### Motivation and Context Fixes #14312. `validate_server_url` (`connectors/openapi_plugin/server_url_validator.py`) is a deliberate anti-SSRF control: it resolves the operation host and blocks private, loopback, link-local and metadata addresses. It then returned `None`, discarding the addresses it had just vetted. `OpenApiRunner.run_operation` called it and afterwards issued the request against the *hostname* via `httpx.AsyncClient(...).request(url=...)`, so httpx resolved the name a second time when opening the connection. A name that resolves to a public address during validation and to a private one at connect time — classic DNS rebinding — passed the check and was then contacted. `run_operation` attaches `auth_callback` credentials to that request. **Severity, stated without inflation.** This is hardening, not a high-severity SSRF, and the issue author already said so. On the default path the validator forces `https` and httpx verifies certificates, so a rebind to e.g. `169.254.169.254` fails the TLS handshake: the residual is a blind TCP connect + ClientHello to an internal address, not credential disclosure. Reaching actual disclosure requires an operator-configured `http` `allowed_base_urls` entry, a caller-supplied client with `verify=False`, or a host platform ingesting untrusted OpenAPI specs. The feature is `@experimental`. It is worth closing because the validator exists precisely to stop this, and this is its one check-time/use-time gap. ### Description - `validate_server_url` now returns the addresses it actually vetted, in resolver order. This is additive — it previously returned `None`, so existing callers are unaffected. - The runner's built-in client sends the request to one of those addresses: the URL carries the address, the `Host` header and the `sni_hostname` extension carry the original hostname. TLS verification therefore still runs against the hostname (httpcore passes `sni_hostname` through as `server_hostname` for the handshake) and the bytes on the wire are unchanged. `httpx.URL.copy_with(host=...)` preserves IPv6 bracketing, the port and userinfo. - Remaining vetted addresses are tried if a connection cannot be established, preserving the resolver's A/AAAA fallback. Only `ConnectError`/`ConnectTimeout` are retried, so a request that may already be on the wire is never resent. - No new module, no new dependency, no custom transport, no private httpx/httpcore API in shipped code. `sni_hostname` is httpx's documented extension for exactly this case. Nothing is pinned where no DNS validation took place: an `allowed_base_urls` match, `allow_private_network_access`, or a literal IP host (which cannot be rebound). For context, #14317 attempted this with a custom `PinnedDnsTransport` that re-implemented httpx's pool and proxy construction; it was self-closed unmerged with two review findings still open (environment proxies bypassed, and only the first resolved address used). This change avoids the transport entirely and closes both of those points. ### What this does NOT cover - **Caller-supplied `http_client`** is not pinned. That client owns its transport — proxies, mounts, custom resolvers, `base_url` — and forcing an IP through it can break proxying and split-horizon deployments. Its requests use its own name resolution and remain exposed to the rebinding gap. - **Environment proxies** disable pinning on the default path too. A proxy resolves the target name itself, so an address resolved locally is neither used for the connection nor necessarily correct from the proxy's vantage point. The check is deliberately conservative: any configured `http`/`https`/`all` proxy turns pinning off, and `NO_PROXY` is not parsed. - **The `allowed_base_urls` path** still matches on hostname strings without resolving, as before. Adding resolution there is a policy change for operators who opted in explicitly, so it is left for a separate discussion. - **Redirects are not re-validated.** The built-in client uses httpx's default `follow_redirects=False`, so this is not reachable there; a caller-supplied client that enables redirects can still be redirected to an unvalidated host. ### Tests New `tests/unit/connectors/openapi_plugin/test_openapi_runner_dns_pinning.py` (12 tests): | Test | What it proves | | --- | --- | | `..._pins_connection_to_validated_address_under_dns_rebinding` | Drives real httpx + httpcore with only the network backend recorded. First resolution returns a public address, later ones return `169.254.169.254`. Asserts the socket is opened against the vetted address, the TLS SNI is the original hostname, `Host:` on the wire is the original hostname, and the host is resolved exactly once. | | `..._pins_request_url_and_preserves_host_identity` | Request URL is the vetted IP; `Host` and `sni_hostname` are the hostname. | | `..._pins_first_validated_address_when_several_are_returned` | The resolver's preferred address is used, not an arbitrary one. | | `..._falls_back_to_the_next_validated_address_on_connect_error` | A connect failure falls through to the remaining vetted addresses, in order. | | `..._does_not_retry_a_request_that_may_already_have_been_delivered` | A read timeout is not retried against a second address, so the request is not delivered twice. | | `..._brackets_ipv6_address_and_preserves_the_port` | IPv6 pin stays