API Security : Patterns and Practices

Prabath Siriwardena
Director of Security Architecture
WSO2
API Security
Patterns and Practices

Gateway Pattern
• Decouple clients from the actual API implementation
• No point-to-point to connection
• Centralized security enforcing
• Centralized auditing & monitoring
• Version controlling

Six key attributes of a secured design
• Only legitimate users can access the system (authentication)
• The system won’t allow users to do anything more than what they
are supposed to do (authorization)
• Confidential data can only be seen by the intended recipients,
nobody else (confidentiality)
• Integrity of the transactions are protected (integrity)
• Protected for non-repudiation
• They system is available for legitimate users to access, all the time
(availability)

Direct Authentication
• HTTP Basic Authentication
• HTTP Digest Authentication
• TLS Mutual Authentication
• OAuth 2.0 (for authentication ?)

HTTP Basic Authentication
curl -I
-u $GitHubUserName:GitHubPassword
-X POST -H 'Content-Type: application/x-www-form-urlencoded’
-d '{"name": "my_github_repo"}'
https://api.github.com/user/repos
 Creating a GitHub repository

HTTP Digest Authentication
curl -k –-digest –u userName:password -v https://localhost:8443/recipe
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest realm="cute-cupcakes.com",
qop="auth”,
nonce="1390781967182:c2db4ebb26207f6ed38bb08eeffc7422",
opaque="F5288F4526B8EAFFC4AC79F04CA8A6ED"
Authorization: Digest username="prabath", realm="cute-cupcakes.com",
nonce="1390781967182:c2db4ebb26207f6ed38bb08eeffc7422", uri="/recipe",
cnonce="MTM5MDc4", nc=00000001, qop="auth",
response="f5bfb64ba8596d1b9ad1514702f5a062",
opaque="F5288F4526B8EAFFC4AC79F04CA8A6ED"

HTTP Basic vs. Digest Authentication

TLS Mutual Authentication
 Gateway itself does the certificate validation
 Fine-grained access validations can be done by the authorization server.
curl -k --cert client.pem https://localhost:8443/recipe

OAuth 2.0 (authorization code grant type)

OAuth 2.0 (implicit grant type)

OAuth 2.0 (password grant type)

OAuth 2.0 (client credentials grant type)

OAuth 2.0 (chained grant type)

OAuth 2.0 Tokens
 AccessTokens
 Bearer tokens vs. Mac
 TLS is a must
 Pass the access token in the HTTP Authorization header
 Authorization: Bearer <token>
 Pass the access token in as a URL query parameter
 Avoid this
 Request  Cache-Control: no-store
 Response  Cache-Control: private
 E.g. https://www.googleapis.com/oauth2/v1/userinfo?access_token=ya29.1.
 Shorter life-time – in minutes or hours
 Do not store in cookies
 Issue scoped tokens

OAuth 2.0 Tokens
 RefreshTokens
 Must useTLS
 Long-lasting
 No refresh tokens under
 implicit grant type
 client credentials grant type
 SAML grant type
 JWT grant type

Self-contained Access Tokens
 JWT
 RFC 7519
 Encodes claims to be transmitted as a JSON object
 Can be signed using JWS (JSON Web Signature)
 Can be encrypted using JWE (JSON Web Encryption)
 Represented as a sequence of URL-safe parts separated by period
('.') characters.
 Each part contains a base64url-encoded value
 Example
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
.eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
.dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

Self-issued Access Tokens
 Same as self-contained access tokens
 Issued by the client itself

Brokered Authentication
• TLS Mutual Authentication
• OAuth 2.0

OAuth 2.0
(decoupling end user authentication from the authorization server)

OAuth & XACML
 A given access token has a scope associated with it and it governs the
access token’s capabilities
 A user delegates access to his Facebook profile to a third party, under the
scope “user_activities”. This provides access to the user's list of activities as
the activities’ connection. To achieve fine-grained access control, this can
be represented in an XACML policy.
 token=gfgew789hkhjkew87
resource_id=GET https://graph.facebook.com/prabathsiriwardena/activities

Token Introspection
POST /introspection HTTP/1.1
Accept: application/x-www-form-urlencoded
Host: server.example.com
Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3
token=X3241Affw.4233-99JXJ&resource_id=…
{
"active": true,
"client_id":"s6BhdRkqt3",
"scope": "read write dolphin",
"sub": "2309fj32kl",
"aud": http://example.org/protected-resource/*
}

XACML Policy
<Policy>
<Target>
<AnyOf>
<AllOf>
<Match MatchId="urn:oasis:names:tc:xacml:1.0:function:string-equal">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">
user_activities</AttributeValue>
<AttributeDesignator MustBePresent="false"
Category="urn:oasis:names:tc:xacml:3.0:attribute-category:scope"
AttributeId="urn:oasis:names:tc:xacml:1.0:scope:scope-id"
DataType="http://www.w3.org/2001/XMLSchema#string"></AttributeDesignator>
</Match>
</AllOf>
</AnyOf>
</Target>
<Rule RuleId="permit_rule" Effect="Permit">
</Rule>
<Rule RuleId="deny_rule" Effect="Deny">
</Rule>
</Policy>

XACML Request
<Request>
<Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:oauth-client">
<Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:client:client-id">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">32324343434</AttributeValue>
</Attribute>
<Attributes>
<Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:action">
<Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:action:action-id">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">GET</AttributeValue>
</Attribute>
</Attributes>
<Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:scope">
<Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:scope:scope-id">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">user_activities</AttributeValue>
</Attribute>
</Attributes>
<Attributes Category="urn:oasis:names:tc:xacml:3.0:attribute-category:resource">
<Attribute AttributeId="urn:oasis:names:tc:xacml:1.0:resource:resource-id">
<AttributeValue
DataType="http://www.w3.org/2001/XMLSchema#string">
https://graph.facebook.com/prabathsiriwardena/activities</AttributeValue>
</Attribute>
</Attributes>
</Request>

Confidentiality
• TLS
• JWE

High Availability
• Network level measures
• Throttling
• Client level
• User level

API Security : Patterns and Practices

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