Recent discoveries of widespread vulnerabilities in the SSL/TLS protocol stack, particular with regard to the verification of server certificates, has left the security of the Internet’s communications in doubt. Newly proposed SSL trust enhancements address many of these vulnerabilities, but are slow to be deployed and do not solve the problem of securing existing software. In this work, we provide new mechanisms that offer immediate solutions to addressing vulnerabilities in legacy code. We introduce CertShim, a lightweight retrofit to SSL implementations that protects against SSL vulnerabilities, including those surveyed by Georgiev et. al., in a manner that is transparent to the application. We demonstrate CertShim’s extensibility by adapting it to work with Convergence, DANE, and Client-Based Key Pinning. CertShim imposes just 20 ms overhead for an SSL verification call, and hooks the SSL dependencies of 94% of Ubuntu’s most popular packages with no changes necessary to existing applications. This work significantly increases system-wide security of SSL communications in non-browser software, while simultaneously reducing the barriers to evaluating and adopting the myriad alternative proposals to the certificate authority system.