What is CVE-2021-39155?

CVE-2021-39155 is a high-severity vulnerability in Istio, classified under Improper Handling of Case Sensitivity. CVSS score: 8.3/10. Published 2021-08-24.

How severe is CVE-2021-39155?

High severity. CVSS v3 base score is 8.3 out of 10.

Is CVE-2021-39155 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Auth bypass in Istio

CVE-2021-39155

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.

Published 2021-08-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.3 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L.

EPSS exploit prediction

EPSS: 0.012 (66.7th percentile), scored 2026-10-01.

Low probability of exploitation in the next 30 days, but still tracked. 67th percentile — 66.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.012 · change: 0.000

Affected products

Istio — versions <= 1.9.8, >= 1.10.0, < 1.10.4, >= 1.11.0, < 1.11.1

Weakness classification (CWE)

Public proof-of-concept exploits

runwhen-contrib/helm-charts

References

github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j (Third Party Advisory)
datatracker.ietf.org/doc/html/rfc4343 (Third Party Advisory)

Frequently asked questions

What is CVE-2021-39155?: CVE-2021-39155 is a high-severity vulnerability in Istio, classified under Improper Handling of Case Sensitivity. CVSS score: 8.3/10. Published 2021-08-24.
How severe is CVE-2021-39155?: High severity. CVSS v3 base score is 8.3 out of 10.
Is CVE-2021-39155 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.