diff --git a/maven-artifact/src/main/java/org/apache/maven/artifact/DefaultArtifact.java b/maven-artifact/src/main/java/org/apache/maven/artifact/DefaultArtifact.java
index 1167e91a53..3fa1907c31 100644
--- a/maven-artifact/src/main/java/org/apache/maven/artifact/DefaultArtifact.java
+++ b/maven-artifact/src/main/java/org/apache/maven/artifact/DefaultArtifact.java
@@ -273,7 +273,7 @@ public class DefaultArtifact
             return Collections.emptyList();
         }
 
-        return metadataMap.values();
+        return Collections.unmodifiableCollection( metadataMap.values() );
     }
 
     // ----------------------------------------------------------------------
diff --git a/maven-artifact/src/main/java/org/apache/maven/artifact/versioning/VersionRange.java b/maven-artifact/src/main/java/org/apache/maven/artifact/versioning/VersionRange.java
index 56343b2f6d..e9196213fb 100644
--- a/maven-artifact/src/main/java/org/apache/maven/artifact/versioning/VersionRange.java
+++ b/maven-artifact/src/main/java/org/apache/maven/artifact/versioning/VersionRange.java
@@ -262,7 +262,7 @@ public class VersionRange
         }
         else
         {
-            restrictions = intersection( r1, r2 );
+            restrictions = Collections.unmodifiableList( intersection( r1, r2 ) );
         }
 
         ArtifactVersion version = null;
diff --git a/maven-compat/src/main/java/org/apache/maven/artifact/repository/DefaultArtifactRepository.java b/maven-compat/src/main/java/org/apache/maven/artifact/repository/DefaultArtifactRepository.java
index f5db5ef634..16b82c5070 100644
--- a/maven-compat/src/main/java/org/apache/maven/artifact/repository/DefaultArtifactRepository.java
+++ b/maven-compat/src/main/java/org/apache/maven/artifact/repository/DefaultArtifactRepository.java
@@ -256,7 +256,7 @@ public class DefaultArtifactRepository
     {
         if ( mirroredRepositories != null )
         {
-            this.mirroredRepositories = mirroredRepositories;
+            this.mirroredRepositories = Collections.unmodifiableList( mirroredRepositories );
         }
         else
         {
diff --git a/maven-compat/src/main/java/org/apache/maven/repository/MetadataResolutionResult.java b/maven-compat/src/main/java/org/apache/maven/repository/MetadataResolutionResult.java
index 70b9c3fdc5..7be1d96950 100644
--- a/maven-compat/src/main/java/org/apache/maven/repository/MetadataResolutionResult.java
+++ b/maven-compat/src/main/java/org/apache/maven/repository/MetadataResolutionResult.java
@@ -118,7 +118,10 @@ public class MetadataResolutionResult
 
     public List<Artifact> getMissingArtifacts()
     {
-        return missingArtifacts == null ? Collections.<Artifact>emptyList() : missingArtifacts;
+        return missingArtifacts == null
+                   ? Collections.<Artifact>emptyList()
+                   : Collections.unmodifiableList( missingArtifacts );
+
     }
 
     public MetadataResolutionResult addMissingArtifact( Artifact artifact )
@@ -148,7 +151,10 @@ public class MetadataResolutionResult
 
     public List<Exception> getExceptions()
     {
-        return exceptions == null ? Collections.<Exception>emptyList() : exceptions;
+        return exceptions == null
+                   ? Collections.<Exception>emptyList()
+                   : Collections.unmodifiableList( exceptions );
+
     }
 
     // ------------------------------------------------------------------------
@@ -185,7 +191,10 @@ public class MetadataResolutionResult
 
     public List<Exception> getVersionRangeViolations()
     {
-        return versionRangeViolations == null ? Collections.<Exception>emptyList() : versionRangeViolations;
+        return versionRangeViolations == null
+                   ? Collections.<Exception>emptyList()
+                   : Collections.unmodifiableList( versionRangeViolations );
+
     }
 
