diff --git a/src/certificate.py b/src/certificate.py
index 2a9fb4ce9..05c4efaa6 100644
--- a/src/certificate.py
+++ b/src/certificate.py
@@ -34,7 +34,7 @@ from datetime import datetime
 
 from moulinette import m18n
 from moulinette.utils.log import getActionLogger
-from moulinette.utils.filesystem import read_file
+from moulinette.utils.filesystem import read_file, chown, chmod
 
 from yunohost.vendor.acme_tiny.acme_tiny import get_crt as sign_certificate
 from yunohost.utils.error import YunohostError, YunohostValidationError
@@ -719,11 +719,8 @@ def _generate_key(destination_path):
 
 
 def _set_permissions(path, user, group, permissions):
-    uid = pwd.getpwnam(user).pw_uid
-    gid = grp.getgrnam(group).gr_gid
-
-    os.chown(path, uid, gid)
-    os.chmod(path, permissions)
+    chown(path, user, group)
+    chmod(path, permissions)
 
 
 def _enable_certificate(domain, new_cert_folder):
diff --git a/src/diagnosers/21-web.py b/src/diagnosers/21-web.py
index 584505ad1..5106e26cc 100644
--- a/src/diagnosers/21-web.py
+++ b/src/diagnosers/21-web.py
@@ -5,7 +5,7 @@ import random
 import requests
 from typing import List
 
-from moulinette.utils.filesystem import read_file
+from moulinette.utils.filesystem import read_file, mkdir, rm
 
 from yunohost.diagnosis import Diagnoser
 from yunohost.domain import domain_list
@@ -46,8 +46,8 @@ class MyDiagnoser(Diagnoser):
                 domains_to_check.append(domain)
 
         self.nonce = "".join(random.choice("0123456789abcedf") for i in range(16))
-        os.system("rm -rf /tmp/.well-known/ynh-diagnosis/")
-        os.system("mkdir -p /tmp/.well-known/ynh-diagnosis/")
+        rm("/tmp/.well-known/ynh-diagnosis/", recursive=True, force=True)
+        mkdir("/tmp/.well-known/ynh-diagnosis/", parents=True)
         os.system("touch /tmp/.well-known/ynh-diagnosis/%s" % self.nonce)
 
         if not domains_to_check: