CVE-2020-5300: Disallow replay of `private_key_jwt` by blacklisting JTIs in Hydra

Impact

When using client authentication method "privatekeyjwt" [1], OpenId specification says the following about assertion jti:

A unique identifier for the token, which can be used to prevent reuse of the token. These tokens MUST only be used once, unless conditions for reuse were negotiated between the parties

Hydra does not seem to check the uniqueness of this jti value. Here is me sending the same token request twice, hence with the same jti assertion, and getting two access tokens:

$ curl --insecure --location --request POST 'https://localhost//oauth2/token' \ --header 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'granttype=clientcredentials' \ --data-urlencode 'clientid=c001d00d-5ecc-beef-ca4e-b00b1e54a111' \ --data-urlencode 'scope=application openid' \ --data-urlencode 'clientassertiontype=urn:ietf:params:oauth:client-assertion-type:jwt-bearer' \ --data-urlencode 'clientassertion=eyJhb [...] jTw' {"accesstoken":"zeG0NoqOtlACl8q5J6A-TIsNegQRRUzqLZaYrQtoBZQ.VR6iUcJQYp3uj7pwvL7YtPqGhtyQe5OhnBE2KCp5pM","expiresin":3599,"scope":"application openid","tokentype":"bearer"}⏎ ~$ curl --insecure --location --request POST 'https://localhost//oauth2/token' \ --header 'Content-Type: application/x-www-form-urlencoded' \ --data-urlencode 'granttype=clientcredentials' \ --data-urlencode 'clientid=c001d00d-5ecc-beef-ca4e-b00b1e54a111' \ --data-urlencode 'scope=application openid' \ --data-urlencode 'clientassertiontype=urn:ietf:params:oauth:client-assertion-type:jwt-bearer' \ --data-urlencode 'clientassertion=eyJhb [...] jTw' {"accesstoken":"wOYtgCLxLXlELORrwZlmeiqqMQ4kRzV-STU2Sollas.mwlQGCZWXN7G2IoegUe1P0Vw5iGoKrkOzOaplhMSjm4","expiresin":3599,"scope":"application openid","tokentype":"bearer"}

Severity

We rate the severity as medium because the following reasons make it hard to replay tokens without the patch:

- TLS protects against MITM which makes it difficult to intercept valid tokens for replay attacks - The expiry time of the JWT gives only a short window of opportunity where it could be replayed

Patches

This will be patched with v1.4.0+oryOS.17

Workarounds

Two workarounds have been identified:

- Do not allow clients to use privatekeyjwt - Use short expiry times for the JWTs

References

https://openid.net/specs/openid-connect-core-10.html#ClientAuthentication

Upstream

This issue will be resolved in the upstream repository https://github.com/ory/fosite