How severe is CVE-2021-31921?

CVE-2021-31921 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-31921 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (70th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-31921?

CVE-2021-31921 affects Istio. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-31921?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-31921 published?

CVE-2021-31921 was published on 2021-06-02 and last updated on 2026-06-17.

CVE-2021-31921

CVE-2021-31921 is a critical-severity vulnerability in Istio with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-862.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 6.8
EPSS exploit prediction: 1% (70th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-862
Affected product: Istio
Published: 2021-06-02
Last modified: 2026-06-17

Description

Istio before 1.8.6 and 1.9.x before 1.9.5 contains a remotely exploitable vulnerability where an external client can access unexpected services in the cluster, bypassing authorization checks, when a gateway is configured with AUTO_PASSTHROUGH routing configuration.

Frequently asked questions

What is CVE-2021-31921?: Istio before 1.8.6 and 1.9.x before 1.9.5 contains a remotely exploitable vulnerability where an external client can access unexpected services in the cluster, bypassing authorization checks, when a gateway is configured with AUTO_PASSTHROUGH routing configuration.
How severe is CVE-2021-31921?: CVE-2021-31921 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-31921 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (70th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-31921?: CVE-2021-31921 affects Istio. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-31921?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-31921 published?: CVE-2021-31921 was published on 2021-06-02 and last updated on 2026-06-17.

References

https://istio.io/latest/news/security/istio-security-2021-006/

Affected products (1)

cpe:2.3:a:istio:istio:*:*:*:*:*:*:*:*

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