The vulnerability stems from insecure package validation when using mirrors. The provided patches show critical changes to distlib's version parsing logic (PEP 426 compliance). These functions directly process package metadata used in authenticity checks. The regex expansion and tuple handling fixes indicate previous version validation weaknesses that could be exploited via malicious mirror packages. While the DNS/mirror logic isn't shown in these patches, the version parsing improvements in distlib 0.1.6 directly address the authentication aspect of CVE-2013-5123.