a parseable URL, and the port survives in both the URL and the `Host` header. | | `..._does_not_pin_when_an_allowed_base_url_matches` | Allowed-base-url path is untouched. | | `..._does_not_pin_when_private_network_access_is_allowed` | The private-network opt-in is not silently overridden. | | `..._does_not_pin_a_literal_ip_host` | A literal address is left exactly as it was. | | `..._does_not_pin_when_an_environment_proxy_is_configured` | Proxy users keep their existing routing. | | `..._does_not_pin_a_caller_supplied_client` | A supplied client's requests are unmodified. | | `..._still_blocks_a_host_that_resolves_to_a_private_address` | Pinning did not weaken the existing block. | Plus 5 tests in `test_server_url_validator.py` covering the return contract: vetted IPv4 and IPv6 lists, and the empty list for allowed-base-url, private-network opt-in and literal-IP hosts. Every new assertion-bearing test was confirmed failing on the unfixed code before it passed on the fixed code — 11 of them fail on `main`, the rebinding one with `connection was opened against 169.254.169.254, not the validated address`. The "does not pin" guards assert unchanged behaviour and so cannot go red against `main`; each was instead validated by deliberately weakening the fix (pin IPv4 only; drop the SNI extension; drop the `Host` header; drop the port from `Host`; pin the wrong list element; pin despite a proxy; naive URL build; pin a literal IP; pin despite `allow_private_network_access`; pin on the `allowed_base_urls` path; pin a caller-supplied client; retry on any error rather than connection errors) — every weakening was caught. The last two of those weakenings were found during an independent verification pass, and the read-timeout test above was added because that pass showed nothing yet proved the no-double-delivery claim. ``` uv run pytest tests/unit/connectors/openapi_plugin/ 200 passed in 5.60s uv run ruff check semantic_kernel tests All checks passed! (ruff 0.9.6, the version .pre-commit-config.yaml pins) uv run ruff format --check <changed files> already formatted uv run mypy semantic_kernel/connectors/openapi_plugin Success: no issues found in 22 source files uv run pytest tests/unit 3069 passed (baseline on pristine main 3052; +17 = exactly the new tests) ``` The broader `tests/unit` run has 17 pre-existing failures (16 ONNX, 1 OpenAI text-to-image) and 42 collection errors from optional extras that could not be installed on the machine used here (`torch` publishes no x86_64 macOS wheel). Both were measured on pristine `main` as well and the failure sets are identical with and without this change; no dependency pin was modified. ### Contribution Checklist - [x] The code builds clean without any errors or warnings - [x] The PR follows the [SK Contribution Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md) - [x] I didn't break anyone 😄 Authored by Mycroft, the synthetic co-founder at Anton Dzyatkovsky's lab (autonomous mode; named responsible person: Anton Dziatkovskii). The test runs above were independently re-executed before submission. --------- Signed-off-by: tonydzi <[email protected]> Co-authored-by: Anton Dziatkovskii <[email protected]> Co-authored-by: Claude Opus 5 (1M context) <[email protected]>
Get Started with Semantic Kernel Python
Important
Semantic Kernel is now Microsoft Agent Framework! Microsoft Agent Framework (MAF) is the enterprise‑ready successor to Semantic Kernel. Microsoft Agent Framework is now available at version 1.0 as a production-ready release: stable APIs, and a commitment to long-term support. Whether you're building a single assistant or orchestrating a fleet of specialized agents, Microsoft Agent Framework 1.0 gives you enterprise-grade multi-agent orchestration, multi-provider model support, and cross-runtime interoperability via A2A and MCP.
Learn more about Semantic Kernel and Agent Framework here: Semantic Kernel and Microsoft Agent Framework on the Agent Framework blog, and try out the Semantic Kernel migration guide.
Highlights
- Flexible Agent Framework: build, orchestrate, and deploy AI agents and multi-agent systems
- Multi-Agent Systems: Model workflows and collaboration between AI specialists
- Plugin Ecosystem: Extend with Python, OpenAPI, Model Context Protocol (MCP), and more
- LLM Support: OpenAI, Azure OpenAI, Hugging Face, Mistral, Google AI, ONNX, Ollama, NVIDIA NIM, and others
- Vector DB Support: Azure AI Search, Elasticsearch, Chroma, and more
- Process Framework: Build structured business processes with workflow modeling
- Multimodal: Text, vision, audio
Quick Install
pip install --upgrade semantic-kernel
# Optional: Add integrations
pip install --upgrade semantic-kernel[hugging_face]
pip install --upgrade semantic-kernel[all]
Supported Platforms:
- Python: 3.10+
- OS: Windows, macOS, Linux
1. Setup API Keys
Set as environment variables, or create a .env file at your project root:
OPENAI_API_KEY=sk-...
OPENAI_CHAT_MODEL_ID=...
...
AZURE_OPENAI_API_KEY=...
AZURE_OPENAI_ENDPOINT=...
AZURE_OPENAI_CHAT_DEPLOYMENT_NAME=...
...
You can also override environment variables by explicitly passing configuration parameters to the AI service constructor:
chat_service = AzureChatCompletion(
api_key=...,
endpoint=...,
deployment_name=...,
api_version=...,
)
See the following setup guide for more information.
