When Squid is compiled with SSL support (i.e. where it can encrypt/decrypt requests/responses rather than using a CONNECT tunnel), the function check_domain is used to check whether a request domain’s name is the same domain (or is equally valid) as in the certificate the server offers. While zero-length domains are rejected before the matchDomainName function as called from check_domain is ever called, if a certificate offers the CNAME as simply * (which would mean all domains), a pointer pointing to an empty/null character is passed to matchDomainName which can either produce junk data, or can cause a crash; or do nothing as it is undefined behavior.