Add draft project security threat-model document

AGENTS.md

@@ -34,6 +34,10 @@ When working on an API area with an explicit design document, follow that
 document as the source of truth instead of inferring policy from the current
 implementation.
 
+For security-sensitive changes, follow [SECURITY.md](SECURITY.md) and
+[THREAT_MODEL.md](THREAT_MODEL.md) as the source of truth for Reqsign's
+security boundary and its relationship to OpenDAL and other host applications.
+
 ### Default Credential Provider
 
 All future refactors and API reviews around `DefaultCredentialProvider` must

SECURITY.md

@@ -0,0 +1,54 @@
+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# Security Policy
+
+## Reporting a Vulnerability
+
+Apache OpenDAL Reqsign follows the
+[Apache Software Foundation security process](https://www.apache.org/security/).
+Please report suspected vulnerabilities privately to
+`private@opendal.apache.org`. If you are unsure where to send the report, use
+the ASF Security Team address, `security@apache.org`. Do not open public GitHub
+issues or pull requests for security reports.
+
+When reporting, include:
+
+- the affected crate name and version;
+- the affected signing service, such as AWS SigV4, Azure Storage, Google,
+  Aliyun OSS, Huawei Cloud OBS, Oracle Cloud, Tencent COS, Volcengine TOS,
+  or another supported service;
+- whether the issue affects direct `reqsign` use, Apache OpenDAL integration,
+  or another embedding application;
+- a minimal reproduction, affected request, credential-provider configuration,
+  and expected versus actual behavior;
+- whether credentials, bearer tokens, private keys, signed URLs, logs, or debug
+  output were exposed.
+
+## Threat Model
+
+The security boundary, in-scope findings, out-of-scope deployment issues, and
+triage guidance for this repository are documented in
+[THREAT_MODEL.md](./THREAT_MODEL.md).
+
+Apache OpenDAL is Reqsign's primary integration for custom key signing and
+cloud-provider authentication, but Reqsign can also be embedded directly by
+other applications. OpenDAL-specific storage behavior, operator authorization,
+path policy, and storage-service trust boundaries are covered by OpenDAL's own
+security documentation.