     // ------------------------------------------------------------------------
@@ -217,8 +226,10 @@ public class MetadataResolutionResult
 
     public List<ArtifactResolutionException> getMetadataResolutionExceptions()
     {
-        return metadataResolutionExceptions == null ? Collections.<ArtifactResolutionException>emptyList()
-                        : metadataResolutionExceptions;
+        return metadataResolutionExceptions == null
+                   ? Collections.<ArtifactResolutionException>emptyList()
+                   : Collections.unmodifiableList( metadataResolutionExceptions );
+
     }
 
     // ------------------------------------------------------------------------
@@ -246,7 +257,7 @@ public class MetadataResolutionResult
             return Collections.emptyList();
         }
 
-        return errorArtifactExceptions;
+        return Collections.unmodifiableList( errorArtifactExceptions );
     }
 
     // ------------------------------------------------------------------------
@@ -283,7 +294,7 @@ public class MetadataResolutionResult
             return Collections.emptyList();
         }
 
-        return circularDependencyExceptions;
+        return Collections.unmodifiableList( circularDependencyExceptions );
     }
 
     // ------------------------------------------------------------------------
@@ -297,7 +308,7 @@ public class MetadataResolutionResult
             return Collections.emptyList();
         }
 
-        return repositories;
+        return Collections.unmodifiableList( repositories );
     }
 
     public MetadataResolutionResult setRepositories( final List<ArtifactRepository> repositories )
diff --git a/maven-core/src/main/java/org/apache/maven/artifact/repository/MavenArtifactRepository.java b/maven-core/src/main/java/org/apache/maven/artifact/repository/MavenArtifactRepository.java
index d4c64f0bf7..f04007ee6e 100644
--- a/maven-core/src/main/java/org/apache/maven/artifact/repository/MavenArtifactRepository.java
+++ b/maven-core/src/main/java/org/apache/maven/artifact/repository/MavenArtifactRepository.java
@@ -405,7 +405,7 @@ public class MavenArtifactRepository
     {
         if ( mirroredRepositories != null )
         {
-            this.mirroredRepositories = mirroredRepositories;
+            this.mirroredRepositories = Collections.unmodifiableList( mirroredRepositories );
         }
         else
         {
diff --git a/maven-core/src/main/java/org/apache/maven/artifact/resolver/ArtifactResolutionResult.java b/maven-core/src/main/java/org/apache/maven/artifact/resolver/ArtifactResolutionResult.java
index 6de04f3cbd..ffae1d3560 100644
--- a/maven-core/src/main/java/org/apache/maven/artifact/resolver/ArtifactResolutionResult.java
+++ b/maven-core/src/main/java/org/apache/maven/artifact/resolver/ArtifactResolutionResult.java
@@ -130,7 +130,10 @@ public class ArtifactResolutionResult
 
     public List<Artifact> getMissingArtifacts()
     {
-        return missingArtifacts == null ? Collections.<Artifact>emptyList() : missingArtifacts;
+        return missingArtifacts == null
+                   ? Collections.<Artifact>emptyList()
+                   : Collections.unmodifiableList( missingArtifacts );
+
     }
 
     public ArtifactResolutionResult addMissingArtifact( Artifact artifact )
@@ -165,7 +168,10 @@ public class ArtifactResolutionResult
 
     public List<Exception> getExceptions()
     {
-        return exceptions == null ? Collections.<Exception>emptyList() : exceptions;
+        return exceptions == null
+                   ? Collections.<Exception>emptyList()
+                   : Collections.unmodifiableList( exceptions );
+
     }
 
     // ------------------------------------------------------------------------
@@ -202,7 +208,10 @@ public class ArtifactResolutionResult
 
     public List<Exception> getVersionRangeViolations()
     {
-        return versionRangeViolations == null ? Collections.<Exception>emptyList() : versionRangeViolations;
+        return versionRangeViolations == null
+                   ? Collections.<Exception>emptyList()
+                   : Collections.unmodifiableList( versionRangeViolations );
+
     }
 
