MSRXCVE-2026-48710
CVE-2026-48710
Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
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.
CVSS 3.1 · CNA
chance of exploitation in the next 30 days (EPSS; in the top 1% of all CVEs)
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.
Scores
Affected packages and fixed versions
| Package | Ecosystem | Fixed in |
|---|---|---|
| starlette | PyPI | 1.0.1 |
Affected products
- Kludex starlette: < 1.0.1
Weakness: CWE-444
Also known as: GHSA-86qp-5c8j-p5mr, PYSEC-2026-161, X41-2026-002
7 references (advisories, patches, write-ups)
- https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr · x_refsource_CONFIRM
- https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6 · x_refsource_MISC
- https://badhost.org · x_refsource_MISC
- https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml · x_refsource_MISC
- https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette · x_refsource_MISC
- https://www.secwest.net/starlette · x_refsource_MISC
- https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette · x_refsource_MISC
Sources: CVE.org, OSV.dev, FIRST EPSS. Missing ones did not answer or have no record yet.