diff --git a/lib/configuration/variables/sources/resolve-external-plugin-sources.js b/lib/configuration/variables/sources/resolve-external-plugin-sources.js
new file mode 100644
index 00000000000..39bbc818ac9
--- /dev/null
+++ b/lib/configuration/variables/sources/resolve-external-plugin-sources.js
@@ -0,0 +1,66 @@
+'use strict';
+
+const ensurePlainObject = require('type/plain-object/ensure');
+const ensurePlainFunction = require('type/plain-function/ensure');
+const ServerlessError = require('../../../serverless-error');
+const logDeprecation = require('../../../utils/logDeprecation');
+
+module.exports = (configuration, resolverConfiguration, externalPlugins) => {
+  for (const externalPlugin of externalPlugins) {
+    const pluginName = externalPlugin.constructor.name;
+    if (externalPlugin.configurationVariablesSources != null) {
+      ensurePlainObject(externalPlugin.configurationVariablesSources, {
+        errorMessage:
+          'Invalid "configurationVariablesSources" ' +
+          `configuration on "${pluginName}", expected object, got: %v"`,
+        Error: ServerlessError,
+        errorCode: 'INVALID_VARIABLE_SOURCES_CONFIGURATION',
+      });
+
+      for (const [sourceName, sourceConfig] of Object.entries(
+        externalPlugin.configurationVariablesSources
+      )) {
+        if (resolverConfiguration.sources[sourceName]) {
+          throw new ServerlessError(
+            `Cannot add "${sourceName}" configuration variable source ` +
+              `(through "${pluginName}" plugin) as resolution rules ` +
+              'for this source name are already configured',
+            'DUPLICATE_VARIABLE_SOURCE_CONFIGURATION'
+          );
+        }
+        ensurePlainFunction(
+          ensurePlainObject(sourceConfig, {
+            errorMessage:
+              `Invalid "configurationVariablesSources.${sourceName}" ` +
+              `configuration on "${pluginName}", expected object, got: %v"`,
+            Error: ServerlessError,
+            errorCode: 'INVALID_VARIABLE_SOURCE_CONFIGURATION',
+          }).resolve,
+          {
+            errorMessage:
+              `Invalid "configurationVariablesSources.${sourceName}.resolve" ` +
+              `value on "${pluginName}", expected function, got: %v"`,
+            Error: ServerlessError,
+            errorCode: 'INVALID_VARIABLE_SOURCE_RESOLVER_CONFIGURATION',
+          }
+        );
+
+        resolverConfiguration.sources[sourceName] = sourceConfig;
+        resolverConfiguration.fulfilledSources.add(sourceName);
+      }
+    } else if (
+      externalPlugin.variableResolvers &&
+      configuration.variablesResolutionMode < 20210326
+    ) {
+      logDeprecation(
+        'NEW_VARIABLES_RESOLVER',
+        `Plugin "${pluginName}" attempts to extend old variables resolver. ` +
+          'Ensure to rely on latest version of a plugin and if this warning is ' +
+          'still displayed please report the problem at plugin issue tracker' +
+          'Starting with next major release, ' +
+          'old variables resolver will not be supported.\n',
+        { serviceConfig: configuration }
+      );
+    }
+  }
+};
diff --git a/scripts/serverless.js b/scripts/serverless.js
index a11350f57a6..18840d9b0c4 100755
--- a/scripts/serverless.js
+++ b/scripts/serverless.js
@@ -554,67 +554,13 @@ const processSpanPromise = (async () => {
           }
         }
 
-        const ensurePlainFunction = require('type/plain-function/ensure');
-        const ensurePlainObject = require('type/plain-object/ensure');
-
         // Register variable source resolvers provided by external plugins
-        for (const externalPlugin of serverless.pluginManager.externalPlugins) {
-          const pluginName = externalPlugin.constructor.name;
-          if (externalPlugin.configurationVariablesSources != null) {
-            ensurePlainObject(externalPlugin.configurationVariablesSources, {
-              errorMessage:
-                'Invalid "configurationVariablesSources" ' +
-                `configuration on "${pluginName}", expected object, got: %v"`,
-              Error: ServerlessError,
-              errorCode: 'INVALID_VARIABLE_SOURCES_CONFIGURATION',
-            });
-
-            for (const [sourceName, sourceConfig] of Object.entries(
-              externalPlugin.configurationVariablesSources
-            )) {
-              if (resolverConfiguration.sources[sourceName]) {
-                throw new ServerlessError(
-                  `Cannot add "${sourceName}" configuration variable source ` +
-                    `(through "${pluginName}" plugin) as resolution rules ` +
-                    'for this source name are already configured',
-                  'DUPLICATE_VARIABLE_SOURCE_CONFIGURATION'
-                );
-              }
-              ensurePlainFunction(
-                ensurePlainObject(sourceConfig, {
-                  errorMessage:
-                    `Invalid "configurationVariablesSources.${sourceName}" ` +
