CriticalCVE-2026-48710

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

Published 26.05.2026Last modified 01.10.2026In CISA KEV since 02.09.2026Sources: pwn.watch, Red Hat, OSV, CVE.org, EUVD, Debian

What is affected, what to do

The vulnerability is being actively exploited; treat it as a priority. Update Kludex Starlette to version 1.0.1. Immediate actions, mitigation and detection rules are in the runbook below.

Affected: Kludex Starlette lt *..1.0.1
Fixed in: Kludex Starlette 1.0.1

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

6.5

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: NVD, CVSS v3

6.5

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: pwn.watch, Red Hat, OSV, CVE.org, EUVD, Debian

70.3 %

EPSS score

High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

Source: FIRST EPSS, percentile 99.38

Show all metrics

Severity: Critical
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 229.57 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 6.5 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 6.5
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 70.3 %; High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 99.38; Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: Actively exploited
Published: 26.05.2026
In CISA KEV since The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: 02.09.2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-444

Response and mitigation

Why act now?

Prioritisation rationale

This flaw is reachable over the network with no authentication or user interaction. On its own, its confidentiality and integrity impact is limited. Its real value to attackers is as a bypass primitive: it defeats path-based access controls and chains with CVE-2026-42271, which explains its KEV listing despite a moderate base rating. Exploitation likelihood is well above that of most CVEs, while CISA does not flag ransomware use. Treat internet-exposed FastAPI/Starlette APIs and AI platforms with custom auth middleware, such as OpenShift AI and AI Inference Server, as patch-or-filter within 24–72 h. Internal-only services can follow the normal patch cycle.

01
Runbook step 1
Immediate response (0-24 h)
- Inventory: Locate every Python service using Starlette, whether directly or transitively via FastAPI. Use pip show starlette, SBOM queries, or container scans with Trivy/Grype. Affected versions are 0.8.3 through 1.0.1. Prioritise internet-facing services that rely on authentication or authorisation middleware.
- Patch: Upgrade Starlette to the fixed release named in the upstream advisory. The patch is available from the vendor; look up the exact version in the advisory, as it is later than 1.0.1. Regenerate lockfiles, check FastAPI dependency constraints, then rebuild and redeploy the images.
- Red Hat products: Apply the relevant RHSA errata for AI Inference Server 3.3.x, OpenShift AI 3.3.x/3.4.x, Enterprise Linux AI, Ansible Automation Platform and Migration Toolkit for Applications 8.2.0. Check the Red Hat Customer Portal for the fixed builds.
- Filter Host headers at the edge: On reverse proxies, load balancers and ingress controllers, drop any request whose Host header contains /, ?, #, @, \ or whitespace. This is faster than a rollout and kills the attack vector.
- Assess the chain: CISA notes this can be chained with CVE-2026-42271. Check exposure and patch state for both CVEs together.
02
Runbook step 2
Mitigation layers
- Network / reverse proxy: Allow-list valid hostnames only. In nginx, define explicit server_name blocks and add a default server with return 444;. In Envoy or ingress-nginx, validate the header against ^[A-Za-z0-9.-]+(:[0-9]+)?$.
- WAF/IPS: ModSecurity: SecRule REQUEST_HEADERS:Host "@rx [/?#@\\\s]" "id:1048710,phase:1,deny,status:400". Suricata: http.host; pcre:"/[\/?#@]/"; in alert or drop mode.
- Application config: Enable TrustedHostMiddleware with an explicit allowed_hosts list (no *) as the outermost middleware. This rejects forged Host headers before URL reconstruction happens.
- Code hardening: Never base access decisions on request.url or str(request.url); use request.scope["path"] instead. Grep custom middleware for request.url to find vulnerable checks.
- IAM / least privilege: Don't protect admin or internal endpoints by path checks alone, such as model management or inference admin APIs. Add segmentation plus independent authentication, e.g. mTLS or an OIDC proxy.
03
Runbook step 3
Detection rules
- Web/proxy access logs: Make sure $http_host is logged. SPL: index=web | regex http_host="[\/?#@\\\s]" | stats count by src_ip, http_host, uri_path.
- Zeek / network telemetry: Inspect http.log host values containing path characters. KQL shape: ZeekHttp | where Host matches regex @"[/?#@]" | summarize count() by SourceIp, Host, Uri. Where TLS terminates at the ingress, log there instead.
- Authorisation anomalies: Correlate 2xx responses on protected paths that have no valid session or preceding login event, especially where the Host header deviates from the expected FQDN. Sigma shape: cs-host|contains: ['/', '?', '#'] AND sc-status: 200.
- Application logs: Once TrustedHostMiddleware is enabled, alert on bursts of "Invalid host header" (HTTP 400) from a single source. These indicate scanning or exploitation attempts.
- Follow-on activity: After a hit, review the affected API for config changes, model uploads, Ansible job launches or bulk data reads.

