REST API Security
Les Hazlewood @lhazlewoodPMC Chair, Apache ShiroExpert Group Member, JEE Application Security (JSR-375)Founder & CTO, Stormpath
About Stormpath• User Management API for Developers• Password security • Authentication and Authorization• Multi Tenancy• MFA, SAML, OAuth2• LDAP & Active Directory Cloud Sync• Instant-on, scalable, and highly available• Free for developers
...
Overview
• HTTP Authentication
• HTTP Authentication Schemes Comparison
• API Key Authentication
• Token Authentication
• Authorization
REST API Focus
• Eliminate server state
• Secure user credentials
• Secure server endpoints
• Expose access control rules
• SPAs and Mobile: ‘Untrusted Clients’
HTTP(S) Authentication & Authorization
Authentication
Proving you are who you say you are.
Authorization
Ensuring someone is allowed to do what they are trying to do.
HTTP Authentication & Authorization
• Authorization header
• No Custom Headers!• Stay spec-standard• No pre-flight CORS requests (browsers) req’d• Custom schemes easily supported
Authorization header
How does it work?
Authorization header
How does it work?
Challenge Response protocol
1. Request
GET /admin HTTP/1.1
2. Challenge
HTTP/1.1 401 UnauthorizedWWW-Authenticate: scheme-name <stuff>
*multiple schemes allowed, typically set as multiple WWW-Authenticate headers
3. Re-Request
GET /admin HTTP/1.1Authorization: scheme-name <stuff>
Example: HTTP Basic Authentication
1. Request (Basic)
GET /admin HTTP/1.1
2. Challenge (Basic)
HTTP/1.1 401 UnauthorizedWWW-Authenticate: Basic realm=“MyApp”
3. Re-Request (Basic)
GET /admin HTTP/1.1Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
Schemes
• Basic• Digest Schemes (OAuth 1.0a)• Bearer Token Schemes (OAuth2)• Custom
HTTP Basic
HTTP Basic
String value = username + ‘:’ + raw_passwordString schemeValue = base64_encode(value)
...
GET /admin HTTP/1.1Authorization: Basic schemeValue
HTTP Basic
Pros:• Very easy to use• Supported by everything
Cons:• Raw password always transmitted• Easy to leak raw password if not careful (logging)• Susceptible to Man-In-The-Middle attacks• HTTPS *always* required• Client must constantly retain/reference the raw password (server clients
usually ok, browser clients not ok)
Digest Schemes
Digest Schemes: Client
request.headers[‘Client-Id’] = getMyId()
String digest = hmacSha256(request, password)
request.headers[‘Authorization’] = ‘Foo ‘ + digest
send(request)
Digest Schemes: Server
String clientId = request.headers[‘Client-Id’]byte[] password = lookupPassword(clientId);
String serverComputedDigest = hmacSha256(request, password)
String val = request.headers[‘Authorization’]String clientSpecifiedDigest = val.remove(‘Foo ‘)
if (clientSpecifiedDigest != serverComputedDigest) { sendError(401, response) return}
//otherwise request is authenticated
Digest Schemes: OAuth 1.0a example
Authorization: OAuth realm="http://sp.example.com/", oauth_consumer_key="0685bd9184jfhq22”, oauth_token="ad180jjd733klru7", oauth_signature_method="HMAC-SHA1", oauth_signature="wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%3D", oauth_timestamp="137131200", oauth_nonce="4572616e48616d6d65724c61686176”
Digest Schemes
Pros:• Probably most secure• Password never sent over the wire• HTTPS not required (but your data may still require HTTPS)• Can guarantee end-to-end HTTP message authenticity
(HTTPS cannot do this)• Not susceptible to Man-In-The-Middle attacks
Cons:• Very difficult to design safely• Difficult to understand and use• Difficult to implement libraries• Client needs to retain a constant reference to the password
(server clients usually ok, browser clients maybe not?)
