{"cve":"CVE-2021-39155","nvd":{"published":"2021-08-24T23:15:07.093","modified":"2026-06-17T04:03:12.617","status":"Modified","description":"Istio is an open source platform for providing a uniform way to integrate microservices, manage traffic flow across microservices, enforce policies and aggregate telemetry data. According to [RFC 4343](https://datatracker.ietf.org/doc/html/rfc4343), Istio authorization policy should compare the hostname in the HTTP Host header in a case insensitive way, but currently the comparison is case sensitive. The proxy will route the request hostname in a case-insensitive way which means the authorization policy could be bypassed. As an example, the user may have an authorization policy that rejects request with hostname \"httpbin.foo\" for some source IPs, but the attacker can bypass this by sending the request with hostname \"Httpbin.Foo\". Patches are available in Istio 1.11.1, Istio 1.10.4 and Istio 1.9.8. As a work around a Lua filter may be written to normalize Host header before the authorization check. This is similar to the Path normalization presented in the [Security Best Practices](https://istio.io/latest/docs/ops/best-practices/security/#case-normalization) guide.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","score":7.5},"cwes":["CWE-178","CWE-863"],"references":["https://datatracker.ietf.org/doc/html/rfc4343","https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j","https://datatracker.ietf.org/doc/html/rfc4343","https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j"],"products":["istio:istio"],"source":"nvd","synced_at":"2026-10-19T21:21:57.125947417+00:00"},"epss":{"cve":"CVE-2021-39155","score":0.01242,"percentile":0.69221,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-7774-7vr3-cc8j","url":"https://osv.dev/vulnerability/GHSA-7774-7vr3-cc8j","summary":"Authorization Policy Bypass Due to Case Insensitive Host Comparison","modified":"2026-09-10T03:49:07.371161372Z","packages":[{"ecosystem":"Go","name":"istio.io/istio","purl":"pkg:golang/istio.io/istio","ranges":["<1.9.8"],"fixed":["1.9.8"]},{"ecosystem":"Go","name":"istio.io/istio","purl":"pkg:golang/istio.io/istio","ranges":[">=1.10.0 <1.10.4"],"fixed":["1.10.4"]},{"ecosystem":"Go","name":"istio.io/istio","purl":"pkg:golang/istio.io/istio","ranges":[">=1.11.0 <1.11.1"],"fixed":["1.11.1"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}