GHSA-2p2x-hpg8-cqp2 Litestar's CORS origin allowlist has a bypass due to unescaped regex metacharacters in allowed origins

• PyPI litestar: >= 2.19.0, < 2.20.0 → fixed in 2.20.0

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.