Bearer Token Schemes
Bearer Token Schemes
Authorization: Bearer opaque-token
Bearer Token Schemes
opaque-token can be whatever you want*
Bearer Token Schemes
opaque-token can be whatever you want*
*should always be cryptographically-signed and expire
Bearer Token Schemes: OAuth 2 Example
Authorization: Bearer eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ.dBjftJeZ4CVP-92K27uhbUJU1p1r_wW1gFWFOEjXk
Bearer Token Schemes
Pros:• Easier to use than digest• De-facto standard token format (JWT)• Can contain state – no server sessions needed• Does not require constant access to the user password
Cons:• HTTPS always required, during and always after login (not a big deal nowadays)• Cannot guarantee end-to-end HTTP message authenticity (like digest schemes can)• Susceptible to Man-In-The-Middle attacks• Token creation and renewal workflows can be very complicated and confusing depending on use case
(OAuth2 confuses many people).• When used for Browser or Mobile, additional security still required (Origin checks, CSRF-protection, etc)• Token content is not standard – applications can open themselves to attack
Custom Scheme
Custom Scheme
• Only if you really, Really, REALLY know what you’re doing.
Seriously.
No, rly. Srsly.
• Non-standard, so you essentially must provide your own client libraries.
Custom Scheme
• Stormpath has a custom SAUTHC1 digest scheme• Authenticates the entire HTTP Message, including the Body (OAuth 1.0a does not)• Uses nonces to prevent replay attacks• Uses key derivation algorithms and HMAC-SHA-256• We use it for our own SDKs*• If you’re curious:
https://github.com/stormpath
(search for ‘sauthc1’ in any stormpath-sdk-* project)
*Basic still supported for non-SDK clients or ‘weird’ environments
API Key Authentication
API Key Example
ID : YLNVXG091ZO1BSANZ5U6DCTIXSecret: ZediwUeDCNl13ldjaFKFQzz0eD13PO931DLAopdeywixaeUAhsip+92iaY
API Keys
• Entropy• Password Independent• Scope• Speed• Limited Exposure• Traceability
API Keys
• Can be thought of as a really long username and password pair.
• Can be used with any HTTP Authentication Scheme that accepts a username and password: Basic, Digest, OAuth2, etc.
• Almost exclusively used for server-to-server communication.
• Never embed API Key secrets in untrusted clients like JavaScript or mobile applications.
HTTP Basic with API Key
String value = apiKeyId + ‘:’ + apiKeySecretString schemeValue = base64_encode(value)
...
GET /admin HTTP/1.1Authorization: Basic schemeValue
Token Authentication
Why not just use Session IDs?
Session ID Problems
• They’re opaque and have no meaning themselves (they’re just ‘pointers’).
• Service-oriented architectures might need a centralized ID de-referencing service
Session ID Problems
• Opaque IDs mean clients can’t inspect them and find out what it is allowed to do or not - it needs to make more requests for this information.
• Susceptible to CSRF attacks
Session ID Problems
• Sessions = Server State!• You need to store that state somewhere• Session ID look up server state on *every request*.• Really not good for distributed/clustered apps• Really not good for scale
Token Authentication to the rescue!
How do you get a Token?
Example: your SPA, your server
1. Token Request
POST /token HTTP/1.1Origin: https://foo.comContent-Type: application/x-www-form-urlencoded
grant_type=password&username=username&password=password
*Assert allowed origin for browser-based apps
2. Token Response
HTTP/1.1 200 OKContent-Type: application/json;charset=UTF-8Cache-Control: no-storePragma: no-cache
{ "access_token":"2YotnFZFEjr1zCsicMWpAA", “token_type":"example", “expires_in":3600, "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA", "example_parameter":"example_value”}
3. Resource Request
GET /admin HTTP/1.1Authorization: Bearer 2YotnFZFEjr1zCsicMWpAA
Example: Token Request using an API Key
POST /token HTTP/1.1Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials&client_id=apiKeyId&client_secret=apiKeySecret
*Assert allowed origin for browser-based apps
How does the server create a Token?
