ssl_verify_fun: partial-label wildcard certificate matches hostnames across a label boundary

Overview

Improper Validation of Certificate with Host Mismatch vulnerability in deadtrickster ssl_verify_fun allows an adversary in the middle to impersonate a TLS peer via a certificate that carries a partial-label wildcard. try_match_wildcard/4 in src/ssl_verify_hostname.erl anchors only the text before and after the * of an identifier such as f*.example.net, so the wildcard also consumes dots and the certificate is accepted for f.evil.example.net. RFC 6125 confines a wildcard to the left-most label. The same matcher lets *.168.1.1 match the IP literal 192.168.1.1. Whole-label wildcards are matched correctly.

Erlang/OTP 20.0 and later check the hostname in ssl before a user verify_fun sees valid_peer, so a client that keeps server_name_indication set rejects the mismatch regardless. Exposed are clients that disable SNI, servers that verify client certificate hostnames with this library, and direct callers of verify_cert_hostname/2.

This issue affects ssl_verify_fun: from 1.0.1 before 1.1.8.

Affected

• ssl_verify_fun >= 1.0.1 < 1.1.8
• deadtrickster/ssl_verify_fun.erl >= a49ca56923356c7a3981692b23c42ef32c2990e0 < ca34031299cd2e0c98794c0da4ba556b94e39aad

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

On Erlang/OTP 21 or later, keep server_name_indication set to the target hostname and pass {customize_hostname_check, [{match_fun, public_key:pkix_verify_hostname_match_fun(https)}]} in the ssl options. The hostname check in ssl then rejects the mismatch before ssl_verify_fun runs, and legitimate wildcard certificates continue to match. hackney and tls_certificate_check already set both options.