diff --git a/maven-core/src/main/java/org/apache/maven/project/artifact/MavenMetadataSource.java b/maven-core/src/main/java/org/apache/maven/project/artifact/MavenMetadataSource.java
index f2fc322b34..1750a7b15a 100644
--- a/maven-core/src/main/java/org/apache/maven/project/artifact/MavenMetadataSource.java
+++ b/maven-core/src/main/java/org/apache/maven/project/artifact/MavenMetadataSource.java
@@ -68,6 +68,7 @@
 import org.apache.maven.project.ProjectBuildingException;
 import org.apache.maven.project.ProjectBuildingRequest;
 import org.apache.maven.properties.internal.EnvironmentUtils;
+import org.apache.maven.properties.internal.SystemProperties;
 import org.apache.maven.repository.legacy.metadata.DefaultMetadataResolutionRequest;
 import org.apache.maven.repository.legacy.metadata.MetadataResolutionRequest;
 import org.codehaus.plexus.PlexusContainer;
@@ -752,7 +753,7 @@ private Properties getSystemProperties()
 
         EnvironmentUtils.addEnvVars( props );
 
-        props.putAll( System.getProperties() );
+        SystemProperties.addSystemProperties( props );
 
         return props;
     }
diff --git a/maven-core/src/main/java/org/apache/maven/properties/internal/SystemProperties.java b/maven-core/src/main/java/org/apache/maven/properties/internal/SystemProperties.java
new file mode 100644
index 0000000000..9cb07c4d12
--- /dev/null
+++ b/maven-core/src/main/java/org/apache/maven/properties/internal/SystemProperties.java
@@ -0,0 +1,51 @@
+package org.apache.maven.properties.internal;
+
+import java.util.Properties;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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
+ *
+ *   http://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.
+ */
+
+/**
+ * @since 3.2.3
+ */
+public class SystemProperties
+{
+    /**
+     * Thread-safe System.properties copy implementation.
+     * 
+     * @see http://jira.codehaus.org/browse/MNG-5670
+     */
+    public static void addSystemProperties( Properties props )
+    {
+        for ( String key : System.getProperties().stringPropertyNames() )
+        {
+            props.put( key, System.getProperty( key ) );
+        }
+    }
+
+    /**
+     * Returns System.properties copy.
+     */
+    public static Properties getSystemProperties()
+    {
+        Properties systemProperties = new Properties();
+        addSystemProperties( systemProperties );
+        return systemProperties;
+    }
+}
diff --git a/maven-core/src/main/java/org/apache/maven/settings/DefaultMavenSettingsBuilder.java b/maven-core/src/main/java/org/apache/maven/settings/DefaultMavenSettingsBuilder.java
index f11c4418fd..820d886e29 100644
--- a/maven-core/src/main/java/org/apache/maven/settings/DefaultMavenSettingsBuilder.java
+++ b/maven-core/src/main/java/org/apache/maven/settings/DefaultMavenSettingsBuilder.java
@@ -23,6 +23,7 @@
 import java.io.IOException;
 
 import org.apache.maven.execution.MavenExecutionRequest;
+import org.apache.maven.properties.internal.SystemProperties;
 import org.apache.maven.settings.building.DefaultSettingsBuildingRequest;
 import org.apache.maven.settings.building.SettingsBuilder;
 import org.apache.maven.settings.building.SettingsBuildingException;
@@ -71,7 +72,7 @@ public Settings buildSettings( File userSettingsFile )
         SettingsBuildingRequest request = new DefaultSettingsBuildingRequest();
         request.setUserSettingsFile( userSettingsFile );
         request.setGlobalSettingsFile( globalSettingsFile );
-        request.setSystemProperties( System.getProperties() );
+        request.setSystemProperties( SystemProperties.getSystemProperties() );
         return build( request );
     }
 
diff --git a/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java b/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java
index 123b01aa81..9e64dd1aa3 100644
--- a/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java
+++ b/maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java
@@ -59,6 +59,7 @@
 import org.apache.maven.model.building.ModelProcessor;
 import org.apache.maven.project.MavenProject;
 import org.apache.maven.properties.internal.EnvironmentUtils;
+import org.apache.maven.properties.internal.SystemProperties;
 import org.apache.maven.settings.building.DefaultSettingsBuildingRequest;
 import org.apache.maven.settings.building.SettingsBuilder;
 import org.apache.maven.settings.building.SettingsBuildingRequest;
@@ -1163,8 +1164,8 @@ static void populateProperties( CommandLine commandLine, Properties systemProper
             }
         }
 
-        systemProperties.putAll( System.getProperties() );
-        
+        SystemProperties.addSystemProperties( systemProperties );
+
         // ----------------------------------------------------------------------
         // Properties containing info about the currently running version of Maven
         // These override any corresponding properties set on the command line
diff --git a/maven-settings-builder/src/main/java/org/apache/maven/settings/building/DefaultSettingsBuildingRequest.java b/maven-settings-builder/src/main/java/org/apache/maven/settings/building/DefaultSettingsBuildingRequest.java
index 6b7e2f4faf..08c93fce43 100644
--- a/maven-settings-builder/src/main/java/org/apache/maven/settings/building/DefaultSettingsBuildingRequest.java
+++ b/maven-settings-builder/src/main/java/org/apache/maven/settings/building/DefaultSettingsBuildingRequest.java
@@ -106,7 +106,11 @@ public DefaultSettingsBuildingRequest setSystemProperties( Properties systemProp
         if ( systemProperties != null )
         {
             this.systemProperties = new Properties();
-            this.systemProperties.putAll( systemProperties );
+            // MNG-5670 guard against ConcurrentModificationException
+            for ( String key : System.getProperties().stringPropertyNames() )
+            {
+                this.systemProperties.put( key, System.getProperty( key ) );
+            }
         }
         else
         {