diff --git a/packages/task/src/browser/task-schema-updater.ts b/packages/task/src/browser/task-schema-updater.ts
index b65316f1200a8..14cd65977fd6f 100644
--- a/packages/task/src/browser/task-schema-updater.ts
+++ b/packages/task/src/browser/task-schema-updater.ts
@@ -99,6 +99,7 @@ export class TaskSchemaUpdater {
             });
             customizedDetectedTask.properties!.problemMatcher = problemMatcher;
             customizedDetectedTask.properties!.options = commandOptionsSchema;
+            customizedDetectedTask.additionalProperties = true;
             customizedDetectedTasks.push(customizedDetectedTask);
         });
 
@@ -118,7 +119,8 @@ export class TaskSchemaUpdater {
                     }
                 },
                 inputs: inputsSchema.definitions!.inputs
-            }
+            },
+            additionalProperties: false
         };
     }
 
@@ -251,7 +253,8 @@ const processTaskConfigurationSchema: IJSONSchema = {
             properties: commandAndArgs
         },
         problemMatcher
-    }
+    },
+    additionalProperties: true
 };
 
 const customizedDetectedTasks: IJSONSchema[] = [];
diff --git a/packages/task/src/browser/task-service.ts b/packages/task/src/browser/task-service.ts
index 9d692b0a391bb..7c996444ddc8d 100644
--- a/packages/task/src/browser/task-service.ts
+++ b/packages/task/src/browser/task-service.ts
@@ -58,7 +58,6 @@ export interface QuickPickProblemMatcherItem {
 
 @injectable()
 export class TaskService implements TaskConfigurationClient {
-
     /**
      * The last executed task.
      */
@@ -233,9 +232,26 @@ export class TaskService implements TaskConfigurationClient {
         return [...configuredTasks, ...notCustomizedProvidedTasks];
     }
 
-    /** Returns an array of the task configurations which are configured in tasks.json files */
-    getConfiguredTasks(): Promise<TaskConfiguration[]> {
-        return this.taskConfigurations.getTasks();
+    /** Returns an array of the valid task configurations which are configured in tasks.json files */
+    async getConfiguredTasks(): Promise<TaskConfiguration[]> {
+        const taskConfigs = await this.taskConfigurations.getTasks();
+        const validTaskConfigs = taskConfigs.filter(t => this.isTaskConfigValid(t));
+        if (validTaskConfigs.length !== taskConfigs.length) {
+            this.messageService.warn('Invalid task configurations are found. Open tasks.json and find details in the Problems view.');
+        }
+        return validTaskConfigs;
+    }
+
+    /**
+     * Returns `true` if the given task configuration is valid as per the task schema defined in Theia
+     * or contributed by Theia extensions and plugins, `false` otherwise.
+     */
+    isTaskConfigValid(task: TaskConfiguration): boolean {
+        const schema = this.taskSchemaUpdater.getTaskSchema();
+        const ajv = new Ajv();
+        const validateSchema = ajv.compile(schema);
+        const isValid = !!validateSchema({ tasks: [task] });
+        return isValid;
     }
 
     /** Returns an array of the task configurations which are provided by the extensions. */