Kludex · Starlette
CVE-2026-48710
Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
What it is
Starlette is a lightweight ASGI framework/toolkit. Prior to version 1.0.1, 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. Users should upgrade to a version greater than or equal to version 1.0.1, 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.
CISA: Kludex Starlette contains a HTTP request/response smuggling vulnerability that could allow attackers to inject paths into the host part, prepending the actual path leading to issues such as authentication bypass when the authentication depends on the reconstructed URL’s path. This vulnerability could be chaned with CVE-2026-42271.
What to do
Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
Deadline for US federal agencies: September 16, 2026
- Added to the CISA list
- September 2, 2026
- Published
- May 26, 2026
- Severity
- Medium 6.5 CVSS 3.1
- Chance of attacks in the next 30 days (EPSS)
- 70% in the top 0.6% of all known vulnerabilities
- Weakness
- CWE-444
- More details
- NVD · CVE.org · CISA
All references (7)
- https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr
- https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6
- https://badhost.org
- https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml
- https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette
- https://www.secwest.net/starlette
- https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.