Session F5: Understanding Security Fails CCS’17, October 30-November 3, 2017, Dallas, TX, USA Certified Malware: Measuring Breaches of Trust in the Windows Code-Signing PKI Doowon Kim Bum Jun Kwon Tudor Dumitras, University of Maryland University of Maryland University of Maryland College Park, MD College Park, MD College Park, MD
[email protected] [email protected] [email protected] ABSTRACT To establish trust in third-party software, we currently rely on Digitally signed malware can bypass system protection mechanisms the code-signing Public Key Infrastructure (PKI). This infrastruc- that install or launch only programs with valid signatures. It can ture includes Certification Authorities (CAs) that issue certificates also evade anti-virus programs, which often forego scanning signed to software publishers, vouching for their identity. Publishers use binaries. Known from advanced threats such as Stuxnet and Flame, these certificates to sign the software they release, and users rely this type of abuse has not been measured systematically in the on these signatures to decide which software packages to trust broader malware landscape. In particular, the methods, effective- (rather than maintaining a list of trusted packages). If adversaries ness window, and security implications of code-signing PKI abuse can compromise code signing certificates, this has severe impli- are not well understood. We propose a threat model that highlights cations for end-host security. Signed malware can bypass system three types of weaknesses in the code-signing PKI. We overcome protection mechanisms that install or launch only programs with challenges specific to code-signing measurements by introducing valid signatures, and it can evade anti-virus programs, which often techniques for prioritizing the collection of code-signing certificates neglect to scan signed binaries.