
s = s.Replace('_', '/'); // 63rd char of encoding
switch (s.Length % 4) // Pad with trailing '='s
{
case 0: break; // No pad chars in this case
case 2: s += "=="; break; // Two pad chars
case 3: s += "="; break; // One pad char
default: throw new System.Exception(
"Illegal base64url string!");
}
return Convert.FromBase64String(s); // Standard base64 decoder
}
As per the example code above, the number of '=' padding characters that needs to be added to the end of a base64url encoded string without padding to turn it into one with padding is a deterministic function of the length of the encoded string. Specifically, if the length mod 4 is 0, no padding is added; if the length mod 4 is 2, two '=' padding characters are added; if the length mod 4 is 3, one '=' padding character is added; if the length mod 4 is 1, the input is malformed.
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