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Fencer Vulns
high 8.3

CVE-2021-39155 json

Authorization Policy Bypass Due to Case Insensitive Host Comparison

published 2021-08-24 5 y ago modified 2026-09-10 1 mo ago also GHSA-7774-7vr3-cc8j , EUVD-2021-1611

cvss
8.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.2%
EPSS 1.2% chance of exploitation in 30 days, percentile 69%
public exploits
0
none known

Summary

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.

Patch / fix

  • go istio.io/istio 1.9.8, 1.10.4, 1.11.1
  • istio istio 1.9.8, 1.10.4, 1.11.1
  • servicemesh service mesh 1.1 1.1.17-3.el8
  • servicemesh-mixc service mesh 1.1 1.1.17-3.el8
  • servicemesh-mixs service mesh 1.1 1.1.17-3.el8
  • servicemesh-proxy service mesh 1.1 1.1.17-2.el8
  • servicemesh-galley service mesh 1.1 1.1.17-3.el8
  • servicemesh-citadel service mesh 1.1 1.1.17-3.el8
  • servicemesh-istioctl service mesh 1.1 1.1.17-3.el8
  • servicemesh-pilot-agent service mesh 1.1 1.1.17-3.el8

red hat advisories RHSA-2021:3273RHSA-2021:3272

  • istio Arch Linux 1.11.1-1

arch linux advisories AVG-2321

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-39155 updated cvss 8.3

istio/istio: HTTP request can bypass authorization mechanisms due to case insensitive host comparison

  • Red Hat OpenShift Service Mesh 1.1 servicemesh 1.1.17-3.el8, servicemesh-mixc 1.1.17-3.el8, servicemesh-mixs 1.1.17-3.el8, servicemesh-galley 1.1.17-3.el8, servicemesh-citadel 1.1.17-3.el8, servicemesh-istioctl 1.1.17-3.el8, +3 more
    fixed: vendor fix RHSA-2021:3273
  • Red Hat OpenShift Service Mesh 2.0 servicemesh 2.0.7-3.el8, servicemesh-mixc 2.0.7-3.el8, servicemesh-mixs 2.0.7-3.el8, servicemesh-istioctl 2.0.7-3.el8, servicemesh-pilot-agent 2.0.7-3.el8, servicemesh-pilot-discovery 2.0.7-3.el8
    fixed: vendor fix RHSA-2021:3272
  • Red Hat OpenShift Service Mesh 1.1 servicemesh-proxy 1.1.17-2.el8 Red Hat OpenShift Service Mesh 2.0 servicemesh-proxy 2.0.7-3.el8
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.24% chance of exploitation in the next 30 days, higher than 69% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 8.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:P/I:N/A:N
GitHub 3.1 8.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L

Affected packages

ecosystem package affected fixed in
go istio.io/istio
<1.9.8
>=1.10.0, <1.10.4
>=1.11.0, <1.11.1
1.9.8
1.10.4
1.11.1

Affected products

  • istio istio

Classification

Weakness
  • CWE-178 Improper Handling of Case Sensitivity
  • CWE-863 Incorrect Authorization

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.