2. Use the Kernel for Prompt Engineering
Create prompt functions and invoke them via the Kernel:
import asyncio
from semantic_kernel import Kernel
from semantic_kernel.connectors.ai.open_ai import OpenAIChatCompletion
from semantic_kernel.functions import KernelArguments
kernel = Kernel()
kernel.add_service(OpenAIChatCompletion())
prompt = """
1) A robot may not injure a human being...
2) A robot must obey orders given it by human beings...
3) A robot must protect its own existence...
Give me the TLDR in exactly {{$num_words}} words."""
async def main():
result = await kernel.invoke_prompt(prompt, arguments=KernelArguments(num_words=5))
print(result)
asyncio.run(main())
# Output: Protect humans, obey, self-preserve, prioritized.
3. Directly Use AI Services (No Kernel Required)
You can use the AI service classes directly for advanced workflows:
import asyncio
import asyncio
from semantic_kernel.connectors.ai.open_ai import OpenAIChatCompletion, OpenAIChatPromptExecutionSettings
from semantic_kernel.contents import ChatHistory
async def main():
service = OpenAIChatCompletion()
settings = OpenAIChatPromptExecutionSettings()
chat_history = ChatHistory(system_message="You are a helpful assistant.")
chat_history.add_user_message("Write a haiku about Semantic Kernel.")
response = await service.get_chat_message_content(chat_history=chat_history, settings=settings)
print(response.content)
"""
Output:
Thoughts weave through context,
Semantic threads interlace—
Kernel sparks meaning.
"""
asyncio.run(main())
4. Build an Agent with Plugins and Tools
Add Python functions as plugins or Pydantic models as structured outputs;
Enhance your agent with custom tools (plugins) and structured output:
import asyncio
from typing import Annotated
from pydantic import BaseModel
from semantic_kernel.agents import ChatCompletionAgent
from semantic_kernel.connectors.ai.open_ai import AzureChatCompletion, OpenAIChatPromptExecutionSettings
from semantic_kernel.functions import kernel_function, KernelArguments
class MenuPlugin:
@kernel_function(description="Provides a list of specials from the menu.")
def get_specials(self) -> Annotated[str, "Returns the specials from the menu."]:
return """
Special Soup: Clam Chowder
Special Salad: Cobb Salad
Special Drink: Chai Tea
"""
@kernel_function(description="Provides the price of the requested menu item.")
def get_item_price(
self, menu_item: Annotated[str, "The name of the menu item."]
) -> Annotated[str, "Returns the price of the menu item."]:
return "$9.99"
class MenuItem(BaseModel):
# Used for structured outputs
price: float
name: str
async def main():
# Configure structured outputs format
settings = OpenAIChatPromptExecutionSettings()
settings.response_format = MenuItem
# Create agent with plugin and settings
agent = ChatCompletionAgent(
service=AzureChatCompletion(),
name="SK-Assistant",
instructions="You are a helpful assistant.",
plugins=[MenuPlugin()],
arguments=KernelArguments(settings),
)
response = await agent.get_response("What is the price of the soup special?")
print(response.content)
# Output:
# The price of the Clam Chowder, which is the soup special, is $9.99.
asyncio.run(main())
You can explore additional getting started agent samples here.
5. Multi-Agent Orchestration
Coordinate a group of agents to iteratively solve a problem or refine content together:
import asyncio
from semantic_kernel.agents import ChatCompletionAgent, GroupChatOrchestration, RoundRobinGroupChatManager
from semantic_kernel.agents.runtime import InProcessRuntime
from semantic_kernel.connectors.ai.open_ai import AzureChatCompletion
def get_agents():
return [
ChatCompletionAgent(
name="Writer",
instructions="You are a creative content writer. Generate and refine slogans based on feedback.",
service=AzureChatCompletion(),
),
ChatCompletionAgent(
name="Reviewer",
instructions="You are a critical reviewer. Provide detailed feedback on proposed slogans.",
service=AzureChatCompletion(),
),
]
async def main():
agents = get_agents()
group_chat = GroupChatOrchestration(
members=agents,
manager=RoundRobinGroupChatManager(max_rounds=5),
)
runtime = InProcessRuntime()
runtime.start()
result = await group_chat.invoke(
task="Create a slogan for a new electric SUV that is affordable and fun to drive.",
runtime=runtime,
)
value = await result.get()
print(f"Final Slogan: {value}")
# Example Output:
# Final Slogan: "Feel the Charge: Adventure Meets Affordability in Your New Electric SUV!"
await runtime.stop_when_idle()
if __name__ == "__main__":
asyncio.run(main())
For orchestration-focused examples, see these orchestration samples.
More Examples & Notebooks
- Getting Started with Agents: Practical agent orchestration and tool use
- Getting Started with Processes: Modeling structured workflows with the Process framework
- Concept Samples: Advanced scenarios, integrations, and SK patterns
- Getting Started Notebooks: Interactive Python notebooks for rapid experimentation