diff --git a/src/hookrelay/config.py b/src/hookrelay/config.py
index 978fa8c..a82f464 100644
--- a/src/hookrelay/config.py
+++ b/src/hookrelay/config.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import hashlib
 import json
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -73,10 +74,15 @@ def _parse_signature(data: object, route_name: str) -> Signature | None:
         raise ConfigError(f"route {route_name!r}: signature must be an object")
     if not data.get("secret"):
         raise ConfigError(f"route {route_name!r}: signature.secret is required")
+    algorithm = str(data.get("algorithm", "sha256"))
+    if algorithm not in hashlib.algorithms_available:
+        raise ConfigError(
+            f"route {route_name!r}: unsupported signature.algorithm {algorithm!r}"
+        )
     return Signature(
         secret=str(data["secret"]),
         header=str(data.get("header", "X-Hub-Signature-256")),
-        algorithm=str(data.get("algorithm", "sha256")),
+        algorithm=algorithm,
         prefix=str(data.get("prefix", "")),
     )
 
diff --git a/tests/test_config.py b/tests/test_config.py
index 920a5bf..1576ed5 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -77,6 +77,20 @@ def test_signature_requires_secret():
         parse_config(bad)
 
 
+def test_signature_rejects_unsupported_algorithm():
+    bad = {
+        "routes": [
+            {
+                "name": "r",
+                "signature": {"secret": "shh", "algorithm": "sha265"},
+                "sinks": [{"type": "slack", "url": "http://s"}],
+            }
+        ]
+    }
+    with pytest.raises(ConfigError):
+        parse_config(bad)
+
+
 def test_signature_parsed():
     data = {
         "routes": [