Systematic Parsing of X.509: Eradicating Security Issues with a Parse Tree
This addresses security vulnerabilities in X.509 parsing for systems relying on TLS/HTTPS, though it is incremental as it builds on existing parsing methods.
The authors tackled the problem of insecure X.509 certificate parsing by developing a grammar-based parser with strong termination guarantees, and found that 21.5% of certificates in a large dataset were syntactically invalid, with 5.7% to 10.5% incorrectly validated by existing libraries, enabling an impersonation attack.
X.509 certificate parsing and validation is a critical task which has shown consistent lack of effectiveness, with practical attacks being reported with a steady rate during the last 10 years. In this work we analyze the X.509 standard and provide a grammar description of it amenable to the automated generation of a parser with strong termination guarantees, providing unambiguous input parsing. We report the results of analyzing a 11M X.509 certificate dump of the HTTPS servers running on the entire IPv4 space, showing that 21.5% of the certificates in use are syntactically invalid. We compare the results of our parsing against 7 widely used TLS libraries showing that 631k to 1,156k syntactically incorrect certificates are deemed valid by them (5.7%--10.5%), including instances with security critical mis-parsings. We prove the criticality of such mis-parsing exploiting one of the syntactic flaws found in existing certificates to perform an impersonation attack.