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Fencer Vulns
medium 6.5

CVE-2021-31920 json

Istio Authorization Bypass Vulnerability

published 2021-05-27 5 y ago modified 2026-09-10 1 mo ago also GHSA-6q5m-22mq-q2xv , EUVD-2022-2845

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.3%
EPSS 1.3% chance of exploitation in 30 days, percentile 70%
public exploits
0
none known

Summary

Istio before 1.8.6 and 1.9.x before 1.9.5 has a remotely exploitable vulnerability where an HTTP request path with multiple slashes or escaped slash characters (%2F or %5C) could potentially bypass an Istio authorization policy when path based authorization rules are used.

Patch / fix

  • istio istio 1.8.6, 1.9.5
  • servicemesh service mesh 2.0 2.0.4-1.el8
  • servicemesh-mixc service mesh 2.0 2.0.4-1.el8
  • servicemesh-mixs service mesh 2.0 2.0.4-1.el8
  • servicemesh-proxy service mesh 2.0 2.0.4-1.el8
  • servicemesh-istioctl service mesh 2.0 2.0.4-1.el8
  • servicemesh-operator service mesh 2.0 2.0.4-3.el8
  • servicemesh-pilot-agent service mesh 2.0 2.0.4-1.el8
  • servicemesh-pilot-discovery service mesh 2.0 2.0.4-1.el8

red hat advisories RHSA-2021:1538RHSA-2021:1540

  • istio Arch Linux 1.10.0-1

arch linux advisories AVG-1947

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-2021-31920 updated cvss 8.1

istio/istio: HTTP request with escaped slash characters can bypass authorization mechanisms

  • Red Hat OpenShift Service Mesh 1.1 servicemesh 1.1.14-1.el8, servicemesh-mixc 1.1.14-1.el8, servicemesh-mixs 1.1.14-1.el8, servicemesh-galley 1.1.14-1.el8, servicemesh-citadel 1.1.14-1.el8, servicemesh-istioctl 1.1.14-1.el8, +3 more
    fixed: vendor fix RHSA-2021:1540
  • Red Hat OpenShift Service Mesh 2.0 servicemesh 2.0.4-1.el8, servicemesh-mixc 2.0.4-1.el8, servicemesh-mixs 2.0.4-1.el8, servicemesh-istioctl 2.0.4-1.el8, servicemesh-operator 2.0.4-3.el8, servicemesh-pilot-agent 2.0.4-1.el8, +1 more
    fixed: vendor fix RHSA-2021:1538
  • Red Hat OpenShift Service Mesh 1.1 servicemesh-proxy 1.1.14-1.el8 Red Hat OpenShift Service Mesh 2.0 servicemesh-proxy 2.0.4-1.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

1.32% chance of exploitation in the next 30 days, higher than 70% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • istio.io https://istio.io/latest/news/security/istio-security-2021-005/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
NVD 2.0 4.0 AV:N/AC:L/Au:S/C:P/I:N/A:N

Classification

Weakness
  • CWE-706 Use of Incorrectly-Resolved Name or Reference

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.