diff --git a/core/src/main/java/hudson/ProxyConfiguration.java b/core/src/main/java/hudson/ProxyConfiguration.java
index 6e54fbb0e3..fa4206521e 100644
--- a/core/src/main/java/hudson/ProxyConfiguration.java
+++ b/core/src/main/java/hudson/ProxyConfiguration.java
@@ -248,7 +248,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
 
     @DataBoundSetter
     public void setUserName(String userName) {
-        this.userName = userName;
+        this.userName = Util.fixEmptyAndTrim(userName);
     }
 
     @DataBoundSetter
@@ -290,6 +290,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
             secretPassword = Secret.fromString(Scrambler.descramble(password));
         password = null;
         authenticator = newAuthenticator();
+        userName = Util.fixEmptyAndTrim(userName);
         return this;
     }
 
@@ -363,12 +364,9 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
      *
      * <p>Equivalent to {@code newHttpClientBuilder().followRedirects(HttpClient.Redirect.NORMAL).build()}.
      *
-     * <p>The Jenkins-specific default settings include a proxy server and proxy authentication (as
-     * configured by {@link ProxyConfiguration}) and a connection timeout (as configured by {@link
-     * ProxyConfiguration#DEFAULT_CONNECT_TIMEOUT_MILLIS}).
-     *
      * @return a new {@link HttpClient}
      * @since 2.379
+     * @see #newHttpClientBuilder
      */
     public static HttpClient newHttpClient() {
         return newHttpClientBuilder().followRedirects(HttpClient.Redirect.NORMAL).build();
@@ -383,6 +381,12 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
      * configured by {@link ProxyConfiguration}) and a connection timeout (as configured by {@link
      * ProxyConfiguration#DEFAULT_CONNECT_TIMEOUT_MILLIS}).
      *
+     * <p><strong>Warning:</strong> if both {@link #getName} and {@link #getUserName} are set
+     * (meaning that an authenticated proxy is defined),
+     * you will not be able to pass an {@code Authorization} header to the real server
+     * when running on Java 17 and later
+     * (pending <a href="https://bugs.openjdk.org/browse/JDK-8326949">JDK-8326949</a>.
+     *
      * @return an {@link HttpClient.Builder}
      * @since 2.379
      */
diff --git a/test/src/test/java/hudson/ProxyConfigurationManagerGUITest.java b/test/src/test/java/hudson/ProxyConfigurationManagerGUITest.java
new file mode 100644
index 0000000000..5309e43d04
--- /dev/null
+++ b/test/src/test/java/hudson/ProxyConfigurationManagerGUITest.java
@@ -0,0 +1,69 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2024 CloudBees, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package hudson;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+
+import hudson.util.Secret;
+import java.io.File;
+import org.apache.commons.io.FileUtils;
+import org.junit.Rule;
+import org.junit.Test;
+import org.jvnet.hudson.test.JenkinsSessionRule;
+
+public final class ProxyConfigurationManagerGUITest {
+
+    @Rule public JenkinsSessionRule rr = new JenkinsSessionRule();
+
+    @Test public void configRoundtrip() throws Throwable {
+        rr.then(r -> {
+            assertNull(r.jenkins.proxy);
+            r.jenkins.proxy = new ProxyConfiguration("proxy.mycorp", 80);
+            r.configRoundtrip();
+            FileUtils.copyFile(new File(r.jenkins.root, "proxy.xml"), System.out);
+        });
+        rr.then(r -> {
+            ProxyConfiguration pc = r.jenkins.proxy;
+            assertNotNull(pc);
+            assertEquals("proxy.mycorp", pc.getName());
+            assertEquals(80, pc.getPort());
+            assertNull(pc.getUserName());
+            pc.setUserName("proxyuser");
+            pc.setSecretPassword(Secret.fromString("proxypass"));
+            r.configRoundtrip();
+            FileUtils.copyFile(new File(r.jenkins.root, "proxy.xml"), System.out);
+        });
+        rr.then(r -> {
+            ProxyConfiguration pc = r.jenkins.proxy;
+            assertEquals("proxy.mycorp", pc.getName());
+            assertEquals(80, pc.getPort());
+            assertEquals("proxyuser", pc.getUserName());
+            assertEquals("proxypass", pc.getSecretPassword().getPlainText());
+        });
+    }
+
+}