     // ------------------------------------------------------------------------
@@ -234,8 +243,10 @@ public class ArtifactResolutionResult
 
     public List<ArtifactResolutionException> getMetadataResolutionExceptions()
     {
-        return metadataResolutionExceptions == null ? Collections.<ArtifactResolutionException>emptyList()
-                        : metadataResolutionExceptions;
+        return metadataResolutionExceptions == null
+                   ? Collections.<ArtifactResolutionException>emptyList()
+                   : Collections.unmodifiableList( metadataResolutionExceptions );
+
     }
 
     // ------------------------------------------------------------------------
@@ -267,7 +278,7 @@ public class ArtifactResolutionResult
             return Collections.emptyList();
         }
 
-        return errorArtifactExceptions;
+        return Collections.unmodifiableList( errorArtifactExceptions );
     }
 
     // ------------------------------------------------------------------------
@@ -304,7 +315,7 @@ public class ArtifactResolutionResult
             return Collections.emptyList();
         }
 
-        return circularDependencyExceptions;
+        return Collections.unmodifiableList( circularDependencyExceptions );
     }
 
     // ------------------------------------------------------------------------
@@ -318,7 +329,7 @@ public class ArtifactResolutionResult
             return Collections.emptyList();
         }
 
-        return repositories;
+        return Collections.unmodifiableList( repositories );
     }
 
     public ArtifactResolutionResult setRepositories( final List<ArtifactRepository> repositories )
diff --git a/maven-core/src/main/java/org/apache/maven/artifact/resolver/ResolutionNode.java b/maven-core/src/main/java/org/apache/maven/artifact/resolver/ResolutionNode.java
index a156871de0..8e47d2e0b4 100644
--- a/maven-core/src/main/java/org/apache/maven/artifact/resolver/ResolutionNode.java
+++ b/maven-core/src/main/java/org/apache/maven/artifact/resolver/ResolutionNode.java
@@ -102,6 +102,7 @@ public class ResolutionNode
 
                 children.add( new ResolutionNode( a, remoteRepositories, this ) );
             }
+            children = Collections.unmodifiableList( children );
         }
         else
         {
diff --git a/maven-core/src/main/java/org/apache/maven/exception/ExceptionSummary.java b/maven-core/src/main/java/org/apache/maven/exception/ExceptionSummary.java
index dcc376a4b9..6615af61ce 100644
--- a/maven-core/src/main/java/org/apache/maven/exception/ExceptionSummary.java
+++ b/maven-core/src/main/java/org/apache/maven/exception/ExceptionSummary.java
@@ -53,7 +53,10 @@ public class ExceptionSummary
         this.exception = exception;
         this.message = ( message != null ) ? message : "";
         this.reference = ( reference != null ) ? reference : "";
-        this.children = ( children != null ) ? children : Collections.<ExceptionSummary>emptyList();
+        this.children = ( children != null )
+                            ? Collections.unmodifiableList( children )
+                            : Collections.<ExceptionSummary>emptyList();
+
     }
 
     public Throwable getException()
diff --git a/maven-core/src/main/java/org/apache/maven/execution/DefaultMavenExecutionResult.java b/maven-core/src/main/java/org/apache/maven/execution/DefaultMavenExecutionResult.java
index 87d8676627..112a064923 100644
--- a/maven-core/src/main/java/org/apache/maven/execution/DefaultMavenExecutionResult.java
+++ b/maven-core/src/main/java/org/apache/maven/execution/DefaultMavenExecutionResult.java
@@ -64,8 +64,10 @@ public class DefaultMavenExecutionResult
 
     public List<MavenProject> getTopologicallySortedProjects()
     {
-        return null == topologicallySortedProjects ? Collections.<MavenProject>emptyList()
-                        : topologicallySortedProjects;
+        return null == topologicallySortedProjects
+                   ? Collections.<MavenProject>emptyList()
+                   : Collections.unmodifiableList( topologicallySortedProjects );
+
     }
 
