How severe is CVE-2021-39156?

CVE-2021-39156 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-39156?

CVE-2021-39156 affects the following packages: istio.io/istio (Go), istio.io/istio (Go), istio.io/istio (Go). Ecosystems affected: Go.

How do I fix CVE-2021-39156?

Update istio.io/istio to 1.9.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39156 is resolved across your whole dependency graph.

How do I detect CVE-2021-39156 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for istio.io/istio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-39156 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2021-39156 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-39156 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-39156?

CVE-2021-39156 has an EPSS (Exploit Prediction Scoring System) score of 1.2%, placing it in the 67th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-39156?

CVE-2021-39156 is classified as CWE-863 (CWE-863), CWE-706 (CWE-706). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-39156 published, and has it been updated?

CVE-2021-39156 was published on August 24, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

HIGH severity

CVE-2021-39156 — istio

HIGH

CVE-2021-39156 is a high-severity (CVSS 7.5) CWE-863 vulnerability in istio.io/istio. A fix is available for istio.io/istio — see the affected versions and patch details below.

Istio Fragments in Path May Lead to Authorization Policy Bypass

Also known asGHSA-hqxw-mm44-gc4r

Published

Aug 24, 2021

Updated

Jul 9, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs67th percentile — riskier than 67% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-39156 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

3 pkgs affected

🐹istio.io/istio🐹istio.io/istio🐹istio.io/istio

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Istio 1.11.0, 1.10.3 and below, and 1.9.7 and below contain a remotely exploitable vulnerability where an HTTP request with #fragment in the path may bypass Istio’s URI path based authorization policies.

Patches

Istio 1.11.1 and above
Istio 1.10.4 and above
Istio 1.9.8 and above

Workarounds

A Lua filter may be written to normalize the path. This is similar to the Path normalization presented in the Security Best Practices guide.

References

More details can be found in the Istio Security Bulletin

For more information

If you have any questions or comments about this advisory, please email us at [email protected]

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`istio.io/istio`	all versions	1.9.8`go get istio.io/istio@v1.9.8`
🐹Go	`istio.io/istio`	≥ 1.10.0&&< 1.10.4	1.10.4`go get istio.io/istio@v1.10.4`
🐹Go	`istio.io/istio`	≥ 1.11.0&&< 1.11.1	1.11.1`go get istio.io/istio@v1.11.1`

Affected Products

1 product · 3 configurations

Application

istioistio

≥ 1.11.0 && < 1.11.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for istio.io/istio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update istio.io/istio to 1.9.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39156 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Product	Fixed in	Advisory
OpenShift Service Mesh 1.1	`servicemesh-0:1.1.17-3.el8`	RHSA-2021:3273
OpenShift Service Mesh 2.0	`servicemesh-0:2.0.7-3.el8`	RHSA-2021:3272

Frequently Asked Questions

### Impact Istio 1.11.0, 1.10.3 and below, and 1.9.7 and below contain a remotely exploitable vulnerability where an HTTP request with `#fragment` in the path may bypass Istio’s URI path based authorization policies. ### Patches * Istio 1.11.1 and above * Istio 1.10.4 and above * Istio 1.9.8 and above ### Workarounds A Lua filter may be written to normalize the path. 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. ### References More details can be found in the [Istio