Impact

When validating TLS certificates, Envoy would incorrectly allow a wildcard DNS Subject Alternative Name apply to multiple subdomains. For example, with a SAN of *.example.com, Envoy would incorrectly allow nested.subdomain.example.com, when it should only allow subdomain.example.com.

This defect applies to both validating a client TLS certificate in mTLS, and validating a server TLS certificate for upstream connections.

This vulnerability is only applicable to situations where an untrusted entity can obtain a signed wildcard TLS certificate for a domain of which you only intend to trust a subdomain of. For example, if you intend to trust api.mysubdomain.example.com, and an untrusted actor can obtain a signed TLS certificate for *.example.com or *.com.

Configurations are vulnerable if they use verify_subject_alt_name in any Envoy version, or if they use match_subject_alt_names in version 1.14 or later.

Patches

This issue has been fixed in Envoy versions 1.15.0, 1.14.4, 1.13.4, 1.12.6.

Workarounds

None

References

https://tools.ietf.org/html/rfc6125#section-6.4.3