     public DependencyResolutionResult getDependencyResolutionResult()
diff --git a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
index 766aed1469..b78f54dc42 100644
--- a/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
+++ b/maven-core/src/main/java/org/apache/maven/lifecycle/internal/MojoExecutor.java
@@ -105,32 +105,34 @@ public class MojoExecutor
 
     private Collection<String> toScopes( String classpath )
     {
+        Collection<String> scopes = Collections.emptyList();
+
         if ( StringUtils.isNotEmpty( classpath ) )
         {
             if ( Artifact.SCOPE_COMPILE.equals( classpath ) )
             {
-                return Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED );
+                scopes = Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED );
             }
             else if ( Artifact.SCOPE_RUNTIME.equals( classpath ) )
             {
-                return Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_RUNTIME );
+                scopes = Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_RUNTIME );
             }
             else if ( Artifact.SCOPE_COMPILE_PLUS_RUNTIME.equals( classpath ) )
             {
-                return Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED,
-                                      Artifact.SCOPE_RUNTIME );
+                scopes = Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED,
+                                        Artifact.SCOPE_RUNTIME );
             }
             else if ( Artifact.SCOPE_RUNTIME_PLUS_SYSTEM.equals( classpath ) )
             {
-                return Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_RUNTIME );
+                scopes = Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_RUNTIME );
             }
             else if ( Artifact.SCOPE_TEST.equals( classpath ) )
             {
-                return Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED,
-                                      Artifact.SCOPE_RUNTIME, Artifact.SCOPE_TEST );
+                scopes = Arrays.asList( Artifact.SCOPE_COMPILE, Artifact.SCOPE_SYSTEM, Artifact.SCOPE_PROVIDED,
+                                        Artifact.SCOPE_RUNTIME, Artifact.SCOPE_TEST );
             }
         }
-        return Collections.emptyList();
+        return Collections.unmodifiableCollection( scopes );
     }
 
     public void execute( MavenSession session, List<MojoExecution> mojoExecutions, ProjectIndex projectIndex )
diff --git a/maven-core/src/main/java/org/apache/maven/plugin/internal/DefaultMavenPluginManager.java b/maven-core/src/main/java/org/apache/maven/plugin/internal/DefaultMavenPluginManager.java
index cec6f120db..86304714df 100644
--- a/maven-core/src/main/java/org/apache/maven/plugin/internal/DefaultMavenPluginManager.java
+++ b/maven-core/src/main/java/org/apache/maven/plugin/internal/DefaultMavenPluginManager.java
@@ -461,7 +461,7 @@ public class DefaultMavenPluginManager
                 it.remove();
             }
         }
-        return artifacts;
+        return Collections.unmodifiableList( artifacts );
     }
 
