Authorization Policy Bypass Due to Case Insensitive Host Comparison

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L

EPSS

• CVE-2021-39155: 1.24% estimated exploitation activity in the next 30 days; percentile 69.25 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact 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". Patches Istio 1.11.1 and above Istio 1.10.4 and above Istio 1.9.8 and above 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…

Published by the source

30 Aug 2021, 16:15 UTC

Last modified by the source

22 May 2023, 18:26 UTC

Source last checked

26 Oct 2026, 14:58 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-7774-7vr3-cc8j

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# Patch Next advisory brief: CVE-2021-39155

Advisory: Authorization Policy Bypass Due to Case Insensitive Host Comparison
IDs: CVE-2021-39155, GHSA-7774-7vr3-cc8j
Record: https://patchnext.com/advisories/cve-2021-39155 (revision 1, source checked 26 Oct 2026, 14:58 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: istio.io/istio 1.11.1, 1.10.4, 1.9.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go istio.io/istio: affected < 1.9.8; >= 1.10.0, < 1.10.4; >= 1.11.0, <= 1.11.0; fixed in 1.10.4, 1.11.1, 1.9.8

## Suggested inspection (read-only)
- Go istio.io/istio: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on istio.io/istio.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-7774-7vr3-cc8j (GitHub-reviewed advisory GHSA-7774-7vr3-cc8j lists affected ranges: Go istio.io/istio >=0 =1.10.0 =1.11.0 <=1.11.0)

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