OpenSSL

Major TLS stacks contained severe implementation flaws (OpenSSL Heartbleed 2014, Apple Secure Transport goto fail, Microsoft SChannel Winshock, GnuTLS cert verification bugs), exposing keys and enabling authentication bypasses.

Version drift in OpenSSL across Linux, macOS, and Windows distributions causes hours of debugging. Different versions ship with varying compile flags, and small mismatches break builds or runtime behavior. Breaking API changes hidden in patch releases compound the problem.

Cross-signing of CA certificates creates multiple possible trust chains. Different SSL stacks (Windows, OpenSSL) behave differently during verification, causing some platforms to fail validation while others succeed.

Early TLS versions had vulnerabilities in how renegotiation was handled, allowing man-in-the-middle attacks. Servers using older TLS libraries or failing to implement secure renegotiation remain vulnerable.