     private Map<String, ClassLoader> calcImports( MavenProject project, ClassLoader parent, List<String> imports )
diff --git a/maven-core/src/main/java/org/apache/maven/plugin/prefix/DefaultPluginPrefixRequest.java b/maven-core/src/main/java/org/apache/maven/plugin/prefix/DefaultPluginPrefixRequest.java
index 7ab86cfd16..01194c866d 100644
--- a/maven-core/src/main/java/org/apache/maven/plugin/prefix/DefaultPluginPrefixRequest.java
+++ b/maven-core/src/main/java/org/apache/maven/plugin/prefix/DefaultPluginPrefixRequest.java
@@ -100,7 +100,7 @@ public class DefaultPluginPrefixRequest
     {
         if ( pluginGroups != null )
         {
-            this.pluginGroups = pluginGroups;
+            this.pluginGroups = Collections.unmodifiableList( pluginGroups );
         }
         else
         {
@@ -131,7 +131,7 @@ public class DefaultPluginPrefixRequest
     {
         if ( repositories != null )
         {
-            this.repositories = repositories;
+            this.repositories = Collections.unmodifiableList( repositories );
         }
         else
         {
diff --git a/maven-core/src/main/java/org/apache/maven/plugin/version/DefaultPluginVersionRequest.java b/maven-core/src/main/java/org/apache/maven/plugin/version/DefaultPluginVersionRequest.java
index 9907066b9d..57f4250c63 100644
--- a/maven-core/src/main/java/org/apache/maven/plugin/version/DefaultPluginVersionRequest.java
+++ b/maven-core/src/main/java/org/apache/maven/plugin/version/DefaultPluginVersionRequest.java
@@ -140,7 +140,7 @@ public class DefaultPluginVersionRequest
     {
         if ( repositories != null )
         {
-            this.repositories = repositories;
+            this.repositories = Collections.unmodifiableList( repositories );
         }
         else
         {
diff --git a/maven-core/src/main/java/org/apache/maven/project/DefaultDependencyResolutionResult.java b/maven-core/src/main/java/org/apache/maven/project/DefaultDependencyResolutionResult.java
index dbdf25261d..509e8b48d9 100644
--- a/maven-core/src/main/java/org/apache/maven/project/DefaultDependencyResolutionResult.java
+++ b/maven-core/src/main/java/org/apache/maven/project/DefaultDependencyResolutionResult.java
@@ -98,7 +98,10 @@ class DefaultDependencyResolutionResult
     public List<Exception> getResolutionErrors( Dependency dependency )
     {
         List<Exception> errors = resolutionErrors.get( dependency );
-        return ( errors != null ) ? errors : Collections.<Exception>emptyList();
+        return ( errors != null )
+                   ? Collections.unmodifiableList( errors )
+                   : Collections.<Exception>emptyList();
+
     }
 
     public void setResolutionErrors( Dependency dependency, List<Exception> errors )
diff --git a/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java b/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java
index 1091a084c9..a6590463a0 100644
--- a/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java
+++ b/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java
@@ -809,6 +809,7 @@ public class DefaultProjectBuilder
                         map.put( d.getManagementKey(), artifact );
                     }
                 }
+                map = Collections.unmodifiableMap( map );
             }
             else
             {
diff --git a/maven-core/src/main/java/org/apache/maven/project/DefaultProjectRealmCache.java b/maven-core/src/main/java/org/apache/maven/project/DefaultProjectRealmCache.java
index 987f861a18..a7f06156a3 100644
--- a/maven-core/src/main/java/org/apache/maven/project/DefaultProjectRealmCache.java
+++ b/maven-core/src/main/java/org/apache/maven/project/DefaultProjectRealmCache.java
@@ -51,7 +51,9 @@ public class DefaultProjectRealmCache
 
         public CacheKey( List<? extends ClassRealm> extensionRealms )
         {
-            this.extensionRealms = ( extensionRealms != null ) ? extensionRealms : Collections.<ClassRealm>emptyList();
+            this.extensionRealms = ( extensionRealms != null )
+                                       ? Collections.unmodifiableList( extensionRealms )
+                                       : Collections.<ClassRealm>emptyList();
 
             this.hashCode = this.extensionRealms.hashCode();
         }
diff --git a/maven-core/src/main/java/org/apache/maven/project/MavenProject.java b/maven-core/src/main/java/org/apache/maven/project/MavenProject.java
index dff5f3fdcf..fd7ab40ddc 100644
--- a/maven-core/src/main/java/org/apache/maven/project/MavenProject.java
+++ b/maven-core/src/main/java/org/apache/maven/project/MavenProject.java
@@ -778,7 +778,7 @@ public class MavenProject
         {
             return Collections.emptyList();
         }
-        return getModel().getBuild().getPlugins();
+        return Collections.unmodifiableList( getModel().getBuild().getPlugins() );
     }
 
     public List<String> getModules()
@@ -1079,7 +1079,7 @@ public class MavenProject
         }
         else
         {
-            return build.getExtensions();
+            return Collections.unmodifiableList( build.getExtensions() );
         }
     }
 
@@ -1604,7 +1604,7 @@ public class MavenProject
         {
             // TODO let the scope handler deal with this
             if ( Artifact.SCOPE_COMPILE.equals( a.getScope() ) || Artifact.SCOPE_PROVIDED.equals( a.getScope() )
-                || Artifact.SCOPE_SYSTEM.equals( a.getScope() ) )
+                     || Artifact.SCOPE_SYSTEM.equals( a.getScope() ) )
             {
                 Dependency dependency = new Dependency();
 
