diff --git a/src/Composer/Cache.php b/src/Composer/Cache.php
index cafad5a28..9c4b252bc 100644
--- a/src/Composer/Cache.php
+++ b/src/Composer/Cache.php
@@ -48,7 +48,7 @@ class Cache
      * @param Filesystem  $filesystem optional filesystem instance
      * @param bool        $readOnly   whether the cache is in readOnly mode
      */
-    public function __construct(IOInterface $io, $cacheDir, $allowlist = 'a-z0-9.', Filesystem $filesystem = null, $readOnly = false)
+    public function __construct(IOInterface $io, $cacheDir, $allowlist = 'a-z0-9._', Filesystem $filesystem = null, $readOnly = false)
     {
         $this->io = $io;
         $this->root = rtrim($cacheDir, '/\\') . '/';
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index 45accf11f..9210c235b 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -168,7 +168,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
 
         $this->baseUrl = rtrim(Preg::replace('{(?:/[^/\\\\]+\.json)?(?:[?#].*)?$}', '', $this->url), '/');
         $this->io = $io;
-        $this->cache = new Cache($io, $config->get('cache-repo-dir').'/'.Preg::replace('{[^a-z0-9.]}i', '-', Url::sanitize($this->url)), 'a-z0-9.$~');
+        $this->cache = new Cache($io, $config->get('cache-repo-dir').'/'.Preg::replace('{[^a-z0-9.]}i', '-', Url::sanitize($this->url)), 'a-z0-9.$~_');
         $this->cache->setReadOnly($config->get('cache-read-only'));
         $this->versionParser = new VersionParser();
         $this->loader = new ArrayLoader($this->versionParser);