diff --git a/gradle.properties b/gradle.properties
index 4083ddf7..fa246149 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -6,4 +6,4 @@ org.gradle.parallel=true
 
 javaFormatVersion=0.0.39
 jmustacheVersion=1.15
-springFrameworkVersion=6.0.0
+springFrameworkVersion=6.0.14-SNAPSHOT
diff --git a/spring-restdocs-core/src/main/java/org/springframework/restdocs/cli/CliOperationRequest.java b/spring-restdocs-core/src/main/java/org/springframework/restdocs/cli/CliOperationRequest.java
index d3b2f9f2..a955fcaf 100644
--- a/spring-restdocs-core/src/main/java/org/springframework/restdocs/cli/CliOperationRequest.java
+++ b/spring-restdocs-core/src/main/java/org/springframework/restdocs/cli/CliOperationRequest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2022 the original author or authors.
+ * Copyright 2014-2023 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.
@@ -18,6 +18,7 @@ package org.springframework.restdocs.cli;
 
 import java.net.URI;
 import java.util.Arrays;
+import java.util.Base64;
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
@@ -30,7 +31,6 @@ import org.springframework.http.HttpMethod;
 import org.springframework.restdocs.operation.OperationRequest;
 import org.springframework.restdocs.operation.OperationRequestPart;
 import org.springframework.restdocs.operation.RequestCookie;
-import org.springframework.util.Base64Utils;
 
 /**
  * An {@link OperationRequest} wrapper with methods that are useful when producing a
@@ -137,7 +137,7 @@ final class CliOperationRequest implements OperationRequest {
 		}
 
 		static String decodeBasicAuthHeader(List<String> value) {
-			return new String(Base64Utils.decodeFromString(value.get(0).substring(6)));
+			return new String(Base64.getDecoder().decode(value.get(0).substring(6)));
 		}
 
 	}
diff --git a/spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/HeadersModifyingOperationPreprocessor.java b/spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/HeadersModifyingOperationPreprocessor.java
index 72078847..9f0d4372 100644
--- a/spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/HeadersModifyingOperationPreprocessor.java
+++ b/spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/HeadersModifyingOperationPreprocessor.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2022 the original author or authors.
+ * Copyright 2014-2023 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.
@@ -57,7 +57,7 @@ public class HeadersModifyingOperationPreprocessor implements OperationPreproces
 
 	private HttpHeaders preprocess(HttpHeaders headers) {
 		HttpHeaders modifiedHeaders = new HttpHeaders();
-		modifiedHeaders.putAll(headers);
+		modifiedHeaders.addAll(headers);
 		for (Modification modification : this.modifications) {
 			modification.applyTo(modifiedHeaders);
 		}
diff --git a/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/CurlRequestSnippetTests.java b/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/CurlRequestSnippetTests.java
index 8b32c3b0..2348adc4 100644
--- a/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/CurlRequestSnippetTests.java
+++ b/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/CurlRequestSnippetTests.java
@@ -17,6 +17,7 @@
 package org.springframework.restdocs.cli;
 
 import java.io.IOException;
+import java.util.Base64;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -26,7 +27,6 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.restdocs.AbstractSnippetTests;
 import org.springframework.restdocs.templates.TemplateFormat;
-import org.springframework.util.Base64Utils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -261,7 +261,7 @@ public class CurlRequestSnippetTests extends AbstractSnippetTests {
 	@Test
 	public void basicAuthCredentialsAreSuppliedUsingUserOption() throws IOException {
 		new CurlRequestSnippet(this.commandFormatter).document(this.operationBuilder.request("http://localhost/foo")
-			.header(HttpHeaders.AUTHORIZATION, "Basic " + Base64Utils.encodeToString("user:secret".getBytes()))
+			.header(HttpHeaders.AUTHORIZATION, "Basic " + Base64.getEncoder().encodeToString("user:secret".getBytes()))
 			.build());
 		assertThat(this.generatedSnippets.curlRequest())
 			.is(codeBlock("bash").withContent("$ curl 'http://localhost/foo' -i -u 'user:secret' -X GET"));
diff --git a/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/HttpieRequestSnippetTests.java b/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/HttpieRequestSnippetTests.java
index a387906d..3d4f9ff8 100644
--- a/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/HttpieRequestSnippetTests.java
+++ b/spring-restdocs-core/src/test/java/org/springframework/restdocs/cli/HttpieRequestSnippetTests.java
@@ -17,6 +17,7 @@
 package org.springframework.restdocs.cli;
 
 import java.io.IOException;
+import java.util.Base64;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -26,7 +27,6 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.restdocs.AbstractSnippetTests;
 import org.springframework.restdocs.templates.TemplateFormat;
-import org.springframework.util.Base64Utils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -260,7 +260,7 @@ public class HttpieRequestSnippetTests extends AbstractSnippetTests {
 	@Test
 	public void basicAuthCredentialsAreSuppliedUsingAuthOption() throws IOException {
 		new HttpieRequestSnippet(this.commandFormatter).document(this.operationBuilder.request("http://localhost/foo")
-			.header(HttpHeaders.AUTHORIZATION, "Basic " + Base64Utils.encodeToString("user:secret".getBytes()))
+			.header(HttpHeaders.AUTHORIZATION, "Basic " + Base64.getEncoder().encodeToString("user:secret".getBytes()))
 			.build());
 		assertThat(this.generatedSnippets.httpieRequest())
 			.is(codeBlock("bash").withContent("$ http --auth 'user:secret' GET 'http://localhost/foo'"));