CVE-2021-39155: Authorization Policy Bypass Due to Case Insensitive Host Comparison

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-24 · updated 2026-06-17
Weakness: CWE-178

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.

Affected products

istio: before 1.9.8; from 1.10.0 before 1.10.4; from 1.11.0 before 1.11.1 — fixed in 1.10.4, 1.11.1, 1.9.8

How to fix

Upgrade to a fixed version: istio 1.10.4, 1.11.1, 1.9.8. Check the vendor advisory for mitigations.

References

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