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

Advisory record · `GHSA-86qp-5c8j-p5mr`

- **Identifiers:** `GHSA-86qp-5c8j-p5mr` · `CVE-2026-48710` · `PYSEC-2026-161` · `X41-2026-002`
- **State:** Confirmed
- **Severity:** CVSS 6.5 Medium
- **Critical:** Yes: KEV
- **Weakness (CWE):** [CWE-444](https://cwe.mitre.org/data/definitions/444.html) · [CWE-1289](https://cwe.mitre.org/data/definitions/1289.html)
- **Tags:** [HTTP request smuggling](https://agentprime.io/advisories/tag/request-smuggling.md) · [CWE-1289](https://agentprime.io/advisories/tag/cwe-1289.md)
- **Published:** 4 Jun 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 4 Jun 2026, 13:15 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- In CISA’s KEV catalogue since 2 Sept 2026: it is known to be exploited.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `starlette` (PyPI): affected `<1.0.1`; fixed in `1.0.1`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Starlette versions before 1.0.1 do not validate the HTTP Host header before rebuilding request.url, so a malformed Host value can make request.url.path differ from the path actually routed. An attacker who can send requests to the application can bypass middleware or endpoint checks that rely on request.url.path, such as path-prefix authorization. Upgrade to Starlette 1.0.1 or later. The source gives no other workaround.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.5 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Listed since 2 Sept 2026: known to be exploited.
- **EPSS:** 0.70, the 99th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:11 UTC: First seen by agentprime
- 31 Oct 2026, 14:11 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-86qp-5c8j-p5mr)

## Sources

- [GitHub Advisory Database: GHSA-86qp-5c8j-p5mr](https://github.com/advisories/GHSA-86qp-5c8j-p5mr)
- [OSV: GHSA-86qp-5c8j-p5mr](https://osv.dev/vulnerability/GHSA-86qp-5c8j-p5mr)
- <https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-48710>
- <https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6>
- <https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette>
- <https://www.secwest.net/starlette>
- <https://www.cve.org/CVERecord?id=CVE-2026-48710>
- <https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-48710.json>
- <https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette>
- <https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml>
- <https://github.com/Kludex/starlette>
- <https://bugzilla.redhat.com/show_bug.cgi?id=2481742>
- <https://badhost.org/>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-86qp-5c8j-p5mr>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-86qp-5c8j-p5mr/>. Everything for agents: <https://agentprime.io/llms.txt>.