JSON Web Tokens (JWT)
• A URL-safe, compact, self-contained string with meaningful information that is usually digitally signed or encrypted.
• The string is ‘opaque’ and can be used as a ‘token’.
• Many OAuth2 implementations use JWTs as OAuth2 Access Tokens.
JSON Web Tokens (JWT)
• You can store them in cookies! But all those cookie security rules still apply (CSRF protection, etc).
• You can entirely replace your session ID with a JWT.
JSON Web Tokens (JWT)
In the wild they look like just another ugly string:
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ.dBjftJeZ4CVPmB92K27uhbUJU1p1r_wW1gFWFOEjXk
JSON Web Tokens (JWT)
But they do have a three part structure. Each part is a Base64Url-encoded string:
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ.dBjftJeZ4CVPmB92K27uhbUJU1p1r_wW1gFWFOEjXk
Header
Body (‘Claims’)
Cryptographic Signature
JSON Web Tokens (JWT)Base64Url-decode the parts to find the juicy bits:
{ "typ":"JWT", "alg":"HS256"}
{ "iss”:”http://trustyapp.com/”, "exp": 1300819380, “sub”: ”users/8983462”, “scope”: “self api/buy”}
tß´—™à%O˜v+nî…SZu¯µ€U…8H×
Header
Body (‘Claims’)
Cryptographic Signature
JSON Web Tokens (JWT)The claims body is the best part! It can tell:{
"iss”:”http://trustyapp.com/”,
"exp": 1300819380,
“sub”: ”users/8983462”,
“scope”: “self api/buy”
}
Who issued the token
JSON Web Tokens (JWT)The claims body is the best part! It can tell:{
"iss”:”http://trustyapp.com/”,
"exp": 1300819380,
“sub”: ”users/8983462”,
“scope”: “self api/buy”
}
Who issued the token
When it expires
JSON Web Tokens (JWT)The claims body is the best part! It can tell:{
"iss”:”http://trustyapp.com/”,
"exp": 1300819380,
“sub”: ”users/8983462”,
“scope”: “self api/buy”
}
Who issued the token
When it expires
Who it represents
JSON Web Tokens (JWT)The claims body is the best part! It can tell:{
"iss”:”http://trustyapp.com/”,
"exp": 1300819380,
“sub”: ”users/8983462”,
“scope”: “self api/buy”
}
Who issued the token
When it expires
Who it represents
What they can do
JSON Web Tokens (JWT)Great! Why is this useful?
• Implicitly trusted because it is cryptographically signed (verified not tampered).
• It is structured, enabling inter-op between services
• It can inform your client about basic access control rules (permissions)*
• And the big one: statelessness!*servers must always enforce access control policies
JSON Web Tokens (JWT)So, what’s the catch?
• Implicit trust is a tradeoff – how long should the token be good for? how will you revoke it? (Another talk: refresh tokens)
• You still have to secure your cookies!• You have to be mindful of what you
store in the JWT if they are not encrypted. No sensitive info!
Authorization
Authorization
• JWT Claims can have whatever you want
• Use a scope field that contains a list of permissions for that user
• Client can inspect the claims and scope and turn on or off features based on permissions*
• *Server must always assert permissions
Authorization Failed: 403
HTTP/1.1 403 Forbidden
In addition to user authentication and data security, Stormpath can handle authentication and authorization for your API, SPA or mobile app.
• API Authentication• API Key Management• Authorization• Token Based Authentication• OAuth• JWTs• MFA, SAML, OAuth2• Multi-Tenancy
http://docs.stormpath.com/guides/api-key-management/
Implementations in your Library of choice: https://docs.stormpath.com/home/
Use Stormpath for API Authentication & Security
Follow Us on Twitter
@lhazlewood
@goStormpath