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High · CVSS 8.3CVE-2021-39155GHSA-7774-7VR3-CC8J

Authorization Policy Bypass Due to Case Insensitive Host ComparisonCVE-2021-39155

Answer in brief

CVE-2021-39155 records a High severity (CVSS 8.3) vulnerability in Authorization Policy Bypass Due to Case Insensitive Host Comparison. The current sources do not mark it as known exploited. The current feed maps istio.io/istio (go), istio.io/istio (go), istio.io/istio (go). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Aug 30, 2021Updated Sep 10, 2026Source checked Oct 25, 2026First seen by HOL Sep 10, 2026Material review Sep 10, 2026
Upstream Advisory

Record context

Vulnerability class
Vulnerability
EPSS
Not reported
CWE IDs
Not reported
Source
OSV
Source checked
Oct 25, 2026
References
7 linked sources
Open source record

Key facts

Risk
High · CVSS 8.3
Exploitation
Not marked as known exploited
Affected software
3 mapped packages or products
Fix availability
Available

Why this deserves its current priority

CVSS is 8.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps istio.io/istio (go), istio.io/istio (go), istio.io/istio (go). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
istio.io/istiogo>=0 <1.9.81.9.8
istio.io/istiogo>=1.10.0 <1.10.41.10.4
istio.io/istiogo>=1.11.0 <1.11.11.11.1

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for istio.io/istio, istio.io/istio, istio.io/istio.
  2. 2Review the reported fix. Update istio.io/istio to 1.9.8; istio.io/istio to 1.10.4; istio.io/istio to 1.11.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Aug 30, 2021

    Evidence: source:osv:source_dates:source-dates:record

  2. Source modified

    Sep 10, 2026

    Evidence: source:osv:source_dates:source-dates:record

  3. First seen by HOL

    Sep 10, 2026

Sources and claim methodology

  • Source referencedatatracker.ietf.org
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • NVD vulnerability recordnvd.nist.gov
Upstream source description

### Impact 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 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](https://istio.io/latest/docs/ops/best-practices/security/#case-normalization) guide. ### References More details can be found in the [Istio Security Bulletin](https://istio.io/latest/news/security/istio-security-2021-008). ### For more information If you have any questions or comments about this advisory, please email us at [email protected]

Quoted source text, attributed separately from HOL analysis.