CVE-2021-39155
HIGHDescription
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, 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 guide.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L
Exploit Intelligence
Moderate risk: more likely to be exploited than 65% of all known CVEs.
References
Related Vulnerabilities
Other CWE-178 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-27636 | Medium | 5.6 | 81% | - | Fix |
| CVE-2021-24347 | High | 8.8 | 54% | - | Fix |
| CVE-2020-12812 | Critical | 9.8 | 49% | KEV + Ransom | Fix |
| CVE-2003-0411 | High | 7.5 | 25% | - | - |
| CVE-2018-9845 | Critical | 9.8 | 13% | - | Fix |
| CVE-2001-0766 | Critical | 9.8 | 8.2% | - | - |
Common questions
How do I fix CVE-2021-39155?
Upgrade istio to 1.11.1 or later.
Is CVE-2021-39155 being actively exploited?
Not that we know of. CVE-2021-39155 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.2% is the estimated probability that it will be exploited in the next 30 days. That is higher than 65% of all scored CVEs.
How severe is CVE-2021-39155?
CVE-2021-39155 has a CVSS v3 base score of 8.3, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2021-39155 affect?
Published advisories record a fix for istio (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2024-11-21.