CVE-2021-39155 — istio
HIGHCVE-2021-39155 is a high-severity (CVSS 7.5) CWE-178 vulnerability in istio.io/istio. A fix is available for istio.io/istio — see the affected versions and patch details below.
Authorization Policy Bypass Due to Case Insensitive Host Comparison
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-39155 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,333 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
istio.io/istio🐹istio.io/istio🐹istio.io/istioReal-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
Istio is an open source platform for providing a uniform way to integrate microservices, manage traffic flow across microservices, enforce policies and aggregate telemetry data. According to RFC 4343, Istio authorization policy should compare the hostname in the HTTP Host header in a case insensitive way, but currently the comparison is case sensitive. The proxy will route the request hostname in a case-insensitive way which means the authorization policy could be bypassed. As an example, the user may have an authorization policy that rejects request with hostname "httpbin.foo" for some source IPs, but the attacker can bypass this by sending the request with hostname "Httpbin.Foo". Patches are available in Istio 1.11.1, Istio 1.10.4 and Istio 1.9.8. As a work around a Lua filter may be written to normalize Host header before the authorization check. This is similar to the Path normalization presented in the Security Best Practices guide.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | istio.io/istio | all versions | 1.9.8go get istio.io/istio@v1.9.8 |
| 🐹Go | istio.io/istio | ≥ 1.10.0&&< 1.10.4 | 1.10.4go get istio.io/istio@v1.10.4 |
| 🐹Go | istio.io/istio | ≥ 1.11.0&&< 1.11.1 | 1.11.1go get istio.io/istio@v1.11.1 |
Detection & mitigation playbook
Open-source dependencyDetect
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-2021-39155 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-39155 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-39155. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
| 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
Is CVE-2021-39155 in your dependencies?
O3 Security finds CVE-2021-39155 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.