-                    `configuration on "${pluginName}", expected object, got: %v"`,
-                  Error: ServerlessError,
-                  errorCode: 'INVALID_VARIABLE_SOURCE_CONFIGURATION',
-                }).resolve,
-                {
-                  errorMessage:
-                    `Invalid "configurationVariablesSources.${sourceName}.resolve" ` +
-                    `value on "${pluginName}", expected function, got: %v"`,
-                  Error: ServerlessError,
-                  errorCode: 'INVALID_VARIABLE_SOURCE_RESOLVER_CONFIGURATION',
-                }
-              );
-
-              resolverConfiguration.sources[sourceName] = sourceConfig;
-              resolverConfiguration.fulfilledSources.add(sourceName);
-            }
-          } else if (
-            externalPlugin.variableResolvers &&
-            configuration.variablesResolutionMode < 20210326
-          ) {
-            logDeprecation(
-              'NEW_VARIABLES_RESOLVER',
-              `Plugin "${pluginName}" attempts to extend old variables resolver. ` +
-                'Ensure to rely on latest version of a plugin and if this warning is ' +
-                'still displayed please report the problem at plugin issue tracker' +
-                'Starting with next major release, ' +
-                'old variables resolver will not be supported.\n',
-              { serviceConfig: configuration }
-            );
-          }
-        }
+        const resolverExternalPluginSources = require('../lib/configuration/variables/sources/resolve-external-plugin-sources');
+        resolverExternalPluginSources(
+          configuration,
+          resolverConfiguration,
+          serverless.pluginManager.externalPlugins
+        );
 
         // Having all source resolvers configured, resolve variables
         await resolveVariables(resolverConfiguration);
diff --git a/test/unit/lib/configuration/variables/sources/resolve-external-plugin-resources.test.js b/test/unit/lib/configuration/variables/sources/resolve-external-plugin-resources.test.js
new file mode 100644
index 00000000000..52978b18342
--- /dev/null
+++ b/test/unit/lib/configuration/variables/sources/resolve-external-plugin-resources.test.js
@@ -0,0 +1,90 @@
+'use strict';
+
+const { expect } = require('chai');
+
+const ServerlessError = require('../../../../../../lib/serverless-error');
+const resolveExternalPluginSources = require('../../../../../../lib/configuration/variables/sources/resolve-external-plugin-sources');
+
+describe('test/unit/lib/configuration/variables/sources/resolve-external-plugin-resources.test.js', () => {
+  it('should resolve external plugin sources', () => {
+    const sources = {};
+    const fulfilledSources = new Set();
+    const externalPlugins = [
+      {
+        configurationVariablesSources: {
+          ext1: { resolve: () => {} },
+          ext2: { resolve: () => {} },
+        },
+      },
+      {
+        configurationVariablesSources: {
+          ext3: { resolve: () => {} },
+        },
+      },
+    ];
+    resolveExternalPluginSources({}, { sources, fulfilledSources }, new Set(externalPlugins));
+    expect(sources).to.deep.equal({
+      ...externalPlugins[0].configurationVariablesSources,
+      ...externalPlugins[1].configurationVariablesSources,
+    });
+    expect(fulfilledSources).to.deep.equal(new Set(Object.keys(sources)));
+  });
+
+  it('should reject meaningfully invalid sources configuration', () => {
+    expect(() =>
+      resolveExternalPluginSources(
+        {},
+        { sources: {}, fulfilledSources: new Set() },
+        new Set([
+          {
+            configurationVariablesSources: 'foo',
+          },
+        ])
+      )
+    )
+      .to.throw(ServerlessError)
+      .with.property('code', 'INVALID_VARIABLE_SOURCES_CONFIGURATION');
+
+    expect(() =>
+      resolveExternalPluginSources(
+        {},
+        { sources: { existing: { resolve: () => {} } }, fulfilledSources: new Set(['existing']) },
+        new Set([
+          {
+            configurationVariablesSources: { existing: { resolve: () => {} } },
+          },
+        ])
+      )
+    )
+      .to.throw(ServerlessError)
+      .with.property('code', 'DUPLICATE_VARIABLE_SOURCE_CONFIGURATION');
+
+    expect(() =>
+      resolveExternalPluginSources(
+        {},
+        { sources: {}, fulfilledSources: new Set() },
+        new Set([
+          {
+            configurationVariablesSources: { source: 'foo' },
+          },
+        ])
+      )
+    )
+      .to.throw(ServerlessError)
+      .with.property('code', 'INVALID_VARIABLE_SOURCE_CONFIGURATION');
+
+    expect(() =>
+      resolveExternalPluginSources(
+        {},
+        { sources: {}, fulfilledSources: new Set() },
+        new Set([
+          {
+            configurationVariablesSources: { source: { resolve: 'foo ' } },
+          },
+        ])
+      )
+    )
+      .to.throw(ServerlessError)
+      .with.property('code', 'INVALID_VARIABLE_SOURCE_RESOLVER_CONFIGURATION');
+  });
+});