diff --git a/src/Composer/Cache.php b/src/Composer/Cache.php
index 4e1fa7429..1a216c592 100644
--- a/src/Composer/Cache.php
+++ b/src/Composer/Cache.php
@@ -47,7 +47,7 @@ class Cache
      * @param Filesystem  $filesystem optional filesystem instance
      * @param bool        $readOnly   whether the cache is in readOnly mode
      */
-    public function __construct(IOInterface $io, string $cacheDir, string $allowlist = 'a-z0-9.', ?Filesystem $filesystem = null, bool $readOnly = false)
+    public function __construct(IOInterface $io, string $cacheDir, string $allowlist = 'a-z0-9._', ?Filesystem $filesystem = null, bool $readOnly = false)
     {
         $this->io = $io;
         $this->root = rtrim($cacheDir, '/\\') . '/';
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index fa9ac4c54..a3442720f 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -175,7 +175,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
         assert($baseUrl !== '');
         $this->baseUrl = $baseUrl;
         $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);