What is CVE-2021-31920?

CVE-2021-31920 is a medium-severity vulnerability in Istio, classified under Use of Incorrectly-Resolved Name or Reference. CVSS score: 6.5/10. Published 2021-05-27.

How severe is CVE-2021-31920?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-31920 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Istio

CVE-2021-31920

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.

Published 2021-05-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.012 (66.3th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 66th percentile — 66.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.012 · change: 0.000

Affected products

Istio

Weakness classification (CWE)

CWE-706 — Use of Incorrectly-Resolved Name or Reference

Public proof-of-concept exploits

n0-traces/cve_

References

istio.io/latest/news/security/istio-security-2021-005/ (Exploit, Vendor Advisory)

Frequently asked questions

What is CVE-2021-31920?: CVE-2021-31920 is a medium-severity vulnerability in Istio, classified under Use of Incorrectly-Resolved Name or Reference. CVSS score: 6.5/10. Published 2021-05-27.
How severe is CVE-2021-31920?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-31920 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.