diff --git a/dnstapir/key_resolver.py b/dnstapir/key_resolver.py
index ff8ed7a..455d0d6 100644
--- a/dnstapir/key_resolver.py
+++ b/dnstapir/key_resolver.py
@@ -1,7 +1,8 @@
 import logging
+import re
 from abc import abstractmethod
 from pathlib import Path
-from urllib.parse import urljoin
+from urllib.parse import urljoin, urlparse
 
 import httpx
 from cryptography.hazmat.primitives.asymmetric.ec import EllipticCurvePublicKey
@@ -34,11 +35,16 @@ def key_resolver_from_client_database(client_database: str, key_cache: KeyCache
 class KeyResolver:
     def __init__(self):
         self.logger = logging.getLogger(__name__).getChild(self.__class__.__name__)
+        self.key_id_validator = re.compile(r"^[a-zA-Z0-9_-]+$")
 
     @abstractmethod
     def resolve_public_key(self, key_id: str) -> PublicKey:
         pass
 
+    def validate_key_id(self, key_id: str) -> None:
+        if not self.key_id_validator.match(key_id):
+            raise ValueError(f"Invalid key_id format: {key_id}")
+
 
 class CacheKeyResolver(KeyResolver):
     def __init__(self, key_cache: KeyCache | None):
@@ -69,6 +75,7 @@ def __init__(self, client_database_directory: str, key_cache: KeyCache | None =
 
     def get_public_key_pem(self, key_id: str) -> bytes:
         with tracer.start_as_current_span("get_public_key_pem_from_file"):
+            self.validate_key_id(key_id)
             filename = Path(self.client_database_directory) / f"{key_id}.pem"
             self.logger.debug("Fetching public key for %s from %s", key_id, filename)
             try:
@@ -87,10 +94,16 @@ def __init__(self, client_database_base_url: str, key_cache: KeyCache | None = N
 
     def get_public_key_pem(self, key_id: str) -> bytes:
         with tracer.start_as_current_span("get_public_key_pem_from_url"):
+            self.validate_key_id(key_id)
+
             if self.key_id_pattern in self.client_database_base_url:
                 public_key_url = self.client_database_base_url.replace(self.key_id_pattern, key_id)
             else:
                 public_key_url = urljoin(self.client_database_base_url, f"{key_id}.pem")
+
+            if urlparse(public_key_url).scheme not in ("http", "https"):
+                raise ValueError(f"Invalid URL constructed: {public_key_url}")
+
             self.logger.debug("Fetching public key for %s from %s", key_id, public_key_url)
             try:
                 response = self.httpx_client.get(public_key_url)
diff --git a/tests/test_key_resolver.py b/tests/test_key_resolver.py
index 9958337..8c2a9fe 100644
--- a/tests/test_key_resolver.py
+++ b/tests/test_key_resolver.py
@@ -46,6 +46,9 @@ def test_url_key_resolver(httpx_mock: HTTPXMock):
     request = httpx_mock.get_request()
     assert request.headers["Accept"] == "application/x-pem-file"
 
+    with pytest.raises(ValueError):
+        _ = resolver.resolve_public_key("🔐")
+
     with pytest.raises(KeyError):
         _ = resolver.resolve_public_key("unknown")