CVE-2020-5300

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

Workarounds

Two workarounds have been identified:

• Do not allow clients to use private_key_jwt
• Use short expiry times for the JWTs

References

https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication

Upstream

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

When using client authentication method "private_key_jwt" [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 'grant_type=client_credentials' \
   --data-urlencode 'client_id=c001d00d-5ecc-beef-ca4e-b00b1e54a111' \
   --data-urlencode 'scope=application openid' \
   --data-urlencode 'client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer' \
   --data-urlencode 'client_assertion=eyJhb [...] jTw'
{"access_token":"zeG0NoqOtlACl8q5J6A-TIsNegQRRUzqLZaYrQtoBZQ.VR6iUcJQYp3u_j7pwvL7YtPqGhtyQe5OhnBE2KCp5pM","expires_in":3599,"scope":"application openid","token_type":"bearer"}⏎            ~$ curl --insecure --location --request POST 'https://localhost/_/oauth2/token' \
   --header 'Content-Type: application/x-www-form-urlencoded' \
   --data-urlencode 'grant_type=client_credentials' \
   --data-urlencode 'client_id=c001d00d-5ecc-beef-ca4e-b00b1e54a111' \
   --data-urlencode 'scope=application openid' \
   --data-urlencode 'client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer' \
   --data-urlencode 'client_assertion=eyJhb [...] jTw'
{"access_token":"wOYtgCLxLXlELORrwZlmeiqqMQ4kRzV-STU2_Sollas.mwlQGCZWXN7G2IoegUe1P0Vw5iGoKrkOzOaplhMSjm4","expires_in":3599,"scope":"application openid","token_type":"bearer"}

CVE-2020-5300 has a CVSS score of 5.8 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.0); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2020-5300? CVE-2020-5300 is a medium-severity security vulnerability in github.com/ory/hydra (go), affecting versions < 1.4.0. It is fixed in 1.4.0.
2. How severe is CVE-2020-5300? CVE-2020-5300 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/ory/hydra are affected by CVE-2020-5300? github.com/ory/hydra (go) versions < 1.4.0 is affected.
4. Is there a fix for CVE-2020-5300? Yes. CVE-2020-5300 is fixed in 1.4.0. Upgrade to this version or later.
5. Is CVE-2020-5300 exploitable, and should I be worried? Whether CVE-2020-5300 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2020-5300 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2020-5300? Upgrade github.com/ory/hydra to 1.4.0 or later.