An authorization server that supports PKCE but does not make its use mandatory for all flows can be susceptible to a PKCE downgrade attack.
OAuth 2.0 Security Best Current Practice (draft 25) (SecBCP, section 4.8)
This threat is considered fully mitigated if all the test cases from the following test set succeed.
The impact factor is a measure that indicates how important a given countermeasure is towards mitigating a threat.
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