@@ -1618,7 +1618,7 @@ public class MavenProject
                 list.add( dependency );
             }
         }
-        return list;
+        return Collections.unmodifiableList( list );
     }
 
     @Deprecated
@@ -1662,7 +1662,7 @@ public class MavenProject
 
             list.add( dependency );
         }
-        return list;
+        return Collections.unmodifiableList( list );
     }
 
     @Deprecated // used by the Maven ITs
@@ -1677,7 +1677,7 @@ public class MavenProject
 
         List<Dependency> list = new ArrayList<>( artifacts.size() );
 
-        for ( Artifact a : getArtifacts()  )
+        for ( Artifact a : getArtifacts() )
         {
             // TODO let the scope handler deal with this
             if ( Artifact.SCOPE_COMPILE.equals( a.getScope() ) || Artifact.SCOPE_RUNTIME.equals( a.getScope() ) )
@@ -1694,7 +1694,7 @@ public class MavenProject
                 list.add( dependency );
             }
         }
-        return list;
+        return Collections.unmodifiableList( list );
     }
 
     @Deprecated
@@ -1790,7 +1790,7 @@ public class MavenProject
                 list.add( dependency );
             }
         }
-        return list;
+        return Collections.unmodifiableList( list );
     }
 
     @Deprecated
@@ -1862,8 +1862,7 @@ public class MavenProject
         {
             return Collections.emptyList();
         }
-        return getModel().getReporting().getPlugins();
-
+        return Collections.unmodifiableList( getModel().getReporting().getPlugins() );
     }
 
     @Deprecated
diff --git a/maven-core/src/main/java/org/apache/maven/project/artifact/ProjectArtifact.java b/maven-core/src/main/java/org/apache/maven/project/artifact/ProjectArtifact.java
index 8556a6af64..46e56dc470 100644
--- a/maven-core/src/main/java/org/apache/maven/project/artifact/ProjectArtifact.java
+++ b/maven-core/src/main/java/org/apache/maven/project/artifact/ProjectArtifact.java
@@ -58,8 +58,11 @@ public class ProjectArtifact
 
     public List<Dependency> getManagedDependencies()
     {
-        DependencyManagement depMgmt = project.getDependencyManagement();
-        return ( depMgmt != null ) ? depMgmt.getDependencies() : Collections.<Dependency>emptyList();
+        DependencyManagement depMngt = project.getDependencyManagement();
+        return ( depMngt != null )
+                   ? Collections.unmodifiableList( depMngt.getDependencies() )
+                   : Collections.<Dependency>emptyList();
+
     }
 
     static class PomArtifactHandler
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 ca8e040732..6a1a34ad55 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
@@ -750,7 +750,9 @@ public class MavenCli
 
                 BootstrapCoreExtensionManager resolver = container.lookup( BootstrapCoreExtensionManager.class );
 
-                return resolver.loadCoreExtensions( request, providedArtifacts, extensions );
+                return Collections.unmodifiableList( resolver.loadCoreExtensions( request, providedArtifacts,
+                                                                                  extensions ) );
+
             }
             finally
             {
diff --git a/maven-model-builder/src/main/java/org/apache/maven/model/building/ModelBuildingException.java b/maven-model-builder/src/main/java/org/apache/maven/model/building/ModelBuildingException.java
index 434cb591a2..b5274382be 100644
--- a/maven-model-builder/src/main/java/org/apache/maven/model/building/ModelBuildingException.java
+++ b/maven-model-builder/src/main/java/org/apache/maven/model/building/ModelBuildingException.java
@@ -136,7 +136,7 @@ public class ModelBuildingException
         {
             return Collections.emptyList();
         }
-        return result.getProblems();
+        return Collections.unmodifiableList( result.getProblems() );
     }
 
     private static String toMessage( ModelBuildingResult result )