Fix OAuth2 client-secret encoding with Base64 padding

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/AbstractRestClientOAuth2AccessTokenResponseClient.java

@@ -20,7 +20,9 @@
 
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
 import org.springframework.http.converter.FormHttpMessageConverter;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.security.oauth2.client.http.OAuth2ErrorResponseErrorHandler;
 import org.springframework.security.oauth2.client.registration.ClientRegistration;
 import org.springframework.security.oauth2.core.ClientAuthenticationMethod;
@@ -66,6 +68,7 @@ public abstract class AbstractRestClientOAuth2AccessTokenResponseClient<T extend
 			.messageConverters((messageConverters) -> {
 				messageConverters.clear();
 				messageConverters.add(new FormHttpMessageConverter());
+				messageConverters.add(new StringHttpMessageConverter());
 				messageConverters.add(new OAuth2AccessTokenResponseHttpMessageConverter());
 			})
 			.defaultStatusHandler(new OAuth2ErrorResponseErrorHandler())
@@ -134,6 +137,22 @@ private RequestHeadersSpec<?> populateRequest(T grantRequest) {
 		}
 		this.parametersCustomizer.accept(parameters);
 
+		// Special handling for CLIENT_SECRET_POST to preserve Base64 padding
+		ClientRegistration clientRegistration = grantRequest.getClientRegistration();
+		if (ClientAuthenticationMethod.CLIENT_SECRET_POST.equals(clientRegistration.getClientAuthenticationMethod())) {
+			String formData = OAuth2ClientCredentialsGrantRequestEntityUtils.encodeFormData(parameters);
+			return this.restClient.post()
+				.uri(clientRegistration.getProviderDetails().getTokenUri())
+				.headers((headers) -> {
+					HttpHeaders headersToAdd = this.headersConverter.convert(grantRequest);
+					if (headersToAdd != null) {
+						headers.addAll(headersToAdd);
+					}
+					headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
+				})
+				.body(formData);
+		}
+
 		return this.restClient.post()
 			.uri(grantRequest.getClientRegistration().getProviderDetails().getTokenUri())
 			.headers((headers) -> {

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/endpoint/OAuth2ClientCredentialsGrantRequestEntityUtils.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.oauth2.client.endpoint;
+
+import java.io.UnsupportedEncodingException;
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
+import java.util.StringJoiner;
+
+import org.springframework.security.oauth2.core.endpoint.OAuth2ParameterNames;
+import org.springframework.util.MultiValueMap;
+import org.springframework.util.StringUtils;
+
+/**
+ * Utility methods for the OAuth 2.0 Client Credentials Grant.
+ *
+ * @author Hyunjoon Kim
+ * @since 6.5
+ */
+final class OAuth2ClientCredentialsGrantRequestEntityUtils {
+
+	private OAuth2ClientCredentialsGrantRequestEntityUtils() {
+	}
+
+	static String encodeFormData(MultiValueMap<String, String> parameters) {
+		StringJoiner result = new StringJoiner("&");
+		parameters.forEach((key, values) -> {
+			for (String value : values) {
+				result.add(encodeFormParameter(key, value));
+			}
+		});
+		return result.toString();
+	}
+
+	private static String encodeFormParameter(String name, String value) {
+		if (!StringUtils.hasText(value)) {
+			return urlEncode(name);
+		}
+
+		// Special handling for client_secret to preserve Base64 padding
+		if (OAuth2ParameterNames.CLIENT_SECRET.equals(name) && value.endsWith("=")) {
+			// For client secrets ending with '=', don't encode the padding character
+			int lastEqualIndex = value.lastIndexOf('=');
+			String beforePadding = value.substring(0, lastEqualIndex);
+			String padding = value.substring(lastEqualIndex);
+			return urlEncode(name) + "=" + urlEncode(beforePadding) + padding;
+		}
+
+		return urlEncode(name) + "=" + urlEncode(value);
+	}
+
+	private static String urlEncode(String value) {
+		try {
+			return URLEncoder.encode(value, StandardCharsets.UTF_8.name());
+		}
+		catch (UnsupportedEncodingException ex) {
+			// Should never happen with UTF-8
+			throw new IllegalArgumentException(ex);
+		}
+	}
+
+}