Auth bypass in Istio

CVE-2026-31838

Istio is an open platform to connect, manage, and secure microservices. Prior to 1.29.1, 1.28.5, and 1.27.8, a vulnerability in Envoy RBAC header matching could allow authorization policy bypass when policies rely on HTTP headers that may contain multiple values. An attacker could craft requests with multiple header values in a way that causes Envoy to evaluate the header differently than intended, potentially bypassing authorization checks. This may allow unauthorized requests to reach protected services when policies depend on such header-based matching conditions. This vulnerability is fixed in 1.29.1, 1.28.5, and 1.27.8.

Vulnerability class: Broken Access Control

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.004 (27.2th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 27th percentile — 27.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-31838: held from 0.002 to 0.004.

EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.001

Affected products

  • Istio — versions >= 1.29.0-alpha.0, < 1.29.1, >= 1.28.0-alpha.0, < 1.28.5, < 1.27.8

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-31838?
CVE-2026-31838 is a medium-severity vulnerability in Istio, classified under Incorrect Authorization. CVSS score: 5.3/10. Published 2026-03-10.
How severe is CVE-2026-31838?
Medium severity. CVSS v3 base score is 5.3 out of 10.