Systematic parsing of X.509: Eradicating security issues with a parse tree

Article type: Research Article

Authors: Barenghi, Alessandro | Mainardi, Nicholas | Pelosi, Gerardo^{; *}

Affiliations: Department of Electronics, Information and Bioengineering – DEIB, Politecnico di Milano, Italy. E-mails: alessandro.barenghi@polimi.it, nicholas.mainardi@polimi.it, gerardo.pelosi@polimi.it

Correspondence: [*] Corresponding author. E-mail: gerardo.pelosi@polimi.it.

Abstract: 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.

Keywords: Digital certificates, X.509, transport layer security, TLS, parsing, security vulnerabilities

DOI: 10.3233/JCS-171110

Journal: Journal of Computer Security, vol. 26, no. 6, pp. 817-849, 2018