diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index 697a742cb..43ddfa14f 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -40,6 +40,7 @@ use Composer\Downloader\TransportException;
 use Composer\Json\JsonValidationException;
 use Composer\Repository\InstalledRepositoryInterface;
 use Seld\JsonLint\JsonParser;
+use UnexpectedValueException;
 use ZipArchive;
 
 /**
@@ -170,18 +171,21 @@ class Factory
 
         // determine and add main dirs to the config
         $home = self::getHomeDir();
-        $config->merge(array('config' => array(
-            'home' => $home,
-            'cache-dir' => self::getCacheDir($home),
-            'data-dir' => self::getDataDir($home),
-        )), Config::SOURCE_DEFAULT);
+        $config->merge(array(
+            'config' => array(
+                'home' => $home,
+                'cache-dir' => self::getCacheDir($home),
+                'data-dir' => self::getDataDir($home),
+            )
+        ), Config::SOURCE_DEFAULT);
 
         // load global config
         $file = new JsonFile($config->get('home').'/config.json');
         if ($file->exists()) {
-            if ($io && $io->isDebug()) {
-                $io->writeError('Loading config file ' . $file->getPath());
+            if ($io) {
+                $io->writeError('Loading config file ' . $file->getPath(), true, IOInterface::DEBUG);
             }
+            self::validateJsonSchema($io, $file);
             $config->merge($file->read(), $file->getPath());
         }
         $config->setConfigSource(new JsonConfigSource($file));
@@ -205,26 +209,30 @@ class Factory
         // load global auth file
         $file = new JsonFile($config->get('home').'/auth.json');
         if ($file->exists()) {
-            if ($io && $io->isDebug()) {
-                $io->writeError('Loading config file ' . $file->getPath());
+            if ($io) {
+                $io->writeError('Loading config file ' . $file->getPath(), true, IOInterface::DEBUG);
             }
+            self::validateJsonSchema($io, $file, JsonFile::AUTH_SCHEMA);
             $config->merge(array('config' => $file->read()), $file->getPath());
         }
         $config->setAuthConfigSource(new JsonConfigSource($file, true));
 
         // load COMPOSER_AUTH environment variable if set
         if ($composerAuthEnv = Platform::getEnv('COMPOSER_AUTH')) {
-            $authData = json_decode($composerAuthEnv, true);
-
+            $authData = json_decode($composerAuthEnv);
             if (null === $authData) {
                 if ($io) {
                     $io->writeError('<error>COMPOSER_AUTH environment variable is malformed, should be a valid JSON object</error>');
                 }
             } else {
-                if ($io && $io->isDebug()) {
-                    $io->writeError('Loading auth config from COMPOSER_AUTH');
+                if ($io) {
+                    $io->writeError('Loading auth config from COMPOSER_AUTH', true, IOInterface::DEBUG);
+                }
+                self::validateJsonSchema($io, $authData, JsonFile::AUTH_SCHEMA, 'COMPOSER_AUTH');
+                $authData = json_decode($composerAuthEnv, true);
+                if (null !== $authData) {
+                    $config->merge(array('config' => $authData), 'COMPOSER_AUTH');
                 }
-                $config->merge(array('config' => $authData), 'COMPOSER_AUTH');
             }
         }
 