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

A flaw was found in Starlette, a lightweight ASGI (Asynchronous Server Gateway Interface) framework. A remote attacker could exploit this vulnerability by sending a specially crafted HTTP `Host` request header. This malformed header could cause the `request.url` to be incorrectly reconstructed, leading to a discrepancy with the actual requested path. Consequently, security restrictions enforced by middleware and endpoints that rely on `request.url` for validation could be bypassed, potentially allowing unauthorized access or actions.

Source: pwn.watch, redhat_csaf, OSV (Open Source Vulnerabilities), cvelistv5, NVD (NIST), CISA KEV Catalog, ENISA EUVD, CVE.org (MITRE), Debian Security Advisories

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
Kludex	starlette	`< 1.0.1`	`1.0.1`
encode	starlette	`0.8.3 – < 1.0.1`	`1.0.1`
redhat	ai_inference_server	`3.3.0 – ≤ 3.3.5`	—
redhat	ansible_automation_platform	`2.6` `2.7`	— —
redhat	migration_toolkit_for_applications	`< 8.2.0`	`8.2.0`
redhat	openshift_ai	`3.4 – < 3.4.2` `3.3 – < 3.3.5`	`3.4.2` `3.3.5`
redhat	openshift_lightspeed	`—`	—
redhat	satellite	`6.17` `6.19` `6.18`	— — —

Vendors and products

Affected operating systems

Affected operating systems

Linux	redhat	enterprise_linux_ai	`3.0`
Linux	debian	starlette	`bookworm`
Linux	debian	starlette	`trixie`

Weakness classes (CWE)

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Base
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Entry at MITRE CWE-444

Public exploit references

Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

Public exploit references

CISA KEV Catalog	CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov	actively exploited	02.09.2026
PoC-in-GitHub (nomi-sec)	CVE-2026-48710 (opens in a new tab)github.com	PoC (publicly reported)reported, not verified	11.08.2026
PoC-in-GitHub (nomi-sec)	repro-OXDEV-77637-uv-workspace (opens in a new tab)github.com	PoC (publicly reported)reported, not verified	04.08.2026
PoC-in-GitHub (nomi-sec)	BadHost-CVE-2026-48710-Exploit (opens in a new tab)github.com	PoC (publicly reported)reported, not verified	28.05.2026
PoC-in-GitHub (nomi-sec)	supply-chain-guard (opens in a new tab)github.com	PoC (publicly reported)reported, not verified	01.04.2026
PoC-in-GitHub (nomi-sec)	starlette-host-header-lab (opens in a new tab)github.com	PoC (publicly reported)reported, not verified	27.05.2026

References & sources

github.comKludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mrweb, x_refsource_CONFIRM, pwn-watch
NVD (NIST)NVD: CVE-2026-48710advisory, pwn-watch
github.comKludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6web, x_refsource_MISC
x41-dsec.delab/advisories/x41-2026-002-starletteweb, x_refsource_MISC
secwest.netstarletteweb, x_refsource_MISC
CVE.org (MITRE)CVERecord?id=CVE-2026-48710web
security.access.redhat.comdata/csaf/v2/vex/2026/cve-2026-48710.jsonweb, x_sadp-csaf-vex
ostif.orgdisclosing-the-badhost-vulnerability-in-starletteweb, x_refsource_MISC
github.compypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161…web, x_refsource_MISC
github.comKludex/starlettepackage
bugzilla.redhat.comRHBZ#2481742web, Issue tracking, x_refsource_REDHAT
badhost.orgweb, x_refsource_MISC
access.redhat.comsecurity/cve/CVE-2026-48710web, Vulnerability database, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:44696web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:43038web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:37275web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:34532web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:34526web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:34456web, Vendor advisory, x_refsource_REDHAT
access.redhat.comerrata/RHSA-2026:30089web, Vendor advisory, x_refsource_REDHAT

