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Security Digest · Vulnerability record · Known exploited

CVE-2026-48710: Kludex Starlette HTTP Request/Response Smuggling Vulnerability

Exploited in the wild (CISA KEV). Starlette is a lightweight ASGI framework/toolkit.

CISA KEV · added 2026-09-02 CVSS 6.5 Medium EPSS 7.1% · p94 Patch this week

Updated By erroneo.us Security Desk32 references Seen in the digest for Sep 2, 2026

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

  • Yes, if you run Kludex Starlette. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-09-02, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-09-16; treat that as the outer limit, not the target.
  • EPSS 7.1%, percentile 94: FIRST’s model estimates a 7.1% chance of exploitation activity in the next 30 days, higher than 94% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
  • Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
  • Editorial verdict: Patch this week. Starlette BadHost: Host Header Auth Bypass in CISA KEV — Bump starlette to 1.0.1 or later in every Python service this week; it is urgent where an ASGI server faces untrusted clients directly and middleware makes security decisions from request.url.

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’s summary

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.

Weakness type: CWE-444, CWE-1289.

Product: Kludex Starlette · Assigned by: GitHub_M · Published: 2026-05-26

Affected and fixed versions

VendorProductAffected versions (per CVE record)
Kludexstarlette< 1.0.1

Severity scores

Scored byCVSSScoreVector
GitHub_M3.16.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
redhat-SADP3.16.5 MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
  • Attack vector: network (remotely reachable)
  • Attack complexity: low
  • Privileges required: none (no login)
  • User interaction: none
  • Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

  • CISA KEV: added 2026-09-02; federal due date 2026-09-16. Known ransomware use: Unknown.
  • CISA required action: 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.
  • CISA SSVC assessment: exploitation active, automatable yes, technical impact partial.
  • FIRST EPSS (model of 2026-10-21): 7.06% probability of exploitation activity in the next 30 days; percentile 94.

Timeline

  • — erroneo.us brief: Starlette BadHost: Host Header Auth Bypass in CISA KEV
  • — CVE record published by GitHub_M
  • — Added to CISA’s Known Exploited Vulnerabilities catalog
  • — CISA remediation deadline for US federal agencies
  • — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

  1. access.redhat.com access.redhat.com
  2. access.redhat.com access.redhat.com
  3. access.redhat.com access.redhat.com
  4. access.redhat.com access.redhat.com
  5. access.redhat.com access.redhat.com
  6. access.redhat.com access.redhat.com
  7. access.redhat.com access.redhat.com
  8. access.redhat.com access.redhat.com
  9. access.redhat.com access.redhat.com
  10. access.redhat.com access.redhat.com
  11. access.redhat.com access.redhat.com
  12. access.redhat.com access.redhat.com
  13. access.redhat.com access.redhat.com
  14. access.redhat.com access.redhat.com
  15. access.redhat.com access.redhat.com
  16. access.redhat.com access.redhat.com
  17. Listed in CISA KEV notes (github.com) github.com

Vulnerability records

  1. CVE.org record CVE-2026-48710 cve.org
  2. NVD entry CVE-2026-48710 nvd.nist.gov

Government

  1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
  2. Listed in CISA KEV notes (cisa.gov) cisa.gov
  3. Listed in CISA KEV notes (cisa.gov) cisa.gov

Other references from the CVE record and advisory

  1. github.com github.com
  2. badhost.org badhost.org
  3. github.com github.com
  4. ostif.org ostif.org
  5. secwest.net secwest.net
  6. x41-dsec.de x41-dsec.de
  7. wiz.io wiz.io
  8. access.redhat.com access.redhat.com
  9. RHBZ#2481742 bugzilla.redhat.com
  10. security.access.redhat.com security.access.redhat.com

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