CVE-2021-39155 Authorization Policy Bypass Due to Case Insensitive Host Comparison
Attackers can bypass Istio authorization policies by using mixed-case hostnames, allowing unauthorized access to protected services.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2021-39155?
CVE-2021-39155 is a high-severity authentication bypass vulnerability in istio, published on 24 Aug 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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](https://datatracker.ietf.org/doc/html/rfc4343), 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](https://istio.io/latest/docs/ops/best-practices/security/#case-normalization) guide.
Which versions are affected by CVE-2021-39155?
6 open source packages ship an affected version, including istio.io/istio. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
istio.io/istio Go | All versions | 1.9.8 |
istio.io/istio Go | All versions | 1.9.8 |
istio.io/istio Go | From 1.10.0 | 1.10.4 |
istio.io/istio Go | From 1.10.0 | 1.10.4 |
istio.io/istio Go | From 1.11.0 | 1.11.1 |
istio.io/istio Go | From 1.11.0 | 1.11.1 |
Products include Istio Istio.
How do I fix CVE-2021-39155?
Upgrade istio.io/istio to a fixed version for your release line: 1.9.8, 1.10.4 or 1.11.1.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2021-39155 being exploited?
There is no known exploitation of CVE-2021-39155 in the wild and no public exploit code.
Other vulnerabilities in istio
- CVE-2021-31921 Missing authorization in n/a
- CVE-2021-34824 Information disclosure in n/a
- CVE-2026-31837 Istio JWKS resolver to prevent private key material from being exposed when JWKS fetch fails.
- CVE-2021-39156 Fragments in Path May Lead to Authorization Policy Bypass
- CVE-2022-39388 Istio may allow identity impersonation if user has localhost access
Sources
Also known as GHSA-7774-7vr3-cc8j, MINI-3m93-h669-qxh6, MINI-hwmp-gm72-33wh.