Oathkeeper duplicates CORS headers

[kamilkloch]

Preflight checklist

• I could not find a solution in the existing issues, docs, nor discussions.
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Describe the bug

We are using oathkeeper and kratos services.
When oathkeeper CORS config looks as follows:

cors:
  enabled: true
  allowed_origins:
    - "https://ui-admin.our-company.com"

then the request

curl -v 'https://webapi-user.our-company.com/api/v1/datafeed/rollingbar?name=ADAUSD' -H 'origin: https://ui-admin.our-company.com'

produces the following output in the headers:

access-control-allow-credentials: true
access-control-allow-credentials: true
access-control-allow-origin: https://ui-admin.our-company.com
access-control-allow-origin: https://ui-admin.our-company.com
vary: Origin
vary: Origin

oathkeeper appends duplicate CORS headers to the response (in addition to CORS headers added by the underlying services).

This is quite problematic, as it breaks CORS requests issued by the browser: https://crashtest-security.com/multiple-values-access-control-allow-origin/
"While the header does support multiple origins, browsers usually do not. "

Reproducing the bug

curl -v 'https://webapi-user.our-company.com/api/v1/datafeed/rollingbar?name=ADAUSD' -H 'origin: https://ui-admin.our-company.com'

Relevant log output

No response

Relevant configuration

cors:
  enabled: true
  allowed_origins:
    - "https://ui-admin.our-company.com"

Version

0.40.2

On which operating system are you observing this issue?

None

In which environment are you deploying?

None

Additional Context

No response

[civilizeddev]

I have experienced the same problem.

oathkeeper appends duplicate CORS headers to the response (in addition to CORS headers added by the underlying services).

[disambiguationuk]

This could be avoided if we could strip upstream headers, however there doesn't seem like there's a supported way to do this.

[aeneasr]

Wouldn't the easier solution be to turn off the cors headers on the upstream, if oathkeeper handles them anyways, or vice versa (turn of in oathkeeper, turn on in upstream)?