[Possible Bug???] KC_LOCALE URI Param inconsistency #48762

Jeeva-z · 2026-05-06T14:02:33Z

Jeeva-z
May 6, 2026

Hi everyone,

I noticed what seems to be a locale handling inconsistency (possibly a bug) with kc_locale in Keycloak authorization flows.

When triggering an authentication flow from OIDC Debugger using:

kc_locale=<locale>
in the initial authorization request query params, the first login/authenticator page still loads in English. It looks like LocaleUtil.processLocaleParam() is not being triggered during the initial request.

However, if the URL is modified later during the flow (for example on the 2nd authenticator page), the locale gets applied correctly and the UI switches to the expected language.

From debugging, I noticed:

buildRegister() explicitly calls:

LocaleUtil.processLocaleParam(...)

but buildAuthorizationCodeAuthorizationResponse() does not.

So during the normal OIDC authorization code flow, the locale parameter seems to be ignored on the first render.

Has anyone faced this before?
Is this expected behavior, or could this be a bug/inconsistency in locale initialization during the authorization flow?

Thanks!

yanxch · 2026-05-10T10:37:07Z

yanxch
May 10, 2026

I think it has historic reasons why buildRegister does support kc_locales.

With the introduction of the OIDC specific endpoints the query param you have to go for is ui_locales

see #47369 (comment)

OIDC Endpoint Examples

Registration Page (prompt=create)

http://localhost:8080/realms/master/protocol/openid-connect/auth?client_id=security-admin-console&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fadmin%2Fmaster%2Fconsole%2F%23%2Fmaster%2Frealm-settings%2Flocalization&state=2c797761-5e94-40ec-9d6e-5e066e0a5d60&response_mode=query&response_type=code&scope=openid&nonce=b375f408-a259-48db-a2e9-8d8dd6f98305&code_challenge=EGfymus4SMMvjBbohYtd_JfVm9WpC-8ZNzgabTw3lDE&code_challenge_method=S256&kc_locale=en&prompt=create

Login Page

http://localhost:8080/realms/master/protocol/openid-connect/auth?client_id=security-admin-console&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fadmin%2Fmaster%2Fconsole%2F%23%2Fmaster%2Frealm-settings%2Flocalization&state=2c797761-5e94-40ec-9d6e-5e066e0a5d60&response_mode=query&response_type=code&scope=openid&nonce=b375f408-a259-48db-a2e9-8d8dd6f98305&code_challenge=EGfymus4SMMvjBbohYtd_JfVm9WpC-8ZNzgabTw3lDE&code_challenge_method=S256&ui_locales=de

Reset Password Page


http://localhost:8080/realms/master/protocol/openid-connect/forgot-credentials?client_id=security-admin-console&redirect_uri=http%3A%2F%2Flocalhost%3A8080%2Fadmin%2Fmaster%2Fconsole%2F%23%2Fmaster%2Frealm-settings%2Flocalization&state=2c797761-5e94-40ec-9d6e-5e066e0a5d60&response_mode=query&response_type=code&scope=openid&nonce=b375f408-a259-48db-a2e9-8d8dd6f98305&code_challenge=EGfymus4SMMvjBbohYtd_JfVm9WpC-8ZNzgabTw3lDE&code_challenge_method=S256&ui_locales=de

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[Possible Bug???] KC_LOCALE URI Param inconsistency #48762

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[Possible Bug???] KC_LOCALE URI Param inconsistency #48762

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Jeeva-z May 6, 2026

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Uh oh!

yanxch May 10, 2026

Jeeva-z
May 6, 2026

yanxch
May 10, 2026