@@ -690,4 +698,26 @@ class Factory
 
         return rtrim(strtr($home, '\\', '/'), '/');
     }
+
+    /**
+     * @param mixed $fileOrData
+     * @param JsonFile::*_SCHEMA $schema
+     */
+    private static function validateJsonSchema(?IOInterface $io, $fileOrData, int $schema = JsonFile::LAX_SCHEMA, ?string $source = null): void
+    {
+        try {
+            if ($fileOrData instanceof JsonFile) {
+                $fileOrData->validateSchema($schema);
+            } else {
+                JsonFile::validateJsonSchema($source, $fileOrData, $schema);
+            }
+        } catch (JsonValidationException $e) {
+            $msg = $e->getMessage().', this may result in errors and should be resolved:'.PHP_EOL.' - '.implode(PHP_EOL.' - ', $e->getErrors());
+            if ($io) {
+                $io->writeError('<warning>'.$msg.'</>');
+            } else {
+                throw new UnexpectedValueException($msg);
+            }
+        }
+    }
 }
diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index 987de5cbb..49f27f6a9 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -30,6 +30,7 @@ class JsonFile
 {
     public const LAX_SCHEMA = 1;
     public const STRICT_SCHEMA = 2;
+    public const AUTH_SCHEMA = 3;
 
     /** @deprecated Use \JSON_UNESCAPED_SLASHES */
     public const JSON_UNESCAPED_SLASHES = 64;
@@ -186,7 +187,9 @@ class JsonFile
      * @param  string|null             $schemaFile a path to the schema file
      * @throws JsonValidationException
      * @throws ParsingException
-     * @return bool                    true on success
+     * @return true                    true on success
+     *
+     * @phpstan-param self::*_SCHEMA $schema
      */
     public function validateSchema(int $schema = self::STRICT_SCHEMA, ?string $schemaFile = null): bool
     {
@@ -197,6 +200,11 @@ class JsonFile
             self::validateSyntax($content, $this->path);
         }
 
+        return self::validateJsonSchema($this->path, $data, $schema, $schemaFile);
+    }
+
+    public static function validateJsonSchema($source, $data, int $schema, ?string $schemaFile = null): bool
+    {
         $isComposerSchemaFile = false;
         if (null === $schemaFile) {
             $isComposerSchemaFile = true;
@@ -216,6 +224,8 @@ class JsonFile
         } elseif ($schema === self::STRICT_SCHEMA && $isComposerSchemaFile) {
             $schemaData->additionalProperties = false;
             $schemaData->required = array('name', 'description');
+        } elseif ($schema === self::AUTH_SCHEMA && $isComposerSchemaFile) {
+            $schemaData = (object) array('$ref' => $schemaFile.'#/properties/config', '$schema'=> "https://json-schema.org/draft-04/schema#");
         }
 
         $validator = new Validator();
@@ -226,7 +236,7 @@ class JsonFile
             foreach ((array) $validator->getErrors() as $error) {
                 $errors[] = ($error['property'] ? $error['property'].' : ' : '').$error['message'];
             }
-            throw new JsonValidationException('"'.$this->path.'" does not match the expected JSON schema', $errors);
+            throw new JsonValidationException('"'.$source.'" does not match the expected JSON schema', $errors);
         }
 
         return true;
diff --git a/tests/Composer/Test/Json/JsonFileTest.php b/tests/Composer/Test/Json/JsonFileTest.php
index fa393fc41..8e79fad6b 100644
--- a/tests/Composer/Test/Json/JsonFileTest.php
+++ b/tests/Composer/Test/Json/JsonFileTest.php
@@ -239,6 +239,20 @@ class JsonFileTest extends TestCase
         unlink($schema);
     }
 
+    public function testAuthSchemaValidationWithCustomDataSource(): void
+    {
+        $json = json_decode('{"github-oauth": "foo"}');
+        $expectedMessage = sprintf('"COMPOSER_AUTH" does not match the expected JSON schema');
+        $expectedError = 'github-oauth : String value found, but an object is required';
+        try {
+            JsonFile::validateJsonSchema('COMPOSER_AUTH', $json, JsonFile::AUTH_SCHEMA);
+            $this->fail('Expected exception to be thrown');
+        } catch (JsonValidationException $e) {
+            $this->assertEquals($expectedMessage, $e->getMessage());
+            $this->assertSame([$expectedError], $e->getErrors());
+        }
+    }
+
     public function testParseErrorDetectMissingCommaMultiline(): void
     {
         $json = '{