Starlette (FastAPI's web framework) BadHost flaw CVE-2026-48710 used in attacks; update to 1.0.1
Starlette, the framework under FastAPI and many AI tools, has a Host-header flaw (CVE-2026-48710, "BadHost") that can bypass path-based login checks. CISA says it is exploited. Fixed in Starlette 1.0.1: developers should update and rebuild their apps.
If you run or build apps on Starlette or FastAPI, update Starlette to 1.0.1 or later and rebuild and redeploy every container and environment that includes it. Otherwise nothing for you to do; ask your IT provider or developer if you use self-hosted AI tools.
What’s happening
Starlette is a widely used Python web framework and the base of FastAPI. In versions up to 1.0.0 it did not validate the HTTP Host header before using it to rebuild the request address. A crafted Host value could make the path Starlette reports differ from the path actually served, so any login or access check that relied on the reported path could be skipped. The Starlette project published its advisory on 21 May 2026 and fixed the flaw in version 1.0.1. The vendor rates it medium (CVSS 6.5). OSTIF, which funded the audit that found it, calls the flaw BadHost and says the rating understates the risk for AI gateways, vLLM, LiteLLM and agent tools built on Starlette. On 2 September 2026 CISA added it to its list of flaws used in attacks and said it could be chained with CVE-2026-42271. CISA's page does not describe who is attacking or how. Most families are not directly affected; it matters for businesses and developers running Starlette or FastAPI apps.
- What
- Flaw
- Affects
- Starlette (Python web framework used by FastAPI, LiteLLM, vLLM and others)
- Affected
- Starlette 1.0.0 and earlier (pip package starlette)
Apps built on Starlette, including FastAPI (OSTIF also names LiteLLM and vLLM) - Fixed in
- Starlette 1.0.1
- Since
- 2 Sept 2026
- Closed
- 2 Sept 2026
Timeline
- CISA added
CISA added CVE-2026-48710 to its Known Exploited Vulnerabilities catalog, which means it is being used in attacks. CISA notes it could be chained with CVE-2026-42271. CISA's page gives no due date for this entry and no details of the attacks.
- Update
OSTIF published its disclosure of the flaw, which it calls BadHost. X41 D-Sec found it during an OSTIF-managed audit of vLLM. OSTIF named FastAPI, LiteLLM, vLLM and many AI proxy and agent tools as affected, and rated it higher than the GitHub advisory does.
- Made publicFixed
The Starlette project published advisory GHSA-86qp-5c8j-p5mr for CVE-2026-48710. Versions up to 1.0.0 did not check the Host header before using it to rebuild request.url, so path-based security checks could be bypassed. Starlette 1.0.1 was released with the fix.
Sources
- Missing Host header validation poisons request.url.path (GHSA-86qp-5c8j-p5mr)Starlette (Kludex) · 21 May 2026
- Disclosing the BadHost vulnerability in StarletteOSTIF · 26 May 2026
- BadHostBadHost project · 26 May 2026
- CISA Adds Seven Known Exploited Vulnerabilities to CatalogCISA · 2 Sept 2026