CVE-2026-48710 is a medium-severity (CVSS 6.5) CWE-444 vulnerability in starlette. It is in CISA's Known Exploited Vulnerabilities catalog (added 2026-09-02) — treat it as actively exploited and patch now. A fix is available for starlette — see the affected versions and patch details below.
Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2026-09-02. Federal agencies were required to remediate by 2026-09-16.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2026-48710.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-48710 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 390,807 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
starletteReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
In affected versions, 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.
Details
When a client requests http://example.com/foo, it sends:
GET /foo HTTP/1.1
Host: example.com
Affected versions reconstructed the URL by concatenating http://{host}{path} and re-parsing the result. The Host value is only valid as a uri-host [ ":" port ] per RFC 9112 §3.2, where uri-host follows the restricted host grammar of RFC 3986 §3.2.2. When it contains characters outside that grammar - notably /, ?, or # - those characters move the path/query/fragment boundaries during re-parsing, so the parsed request.url.path no longer matches the path the server actually received. For example:
GET /foo HTTP/1.1
Host: example.com/abc?bar=
reconstructs to http://example.com/abc?bar=/foo, whose parsed path is /abc - even though routing used the real path /foo. The router still dispatches to /foo and the endpoint executes, but any middleware or code that reads request.url.path sees /abc, so path-based authorization checks can be bypassed.
Impact
Any application running an affected version that relies on request.url (or request.url.path) for security-sensitive decisions is affected. The most common case is middleware that gates access to certain path prefixes based on request.url.path. Deployments fronted by a proxy or load balancer are mitigated only if that proxy rejects or normalizes the malformed Host header before forwarding and the application does not trust attacker-controlled host headers (e.g. X-Forwarded-Host) elsewhere.
Mitigation
Upgrade to a patched version, 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | starlette | all versions | 1.0.1pip install --upgrade 'starlette==1.0.1' |
Affected Products
starletteencodeai inference serverredhatansible automation platformredhatenterprise linux airedhatmigration toolkit for applicationsredhatopenshift airedhatResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for starlette, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update starlette to 1.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48710 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How to detect CVE-2026-48710
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2026-48710 -u https://target- Template
- Starlette - Improper Validation of Unsafe Equivalence in Input
- Severity
- critical
- Impact
- A remote attacker can exploit this vulnerability by sending a specially crafted HTTP Host header in a request to a Starlette-based application. Due to improper validation of the Host header, security restrictions that rely on request.url for enforcement may be bypassed. This allows attackers to gain unauthorized access to endpoints protected by middleware or path-based checks, potentially leading to privilege escalation, information disclosure, or unauthorized actions.
- Remediation
- Upgrade Starlette to >= 1.0.1 to address this vulnerability.If immediate patching is not possible, implement strict Host header validation in a reverse proxy or web server (such as nginx or Apache) to only allow expected Host values, mitigating the risk of URL-based restriction bypass.
Template by ProjectDiscovery nuclei-templates (ritikchaddha), MIT licensed. View the full template. Scan only systems you are authorised to test.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
The impact of this vulnerability is rated Moderate. A malformed Host header can manipulate the reconstructed request.url.path, introducing a discrepancy with the canonical ASGI request path (scope["path"]). However, the vulnerability does not directly grant access, modify routing, or bypass security controls on its…
Deploying an RFC-compliant reverse proxy (such as nginx, Apache, HAProxy, or Caddy) in front of the ASGI server will reject malformed Host headers before they reach the application. This is the most straightforward mitigation that does not require code changes. If custom middleware is present, it should be updated to use `request.scope["path"]` instead of `request.url.path` for any security decisions. The ASGI scope path is derived from the HTTP request line and is not influenced by the Host header, so it reflects the actual request target.Source: Red Hat security advisory for CVE-2026-48710 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat AI Inference Server 3.3 | rhaiis/vllm-rocm-rhel9:1782353093 | RHSA-2026:30088 |
| Red Hat AI Inference Server 3.3 | rhaiis/vllm-cuda-rhel9:1782352847 | RHSA-2026:30089 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-spyre-rhel9:1789681201 | RHSA-2026:69464 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-cpu-rhel9:1789681128 | RHSA-2026:69466 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-cuda-rhel9:1789681126 | RHSA-2026:69467 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-cpu-rhel9:1790075793 | RHSA-2026:70965 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-spyre-rhel9:1790076141 | RHSA-2026:70969 |
| Red Hat AI Inference Server 3.4 | rhaii/vllm-cuda-rhel9:1790090131 | RHSA-2026:70979 |
Frequently Asked Questions
Is CVE-2026-48710 in your dependencies?
Find it across PyPI, including transitive dependencies.