diff --git a/netwerk/cache2/CacheStorage.cpp b/netwerk/cache2/CacheStorage.cpp
index c98c44d9ad52..c420193bb2bf 100644
--- a/netwerk/cache2/CacheStorage.cpp
+++ b/netwerk/cache2/CacheStorage.cpp
@@ -69,6 +69,11 @@ NS_IMETHODIMP CacheStorage::AsyncOpenURI(nsIURI *aURI,
   if (LookupAppCache()) {
     rv = ChooseApplicationCache(noRefURI, getter_AddRefs(appCache));
     NS_ENSURE_SUCCESS(rv, rv);
+
+    if (appCache) {
+      // From a chosen appcache open only as readonly
+      aFlags &= ~nsICacheStorage::OPEN_TRUNCATE;
+    }
   }
 
   if (appCache) {