The ssl_verify_fun library unauthoritatively allows a certificate that contains a partial‑label wildcard to be considered valid even when the hostname does not match the left‑most label. This flaw permits an adversary to present a certificate such as f*.evil.example.net, which the library will treat as matching f.evil.example.net, thereby enabling a man‑in‑the‑middle to impersonate the intended TLS peer. The vulnerability stems from a mis‑implementation of RFC 6125, falling under CWE‑297, and can be used to subvert confidentiality and authenticity in SSL/TLS sessions.

The issue affects the deadtrickster ssl_verify_fun package from version 1.0.1 up to but not including 1.1.8. Systems that incorporate this library for client or server peer verification, particularly those that disable Server Name Indication (SNI) or that directly invoke verify_cert_hostname/2, are at risk. OTP releases 20.0 and later perform hostname checks before the user verify_fun runs, mitigating the flaw for clients that keep server_name_indication set to the target hostname.

The flaw carries a CVSS score of 9.1, classifying it as critical. No EPSS data is available, and the vulnerability is not listed in the CISA KEV catalog. The likely attack vector is a TLS interception scenario or a client that deliberately suppresses SNI, wherein the attacker can supply a crafted wildcard certificate to deceive the ssl_verify_fun verification process. Users who rely on older library versions or who disable SNI face the highest risk. Upgrade or additional configuration is recommended to reduce exposure.