diff --git a/internal/auth/schema.go b/internal/auth/schema.go
index 0056e4cb..62148c12 100644
--- a/internal/auth/schema.go
+++ b/internal/auth/schema.go
@@ -5,8 +5,13 @@
 
 package auth
 
+// configSchema describes the "auth" section of config.json. Every subsection is
+// optional: Init() logs and continues when "jwts", "oidc" or "ldap" is absent,
+// so only the fields within a configured subsection are required.
 var configSchema = `
-	{
+{
+  "type": "object",
+  "properties": {
     "jwts": {
       "description": "For JWT token authentication.",
       "type": "object",
@@ -55,6 +60,7 @@ var configSchema = `
       "required": ["max-age"]
     },
     "oidc": {
+      "description": "For OpenID Connect authentication.",
       "type": "object",
       "properties": {
         "provider": {
@@ -148,6 +154,6 @@ var configSchema = `
         }
       },
       "required": ["url", "user-base", "search-dn", "user-bind", "user-filter"]
-    },
-  "required": ["jwts"]
-	}`
+    }
+  }
+}`
diff --git a/internal/auth/schema_test.go b/internal/auth/schema_test.go
new file mode 100644
index 00000000..6774bbbb
--- /dev/null
+++ b/internal/auth/schema_test.go
@@ -0,0 +1,104 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package auth
+
+import (
+	"encoding/json"
+	"os"
+	"testing"
+
+	"github.com/santhosh-tekuri/jsonschema/v5"
+)
+
+// compileConfigSchema guards against the schema silently degrading into a
+// document with no constraints (which is what happens when the "type"/
+// "properties" wrapper is missing: every instance validates).
+func compileConfigSchema(t *testing.T) *jsonschema.Schema {
+	t.Helper()
+	sch, err := jsonschema.CompileString("schema.json", configSchema)
+	if err != nil {
+		t.Fatalf("compiling auth config schema: %v", err)
+	}
+	return sch
+}
+
+func validateAuthConfig(t *testing.T, sch *jsonschema.Schema, raw string) error {
+	t.Helper()
+	var v any
+	if err := json.Unmarshal([]byte(raw), &v); err != nil {
+		t.Fatalf("invalid test json: %v", err)
+	}
+	return sch.Validate(v)
+}
+
+func TestConfigSchemaAccepts(t *testing.T) {
+	sch := compileConfigSchema(t)
+
+	tests := map[string]string{
+		"jwts only":    `{"jwts":{"max-age":"2000h"}}`,
+		"oidc only":    `{"oidc":{"provider":"http://localhost:8080/realms/cc"}}`,
+		"empty":        `{}`,
+		"ldap":         `{"ldap":{"url":"ldap://127.0.0.1:389","user-base":"ou=users,dc=example,dc=com","search-dn":"cn=admin,dc=example,dc=com","user-bind":"uid={username},ou=users,dc=example,dc=com","user-filter":"(objectclass=posixAccount)","role-filters":{"admin":"(memberOf=cn=cc-admin,ou=groups,dc=example,dc=com)"}}}`,
+		"role-mapping": `{"oidc":{"provider":"http://p","role-mapping":{"cc-admin":"admin"}}}`,
+	}
+
+	for name, raw := range tests {
+		if err := validateAuthConfig(t, sch, raw); err != nil {
+			t.Errorf("%s: unexpected validation error: %v", name, err)
+		}
+	}
+}
+
+func TestConfigSchemaRejects(t *testing.T) {
+	sch := compileConfigSchema(t)
+
+	tests := map[string]string{
+		"jwts without max-age":      `{"jwts":{"public-key":"abc"}}`,
+		"jwts max-age not a string": `{"jwts":{"max-age":2000}}`,
+		"oidc without provider":     `{"oidc":{"client-id":"cc-backend"}}`,
+		"ldap without user-bind":    `{"ldap":{"url":"ldap://127.0.0.1:389","user-base":"ou=users","search-dn":"cn=admin","user-filter":"(objectclass=posixAccount)"}}`,
+		"role-filters not strings":  `{"ldap":{"url":"u","user-base":"b","search-dn":"d","user-bind":"ub","user-filter":"f","role-filters":{"admin":true}}}`,
+		"jwts not an object":        `{"jwts":"2000h"}`,
+	}
+
+	for name, raw := range tests {
+		if err := validateAuthConfig(t, sch, raw); err == nil {
+			t.Errorf("%s: expected validation error, got none", name)
+		}
+	}
+}
+
+// TestConfigSchemaExampleConfigs validates the auth sections of the shipped
+// example configurations.
+func TestConfigSchemaExampleConfigs(t *testing.T) {
+	sch := compileConfigSchema(t)
+
+	for _, path := range []string{
+		"../../configs/config.json",
+		"../../configs/config-demo.json",
+		"../../configs/config-large.json",
+	} {
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var sections map[string]json.RawMessage
+		if err := json.Unmarshal(raw, &sections); err != nil {
+			t.Fatal(err)
+		}
+		authCfg, ok := sections["auth"]
+		if !ok {
+			continue
+		}
+		var v any
+		if err := json.Unmarshal(authCfg, &v); err != nil {
+			t.Fatal(err)
+		}
+		if err := sch.Validate(v); err != nil {
+			t.Errorf("%s: auth section does not validate: %v", path, err)
+		}
+	}
+}
diff --git a/internal/config/validate.go b/internal/config/validate.go
index af8591ca..7321f718 100644
--- a/internal/config/validate.go
+++ b/internal/config/validate.go
@@ -24,6 +24,6 @@ func Validate(schema string, instance json.RawMessage) {
 	}
 
 	if err = sch.Validate(v); err != nil {
-		cclog.Fatalf("%#v", err)
+		cclog.Fatalf("config validation failed: %s", err.Error())
 	}
 }