GHSA-86qp-5c8j-p5mr is a medium-severity (CVSS 6.5) CWE-444 vulnerability in starlette. 1 public exploit reference exists, so weaponization risk is real. O3 Security confirms whether GHSA-86qp-5c8j-p5mr is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-86qp-5c8j-p5mr.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-86qp-5c8j-p5mr plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 0 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 | 1.0.1 |
Research 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update starlette to 1.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-86qp-5c8j-p5mr 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 O3 protects you
O3 pinpoints whether GHSA-86qp-5c8j-p5mr is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-86qp-5c8j-p5mr. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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…
Frequently Asked Questions
Is GHSA-86qp-5c8j-p5mr in your dependencies?
O3 detects GHSA-86qp-5c8j-p5mr across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.