CVE-2021-39155

Authorization Policy Bypass Due to Case Insensitive Host Comparison

Workarounds

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 guide.

References

More details can be found in the Istio Security Bulletin.

For more information

If you have any questions or comments about this advisory, please email us at [email protected]

According to RFC 4343, 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 Envoy 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".

CVE-2021-39155 has a CVSS score of 8.3 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.9.8, 1.10.4, 1.11.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

• Istio 1.11.1 and above
• Istio 1.10.4 and above
• Istio 1.9.8 and above

1. What is CVE-2021-39155? CVE-2021-39155 is a high-severity security vulnerability in istio.io/istio (go), affecting versions < 1.9.8. It is fixed in 1.9.8, 1.10.4, 1.11.1.
2. How severe is CVE-2021-39155? CVE-2021-39155 has a CVSS score of 8.3 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of istio.io/istio are affected by CVE-2021-39155? istio.io/istio (go) versions < 1.9.8 is affected.
4. Is there a fix for CVE-2021-39155? Yes. CVE-2021-39155 is fixed in 1.9.8, 1.10.4, 1.11.1. Upgrade to this version or later.
5. Is CVE-2021-39155 exploitable, and should I be worried? Whether CVE-2021-39155 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-39155 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-39155?
   • Upgrade istio.io/istio to 1.9.8 or later
   • Upgrade istio.io/istio to 1.10.4 or later
   • Upgrade istio.io/istio to 1.11.1 or later

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