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Actively exploited · Published May 26, 2026 · Assigned by GitHub_M

CVE-2026-48710Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks

Medium 6.5 Exploited

In plain English

A medium-severity security flaw in starlette from Kludex. Attackers are actively exploiting it in the wild (listed by CISA on September 2, 2026). The chance it is exploited in the next 30 days is estimated at 70.3% (higher than 99.4% of all known vulnerabilities). Ready-made attack tooling exists: Nuclei, public proof-of-concept code. Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium
Exploit odds (30 days)70.3%top 0.6% of all CVEs
Exploited in the wildYesCISA KEV since Sep 2, 2026
Attack tooling2Nuclei, Public PoC

CISA guidance

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.

Federal agencies must remediate by Sep 16, 2026.

Official description

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.24py3-starlette1.1.0-r0ALPINE-CVE-2026-48710
Debian:12starlette0.26.1-1+deb12u1DEBIAN-CVE-2026-48710
Debian:13starlette0.46.1-3+deb13u2DEBIAN-CVE-2026-48710
Debian:14starlette1.1.0-1DEBIAN-CVE-2026-48710
PyPIstarlette1.0.1PYSEC-2026-161
PyPIstarlette1.0.1GHSA-86qp-5c8j-p5mr
Ubuntu:22.04:LTSstarletteno fix listedUBUNTU-CVE-2026-48710
Ubuntu:24.04:LTSstarletteno fix listedUBUNTU-CVE-2026-48710
Ubuntu:25.10starletteno fix listedUBUNTU-CVE-2026-48710
Ubuntu:26.04:LTSstarletteno fix listedUBUNTU-CVE-2026-48710
Ubuntu:26.10starletteno fix listedUBUNTU-CVE-2026-48710

Affected products (as reported)

VendorProductVersions
Kludex starlette < 1.0.1

Weakness: CWE-444, CWE-1289 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

References