diff --git a/app/alias_utils.py b/app/alias_utils.py
index 77bae501..d05d70ad 100644
--- a/app/alias_utils.py
+++ b/app/alias_utils.py
@@ -1,3 +1,4 @@
+import re
 from typing import Optional
 
 from sqlalchemy.exc import IntegrityError
@@ -207,3 +208,14 @@ def nb_email_log_for_mailbox(mailbox: Mailbox):
         .filter(Contact.alias_id.in_(alias_ids))
         .count()
     )
+
+
+# Only lowercase letters, numbers, dashes (-) and underscores (_) are currently supported
+_ALIAS_PREFIX_PATTERN = r"[0-9a-z-_]{1,}"
+
+
+def check_alias_prefix(alias_prefix) -> bool:
+    if re.fullmatch(_ALIAS_PREFIX_PATTERN, alias_prefix) is None:
+        return False
+
+    return True
diff --git a/app/api/views/new_custom_alias.py b/app/api/views/new_custom_alias.py
index 244bc9a7..8f72d4b5 100644
--- a/app/api/views/new_custom_alias.py
+++ b/app/api/views/new_custom_alias.py
@@ -2,6 +2,7 @@ from flask import g
 from flask import jsonify, request
 from itsdangerous import SignatureExpired
 
+from app.alias_utils import check_alias_prefix
 from app.api.base import api_bp, require_api_auth
 from app.api.serializer import (
     serialize_alias_info,
@@ -236,6 +237,9 @@ def new_custom_alias_v3():
     name = data.get("name")
     alias_prefix = convert_to_id(alias_prefix)
 
+    if not check_alias_prefix(alias_prefix):
+        return jsonify(error="alias prefix format problem"), 400
+
     # check if mailbox is not tempered with
     mailboxes = []
     for mailbox_id in mailbox_ids:
diff --git a/app/dashboard/views/custom_alias.py b/app/dashboard/views/custom_alias.py
index 5440297f..ab9bb4c1 100644
--- a/app/dashboard/views/custom_alias.py
+++ b/app/dashboard/views/custom_alias.py
@@ -5,6 +5,7 @@ from flask_login import login_required, current_user
 from itsdangerous import TimestampSigner, SignatureExpired
 from sqlalchemy.exc import IntegrityError
 
+from app.alias_utils import check_alias_prefix
 from app.config import (
     DISABLE_ALIAS_SUFFIX,
     CUSTOM_ALIAS_SECRET,
@@ -122,6 +123,14 @@ def custom_alias():
         mailbox_ids = request.form.getlist("mailboxes")
         alias_note = request.form.get("note")
 
+        if not check_alias_prefix(alias_prefix):
+            flash(
+                "Only lowercase letters, numbers, dashes (-) and underscores (_) "
+                "are currently supported for alias prefix",
+                "error",
+            )
+            return redirect(url_for("dashboard.custom_alias"))
+
         # check if mailbox is not tempered with
         mailboxes = []
         for mailbox_id in mailbox_ids:
diff --git a/app/oauth/views/authorize.py b/app/oauth/views/authorize.py
index 79435428..988856a3 100644
--- a/app/oauth/views/authorize.py
+++ b/app/oauth/views/authorize.py
@@ -5,6 +5,7 @@ from flask import request, render_template, redirect, flash
 from flask_login import current_user
 from itsdangerous import SignatureExpired
 
+from app.alias_utils import check_alias_prefix
 from app.config import EMAIL_DOMAIN
 from app.dashboard.views.custom_alias import available_suffixes, signer
 from app.extensions import db
@@ -154,6 +155,14 @@ def authorize():
 
                 alias_prefix = alias_prefix.strip().lower().replace(" ", "")
 
+                if not check_alias_prefix(alias_prefix):
+                    flash(
+                        "Only lowercase letters, numbers, dashes (-) and underscores (_) "
+                        "are currently supported for alias prefix",
+                        "error",
+                    )
+                    return redirect(request.url)
+
                 # hypothesis: user will click on the button in the 600 secs
                 try:
                     alias_suffix = signer.unsign(signed_suffix, max_age=600).decode()
diff --git a/tests/test_alias_utils.py b/tests/test_alias_utils.py
index ae684617..6ee883e6 100644
--- a/tests/test_alias_utils.py
+++ b/tests/test_alias_utils.py
@@ -1,4 +1,4 @@
-from app.alias_utils import delete_alias
+from app.alias_utils import delete_alias, check_alias_prefix
 from app.extensions import db
 from app.models import User, Alias, DeletedAlias
 
@@ -46,3 +46,11 @@ def test_delete_alias_already_in_trash(flask_client):
 
     delete_alias(alias, user)
     assert Alias.get_by(email="first@d1.test") is None
+
+
+def test_check_alias_prefix(flask_client):
+    assert check_alias_prefix("ab-cd_")
+    assert not check_alias_prefix("")
+    assert not check_alias_prefix("éè")
+    assert not check_alias_prefix("a b")
+    assert not check_alias_prefix("+👌")