docs: add Iceberg security model (apache#688)

Similar to: apache/iceberg#16538

This PR publishes a security model for Iceberg and adds a detailed
threat model referenced from AGENTS.md so there is a clear project
reference for how Iceberg distinguishes security issues from other bugs.

The goal is to make Iceberg’s security boundaries explicit for both
human triage and automated scanning, and to reduce false positives in
future security reporting.

This PR was drafted with AI assistance and then manually reviewed and
edited to reflect the project’s current approach to security roles,
trust boundaries, and the handling of recent security reports.

### Note:

`AGENTS.md` is currently in `.gitignore`, but this file was force
committed to ensure that agents running automated security scans will be
able to find the security model.

Author: sungwy

AGENTS.md

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+    http://www.apache.org/licenses/LICENSE-2.0
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+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
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+
+# Apache Iceberg C++ — Agent Instructions
+
+This file provides repository-specific guidance for automated agents working
+in this repository.
+
+## Security Model
+
+When assessing potential vulnerabilities or calibrating automated security
+findings, use [`SECURITY-THREAT-MODEL.md`](SECURITY-THREAT-MODEL.md) as the
+authoritative detailed description of this repository's security boundaries,
+trust assumptions, and non-boundaries.

SECURITY-THREAT-MODEL.md

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+<!--
+  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.
+-->
+
+# Apache Iceberg C++ Security Threat Model
+
+This document describes the detailed security threat model for Apache
+Iceberg C++. It is intended for maintainers and automated security triage.
+
+## Purpose
+
+Apache Iceberg C++ is primarily a native library implementing Iceberg table
+format handling, catalog interactions, and related tooling for C++
+applications and services. It is commonly embedded in larger systems that
+provide their own authentication, authorization, and credential management.
+Because of that deployment model, not every unsafe or surprising behavior is a
+security vulnerability in Iceberg C++ itself.
+
+This model is intended to answer:
+
+- what Iceberg C++ generally treats as a security vulnerability
+- what Iceberg C++ generally treats as correctness, hardening, or deployment
+  work
+- which boundaries are primarily owned by Iceberg C++ versus the surrounding
+  catalog, application, or service
+- which issue classes should be downgraded by default by scanners
+
+## Scope
+
+This model is scoped to the Apache Iceberg C++ repository itself:
+
+- table format and metadata handling
+- catalog and REST catalog clients
+- transport, credential, and configuration handling implemented in this repo
+- native parsing, memory management, and helper tooling shipped in this repo
+
+It is not a general threat model for every application that embeds Iceberg
+C++.
+
+In particular, it does not attempt to define the complete security model for:
+
+- applications or services that embed Iceberg C++
+- storage-level authorization enforced outside Iceberg C++
+
+## Security Goals
+
+Iceberg C++ should:
+
+- avoid exposing secrets or delegated credentials to principals that were not
+  already trusted with them
+- avoid creating new unauthorized capabilities in Iceberg C++-owned
+  components
+- avoid violating trust boundaries that Iceberg C++ itself owns, such as
+  leaking auth, transport, or credential-bearing state across catalog or
+  client boundaries in the same process
+- avoid memory-safety violations triggered by untrusted input, including
+  out-of-bounds access, use-after-free, and other memory corruption
+
+Iceberg C++ does not aim to be the primary enforcement point for:
+
+- user-to-user authorization inside the embedding application
+- storage-level authorization
+- service-side credential scoping performed by an external catalog
+
+## Roles
+
+### Operator
+
+The operator configures the surrounding catalog, application, service, and
+storage integration around Iceberg C++. This role is trusted to choose
+endpoints, warehouses, storage integrations, and credentials.
+
+### Catalog control plane
+
+The catalog control plane resolves tables and supplies metadata, locations,
+configuration, and delegated credentials to Iceberg C++. It may be
+implemented by a REST catalog server or another catalog implementation.
+Iceberg C++ assumes this control plane is trusted and outside its primary
+security boundary.
+
+### REST catalog client
+
+The REST catalog client consumes catalog-provided metadata, configuration, and
+credentials. Client-side bugs in routing, caching, or reuse may still be
+security-relevant if they leak credential-bearing state across boundaries that
+the Iceberg C++ client is expected to preserve.
+
+### Embedding application
+
+Applications and services embedding Iceberg C++ are responsible for their own
+user-facing authorization boundaries unless Iceberg C++ explicitly documents
