GHSA-86QP-5C8J-P5MR

Vulnerability from github – Published: 2026-06-04 13:15 – Updated: 2026-06-04 13:15
VLAI
Summary
Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
Details

Summary

In affected versions, the HTTP Host request header was not validated before being used to reconstruct request.url. Because the routing algorithm relies on the raw HTTP path while request.url is rebuilt from the Host header, a malformed header could make request.url.path differ from the path that was actually requested. Middleware and endpoints that apply security restrictions based on request.url (rather than the raw scope path) could therefore be bypassed.

Details

When a client requests http://example.com/foo, it sends:

GET /foo HTTP/1.1
Host: example.com

Affected versions reconstructed the URL by concatenating http://{host}{path} and re-parsing the result. The Host value is only valid as a uri-host [ ":" port ] per RFC 9112 §3.2, where uri-host follows the restricted host grammar of RFC 3986 §3.2.2. When it contains characters outside that grammar - notably /, ?, or # - those characters move the path/query/fragment boundaries during re-parsing, so the parsed request.url.path no longer matches the path the server actually received. For example:

GET /foo HTTP/1.1
Host: example.com/abc?bar=

reconstructs to http://example.com/abc?bar=/foo, whose parsed path is /abc - even though routing used the real path /foo. The router still dispatches to /foo and the endpoint executes, but any middleware or code that reads request.url.path sees /abc, so path-based authorization checks can be bypassed.

Impact

Any application running an affected version that relies on request.url (or request.url.path) for security-sensitive decisions is affected. The most common case is middleware that gates access to certain path prefixes based on request.url.path. Deployments fronted by a proxy or load balancer are mitigated only if that proxy rejects or normalizes the malformed Host header before forwarding and the application does not trust attacker-controlled host headers (e.g. X-Forwarded-Host) elsewhere.

Mitigation

Upgrade to a patched version, which validates the Host header against the grammar of RFC 9112 §3.2 / RFC 3986 §3.2.2 when constructing request.url and falls back to scope["server"] for malformed values.

Severity
6.5 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.0.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "starlette"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.0.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48710"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-444"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-04T13:15:17Z",
    "nvd_published_at": "2026-05-26T22:16:44Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\nIn affected versions, the HTTP `Host` request header was not validated before being used to reconstruct `request.url`. Because the routing algorithm relies on the raw HTTP path while `request.url` is rebuilt from the `Host` header, a malformed header could make `request.url.path` differ from the path that was actually requested. Middleware and endpoints that apply security restrictions based on `request.url` (rather than the raw `scope` path) could therefore be bypassed.\n\n### Details\nWhen a client requests `http://example.com/foo`, it sends:\n\n```http\nGET /foo HTTP/1.1\nHost: example.com\n```\n\nAffected versions reconstructed the URL by concatenating `http://{host}{path}` and re-parsing the result. The `Host` value is only valid as a `uri-host [ \":\" port ]` per [RFC 9112 \u00a73.2](https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2-6), where `uri-host` follows the restricted `host` grammar of [RFC 3986 \u00a73.2.2](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.2.2). When it contains characters outside that grammar - notably `/`, `?`, or `#` - those characters move the path/query/fragment boundaries during re-parsing, so the parsed `request.url.path` no longer matches the path the server actually received. For example:\n\n```http\nGET /foo HTTP/1.1\nHost: example.com/abc?bar=\n```\n\nreconstructs to `http://example.com/abc?bar=/foo`, whose parsed `path` is `/abc` - even though routing used the real path `/foo`. The router still dispatches to `/foo` and the endpoint executes, but any middleware or code that reads `request.url.path` sees `/abc`, so path-based authorization checks can be bypassed.\n\n### Impact\nAny application running an affected version that relies on `request.url` (or `request.url.path`) for security-sensitive decisions is affected. The most common case is middleware that gates access to certain path prefixes based on `request.url.path`. Deployments fronted by a proxy or load balancer are mitigated only if that proxy rejects or normalizes the malformed `Host` header before forwarding and the application does not trust attacker-controlled host headers (e.g. `X-Forwarded-Host`) elsewhere.\n\n### Mitigation\nUpgrade to a patched version, which validates the `Host` header against the grammar of [RFC 9112 \u00a73.2](https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2-6) / [RFC 3986 \u00a73.2.2](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.2.2) when constructing `request.url` and falls back to `scope[\"server\"]` for malformed values.",
  "id": "GHSA-86qp-5c8j-p5mr",
  "modified": "2026-06-04T13:15:18Z",
  "published": "2026-06-04T13:15:17Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48710"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6"
    },
    {
      "type": "WEB",
      "url": "https://badhost.org"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Kludex/starlette"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml"
    },
    {
      "type": "WEB",
      "url": "https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette"
    },
    {
      "type": "WEB",
      "url": "https://www.secwest.net/starlette"
    },
    {
      "type": "WEB",
      "url": "https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks"
}


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