Python: Harden HttpPlugin request validation (#13969)

Improve input validation and request handling in the Python HttpPlugin.

### Changes
- Add explicit opt-in for unrestricted domain access
- Tighten URL validation logic
- Improve redirect handling when domain restrictions are configured
- Add regression tests

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SergeyMenshykh

python/semantic_kernel/core_plugins/http_plugin.py

@@ -15,49 +15,85 @@ class HttpPlugin(KernelBaseModel):
     """A plugin that provides HTTP functionality.
 
     Usage:
-        kernel.add_plugin(HttpPlugin(), "http")
-
-        # With allowed domains for security:
+        # With allowed domains (recommended):
         kernel.add_plugin(HttpPlugin(allowed_domains=["example.com", "api.example.com"]), "http")
 
+        # Explicitly allow all domains (opt-in, less secure):
+        kernel.add_plugin(HttpPlugin(allow_all_domains=True), "http")
+
     Examples:
         {{http.getAsync $url}}
         {{http.postAsync $url}}
         {{http.putAsync $url}}
         {{http.deleteAsync $url}}
+
+    Security:
+        - By default, all requests are blocked unless ``allowed_domains`` is provided
+          or ``allow_all_domains`` is set to True.
+        - When ``allowed_domains`` is set and ``allow_all_domains`` is False, HTTP
+          redirects are disabled to prevent redirect-based domain bypass (SSRF).
+        - When ``allow_all_domains`` is True, redirects are allowed regardless of
+          whether ``allowed_domains`` is also set.
+        - Only ``http`` and ``https`` URL schemes are permitted.
     """
 
     allowed_domains: set[str] | None = None
-    """List of allowed domains to send requests to. If None, all domains are allowed."""
+    """Set of allowed domains to send requests to."""
+
+    allow_all_domains: bool = False
+    """When True, requests to any domain are allowed. Must be explicitly set."""
+
+    _ALLOWED_SCHEMES: frozenset[str] = frozenset({"http", "https"})
+
+    @property
+    def _allow_redirects(self) -> bool:
+        """Whether HTTP redirects should be followed.
+
+        Redirects are only allowed when ``allow_all_domains`` is True.
+        When domain restrictions are configured, redirects are disabled
+        to prevent redirect-based SSRF bypass.
+        """
+        return self.allow_all_domains
 
     def _is_uri_allowed(self, url: str) -> bool:
-        """Check if the URL's host is in the allowed domains list.
+        """Check if the URL's host and scheme are permitted.
 
         Args:
             url: The URL to check.
 
         Returns:
             True if the URL is allowed, False otherwise.
         """
-        if self.allowed_domains is None:
-            return True
-
         parsed = urlparse(url)
+
+        # Validate scheme
+        if parsed.scheme.lower() not in self._ALLOWED_SCHEMES:
+            return False
+
         host = parsed.hostname
-        if host is None:
+        if not host:
             return False
 
-        # Case-insensitive comparison
-        return host.lower() in {domain.lower() for domain in self.allowed_domains}
+        # If allow_all_domains is set, skip domain check
+        if self.allow_all_domains:
+            return True
+
+        # If allowed_domains is set, check against it
+        if self.allowed_domains is not None:
+            return host.lower() in {domain.lower() for domain in self.allowed_domains}
+
+        # Default: deny all
+        return False
 
     def _validate_url(self, url: str) -> None:
-        """Validate the URL, checking if it's not empty and is in the allowed domains.
+        """Validate the URL, checking scheme, emptiness, and allowed domains.
 
         Args:
             url: The URL to validate.
 
         Raises:
-            FunctionExecutionException: If the URL is empty or not in the allowed domains.
+            FunctionExecutionException: If the URL is empty, uses a disallowed scheme,
+                or targets a domain that is not allowed.
         """
         if not url:
             raise FunctionExecutionException("url cannot be `None` or empty")
@@ -77,7 +113,10 @@ async def get(self, url: Annotated[str, "The URL to send the request to."]) -> s
         """
         self._validate_url(url)
 
-        async with aiohttp.ClientSession() as session, session.get(url, raise_for_status=True) as response:
+        async with (
+            aiohttp.ClientSession() as session,
+            session.get(url, raise_for_status=True, allow_redirects=self._allow_redirects) as response,
+        ):
             return await response.text()
 
     @kernel_function(description="Makes a POST request to a uri", name="postAsync")
@@ -100,7 +139,9 @@ async def post(
         data = json.dumps(body) if body is not None else None
         async with (
             aiohttp.ClientSession() as session,
-            session.post(url, headers=headers, data=data, raise_for_status=True) as response,
+            session.post(
+                url, headers=headers, data=data, raise_for_status=True, allow_redirects=self._allow_redirects
+            ) as response,
         ):
             return await response.text()
 
@@ -125,7 +166,9 @@ async def put(
         data = json.dumps(body) if body is not None else None
         async with (
             aiohttp.ClientSession() as session,
-            session.put(url, headers=headers, data=data, raise_for_status=True) as response,
+            session.put(
+                url, headers=headers, data=data, raise_for_status=True, allow_redirects=self._allow_redirects
+            ) as response,
         ):
             return await response.text()
 
@@ -141,5 +184,8 @@ async def delete(self, url: Annotated[str, "The URI to send the request to."]) -
         """
         self._validate_url(url)
 
-        async with aiohttp.ClientSession() as session, session.delete(url, raise_for_status=True) as response:
+        async with (
+            aiohttp.ClientSession() as session,
+            session.delete(url, raise_for_status=True, allow_redirects=self._allow_redirects) as response,
+        ):
             return await response.text()