Answer in brief
CVE-2021-39155 records a High severity (CVSS 8.3) vulnerability in Authorization Policy Bypass Due to Case Insensitive Host Comparison. The current sources do not mark it as known exploited. The current feed maps istio.io/istio (go), istio.io/istio (go), istio.io/istio (go). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
CVSS is 8.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps istio.io/istio (go), istio.io/istio (go), istio.io/istio (go). Check affected ranges and fixed versions before updating.
| Package | Affected range | Fixed version |
|---|---|---|
| istio.io/istiogo | >=0 <1.9.8 | 1.9.8 |
| istio.io/istiogo | >=1.10.0 <1.10.4 | 1.10.4 |
| istio.io/istiogo | >=1.11.0 <1.11.1 | 1.11.1 |
Published upstream
Aug 30, 2021
Evidence: source:osv:source_dates:source-dates:recordSource modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:recordFirst seen by HOL
Sep 10, 2026
### Impact 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 Envoy 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 * 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 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. ### References More details can be found in the [Istio Security Bulletin](https://istio.io/latest/news/security/istio-security-2021-008). ### For more information If you have any questions or comments about this advisory, please email us at [email protected]
Quoted source text, attributed separately from HOL analysis.