Fix OAuth2 client-secret encoding with Base64 padding #17674
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spring-projects-issues
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status: waiting-for-triage
When using CLIENT_SECRET_POST authentication method with a client secret
that ends with Base64 padding characters ('='), the padding was being
URL-encoded to '%3D'. This caused authentication failures with some
OAuth2 providers that expect the padding characters to remain unencoded.
This commit adds special handling for CLIENT_SECRET_POST authentication
to preserve Base64 padding characters in the client secret while still
properly encoding other form parameters.
Closes spring-projectsgh-17629
Signed-off-by: Hyunjoon Kim <[email protected]>
Signed-off-by: academey <[email protected]>
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Summary
This PR adds compatibility support for OAuth2/OIDC providers that expect Base64 padding characters ('=') to remain unencoded in client secrets when using the CLIENT_SECRET_POST authentication method.
Context
As discussed in #17629, the current behavior of URL-encoding '=' to '%3D' is actually standards-compliant with:
application/x-www-form-urlencodedencodingHowever, some OIDC providers don't properly handle URL-encoded padding characters, causing authentication failures.
Details
AbstractRestClientOAuth2AccessTokenResponseClientOAuth2ClientCredentialsGrantRequestEntityUtilsto handle form parameter encoding while preserving Base64 paddingCompatibility Note
This change is a compatibility enhancement rather than a bug fix. It helps Spring Security work with OIDC providers that don't strictly follow the standards.
Test Plan
getTokenResponseWhenClientSecretPostWithBase64PaddingThenPaddingNotEncodedto verify the fixOpen Questions
Should this behavior be configurable to maintain strict standards compliance by default?
Closes gh-17629