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

01.10.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: active, automatable: yes

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	removed	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-02T00:00:00+00:00"}	—
SSVC assessment	added	—	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-03T03:56:07.799548Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

03.09.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: active, automatable: yes

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	changed	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-02T17:44:11.450445Z"}	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-02T00:00:00+00:00"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

02.09.2026CISA KEV entry updated9119a7d8…
4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name
Show 4 original changes
Individual changes of this event as published by NVD
Type Action Before After
Date Added added
—
2026-09-02
Due Date added
—
2026-09-02
Required Action added
—
2026-09-02
Vulnerability Name added
—
2026-09-02
Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

Individual changes of this event as published by NVD
Type	Action	Before	After
Date Added	added	—	2026-09-02
Due Date	added	—	2026-09-02
Required Action	added	—	2026-09-02
Vulnerability Name	added	—	2026-09-02

02.09.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: active, automatable: no

New references:

Show 4 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-48710
Reference	added	—	https://www.wiz.io/blog/ai-infrastructure-honeypot
Reference	added	—	https:/www.microsoft.com/en-us/security/blog/2026/08/26/when-ai-infrastructure-becomes-target-securing-gateways-control-points
SSVC assessment	changed	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-05-27T14:22:19.241769Z"}	{"id":"CVE-2026-48710","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-02T17:44:11.450445Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

28.08.2026Record modifiedGitHub

New references:

and 3 more references

Show 21 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://badhost.org
Reference	added	—	https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6
Reference	added	—	https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr
Reference	added	—	https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml
Reference	added	—	https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette
Reference	added	—	https://www.secwest.net/starlette
Reference	added	—	https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette
Reference	removed	https://badhost.org	—
Reference	removed	https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6	—
Reference	removed	https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr	—
Reference	removed	https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml	—
Reference	removed	https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette	—
Reference	removed	https://www.secwest.net/starlette	—
Reference	removed	https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette	—
Reference type	removed	https://badhost.org Types: Mitigation, Third Party Advisory	—
Reference type	removed	https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6 Types: Patch	—
Reference type	removed	https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr Types: Vendor Advisory	—
Reference type	removed	https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml Types: Third Party Advisory	—
Reference type	removed	https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette Types: Mitigation, Third Party Advisory	—
Reference type	removed	https://www.secwest.net/starlette Types: Exploit, Mitigation, Third Party Advisory	—
Reference type	removed	https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette Types: Exploit, Mitigation, Third Party Advisory	—

Source identifier: security-advisories@github.com

Show 6 older changes

28.08.2026Record modifiedRed Hat ADP

New references:

and 8 more references

Show 36 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/48xxx/CVE-2026-48710.json">CVE-2026-48710</a>
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:22992
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:22993
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:23346
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:24866
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:26226
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:30088
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:30089
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:34456
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:34526
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:34532
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:37275
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:43038
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:44696
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:51357
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:60520
Reference	added	—	https://access.redhat.com/security/cve/CVE-2026-48710
Reference	added	—	https://bugzilla.redhat.com/show_bug.cgi?id=2481742
Reference	added	—	https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-48710.json
Reference	removed	https://access.redhat.com/errata/RHSA-2026:22992	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:22993	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:23346	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:24866	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:26226	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:30088	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:30089	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:34456	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:34526	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:34532	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:37275	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:43038	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:44696	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:51357	—
Reference	removed	https://access.redhat.com/security/cve/CVE-2026-48710	—
Reference	removed	https://bugzilla.redhat.com/show_bug.cgi?id=2481742	—
Reference	removed	https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-48710.json	—