CVE-2025-66220: Envoy’s TLS certificate matcher for `match_typed_subject_alt_names` may incorrectly treat certificates containing an embedded null byte

Summary Envoy’s mTLS certificate matcher for matchtypedsubjectaltnames may incorrectly treat certificates containing an embedded null byte (\0) inside an OTHERNAME SAN value as valid matches.

Details This occurs when the SAN is encoded as a BMPSTRING or UNIVERSALSTRING, and its UTF-8 conversion result is truncated at the first null byte during string assignment. As a result, "victim\0evil" may match an exact: "victim" rule and be accepted by Envoy.

PoC

Create a CA and a server certificate signed by that CA. Create two client certificates signed by the same CA: clientevil with OTHERNAME BMPSTRING = "evil" clientnull with OTHERNAME BMPSTRING = "victim\0evil" Configure Envoy with requireclientcertificate: true and a matchtypedsubjectaltnames entry for the OTHERNAME OID with matcher.exact: "victim". Connect without a client cert → connection rejected. Connect with clientevil → connection rejected. Connect with clientnull → connection accepted (but shouldn't!).

Impact An attacker who can obtain a trusted client certificate with a null byte embedded in an OTHERNAME SAN can exploit this vulnerability. The practical impact is unauthorized impersonation of the matched identity, enabling access to services or APIs protected by that exact OTHERNAME check.

Credit markevich.nikita1@gmail.com