+otherwise.
+
+### Table writer or maintainer
+
+This role may already have legitimate power to write or replace table
+metadata, write or delete files, choose paths under an allowed warehouse or
+table location, and invoke destructive maintenance operations. If a report
+only shows a new way to achieve the same effect this role can already cause
+legitimately, it is usually not a security issue in Iceberg C++.
+
+## Trust Boundaries
+
+### Boundary 1: operator-trusted configuration
+
+The following are generally treated as trusted operator or deployment inputs:
+
+- catalog properties
+- endpoint configuration
+- warehouse and storage roots
+- transport wiring and credential configuration
+
+If a report depends on the attacker controlling those values directly, it is
+usually not a vulnerability in Iceberg C++ itself.
+
+### Boundary 2: catalog-supplied metadata
+
+Iceberg C++ often accepts metadata locations, table properties, namespace
+properties, and related control-plane information from a catalog. By default,
+Iceberg C++ treats those sources as trusted.
+
+This means a malicious catalog supplying incorrect or malicious metadata is
+usually not an Iceberg C++ vulnerability by itself.
+
+### Boundary 3: REST catalog-supplied configuration and delegated storage access
+
+In REST deployments, Iceberg C++ may also accept service endpoints,
+configuration, and delegated storage access from the REST catalog server. By
+default, those are treated as trusted control-plane inputs unless Iceberg C++
+explicitly documents a stronger guarantee.
+
+This means a malicious REST catalog server sending dangerous endpoints is
+usually not an Iceberg C++ vulnerability by itself. It also means many
+credential-selection bugs are often correctness or specification issues rather
+than security boundary failures.
+
+The major exception is secret exposure. If Iceberg C++ surfaces credentials or
+secrets to a new audience that was not already trusted with them, that is
+security-relevant.
+
+### Boundary 4: storage-level authorization
+
+Object store permissions are enforced by the storage provider and the
+credentials the surrounding deployment chooses to hand to Iceberg C++.
+Iceberg C++ is not the root authority for bucket- or object-level
+authorization.
+
+## In-Scope Security Vulnerabilities
+
+The following categories are generally security-relevant in Iceberg C++ when
+the report is credible and reproducible.
+
+### 1. Secret or credential disclosure to a new audience
+
+Examples include:
+
+- catalog or storage credentials exposed through a user-visible surface
+- one catalog's credentials or auth state leaking into another catalog or
+  client
+
+### 2. Iceberg C++-owned trust-boundary violations
+
+Security issues exist when Iceberg C++ itself is expected to separate
+catalogs, clients, or principals and fails to do so.
+
+Examples include:
+
+- process-global auth or transport state crossing catalog instances
+- secret-bearing state from one principal reused for another principal within
+  an Iceberg C++-owned boundary
+
+### 3. Memory-safety violations from untrusted input
+
+Out-of-bounds access, use-after-free, memory corruption, and similar native
+memory-safety issues triggered by untrusted input are generally security-
+relevant in Iceberg C++.
+
+## Usually Out of Scope or Non-Security by Default
+
+These categories may still be real bugs worth fixing, but they are not usually
+security vulnerabilities in Iceberg C++ itself.
+
+### 1. Correctness bugs
+
+Examples include incorrect metadata handling, ambiguous matching semantics,
+and logic bugs that do not create a new trust-boundary violation.
+
+### 2. Parser hardening and malformed-input robustness without memory corruption
+
+Malformed-input crashes, bounded allocation failures, and memory amplification
+without memory corruption are usually treated as robustness or hardening work
+rather than security issues in Iceberg C++ itself.
+
+### 3. Malicious catalog or external service scenarios
+
+Reports that require a malicious catalog or other external control-plane
+service are usually outside Iceberg C++'s primary security boundary.
+
+### 4. Equivalent-harm reports
+
+If the actor already has a legitimate capability that can cause the same harm,
+the new path is usually not a security issue.
+
+## Scanner Calibration Rules
+
+A scanner targeting Iceberg C++ should treat a finding as higher-confidence
+only if it plausibly shows one of the following:
+
+- exposure of a secret or delegated credential to a new audience
+- creation of a new unauthorized capability in an Iceberg C++-owned component
+- violation of an Iceberg C++-owned trust boundary rather than a surrounding
+  catalog, application, service, or operator boundary
+- memory corruption or other native memory-safety violations triggered by
+  untrusted input
+
+A finding should be downgraded or rejected by default if it instead depends
+primarily on:
+
+- malformed-input robustness or denial-of-service behavior without memory
+  corruption
+- a malicious catalog or external service
+- a principal that already has equivalent power through legitimate write or
+  maintenance capabilities