Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
CISA bestätigt die Ausnutzung dieser Schwachstelle in realen Angriffen. Prüfen Sie vorrangig, ob Ihre Anwendung betroffen ist. CISA KEV · CC0 1.0
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.comAffected 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.
Betroffene Software
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| Plattform | Paket | Anfällige Versionen | Behoben in |
|---|---|---|---|
| Python (PyPI) | starlette | < 1.0.1 | 1.0.1 |
Verweise
- https://badhost.org
- https://www.secwest.net/starlette
- https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette/
- https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr
- https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette/
- https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6
- https://nvd.nist.gov/vuln/detail/CVE-2026-48710
- https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette
- https://www.cve.org/CVERecord?id=CVE-2026-48710
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-48710.json
- https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette
- https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml
- https://github.com/Kludex/starlette
- https://bugzilla.redhat.com/show_bug.cgi?id=2481742
- https://access.redhat.com/security/cve/CVE-2026-48710
- https://access.redhat.com/errata/RHSA-2026:60520
- https://access.redhat.com/errata/RHSA-2026:51357
- https://access.redhat.com/errata/RHSA-2026:44696
- https://access.redhat.com/errata/RHSA-2026:43038
- https://access.redhat.com/errata/RHSA-2026:37275
- https://access.redhat.com/errata/RHSA-2026:34532
- https://access.redhat.com/errata/RHSA-2026:34526
- https://access.redhat.com/errata/RHSA-2026:34456
- https://access.redhat.com/errata/RHSA-2026:30089
- https://access.redhat.com/errata/RHSA-2026:30088
- https://access.redhat.com/errata/RHSA-2026:26226
- https://access.redhat.com/errata/RHSA-2026:24866
- https://access.redhat.com/errata/RHSA-2026:23346
- https://access.redhat.com/errata/RHSA-2026:22993
- https://access.redhat.com/errata/RHSA-2026:22992
- CVSS-Vektor
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N- Schwäche (CWE)
CWE-1289, CWE-444- Plattform
- Python (PyPI)
- Paket
starlette- Veröffentlicht
- 22.05.2026
- Aktualisiert
- 11.10.2026 15:17
Quellen und Lizenzen
OSV / GitHub Advisory Database - PYSEC-2026-161 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-86qp-5c8j-p5mr - CC BY 4.0
Schwachstellenhistorie: starlette
Alle veröffentlichten Schwachstellen dieses Pakets (Python (PyPI)), neueste zuerst. Zeigt, ob es ein einmaliger Ausrutscher ist oder ein Paket, das man im Auge behalten muss.
- Einträge insgesamt
- 10
- Erste → letzte
- 02.2023 06.2026
- Ohne Fix
- 0
- In Angriffen ausgenutzt
- 1
Verteilung nach Schweregrad
- Hoch 5
- Mittel 4
- Niedrig 1
Veröffentlichungen pro Jahr
- 2023: 2
- 2024: 1
- 2025: 2
- 2026: 5
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Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks dieser Eintrag
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