By
Patriz Regalado
April 3, 2014
Your organization’s policies—or lack of policies—regarding trusted root CA certificates are exposing you to unnecessary risk. Because certificates serve as credentials for so many mission-critical transactions, attackers are constantly trying to obtain trusted certificates that can be used in targeted attacks. Systems, for their part, refer to their store of trusted root certificates to determine which certificates to trust. If a certificate is signed by a trusted CA, the system trusts the certificate. To compromise a system, therefore, an attacker simply needs to obtain a certificate that is signed by a trusted root CA—whether by tricking the CA into issuing a fraudulent certificate or by compromising the CA. Every CA that your systems trust represents a potential entry point for attackers.
Many organizations expose themselves to unnecessary risk by allowing far too many of these entry points. They retain the default trust stores distributed with most operating systems and application servers, which include far more certificates than are necessary. According to a University Hannover Germany study, common trust stores for various platforms and operating systems—such as Windows, Linux, MacOS, Firefox, iOS and Android—contained more than 400 trusted root certificates. However, only 66% of these certificates were used to sign HTTPS certificates, leaving the other 34% of trusted root certificates susceptible to use in certificate-based attacks.
We are seeing more and more evidence of malware signed with a legitimate CA because an attacker stole a private key or obtained a fake certificate. Consider the following scenario: Your organization is currently trusting AcmeCA on many of your systems simply because AcmeCA is approved by the vendor providing the software for your systems. If a malicious attacker gains access to a fraudulent certificate from AcmeCA, that attacker can use it to attack multiple systems within your organization.
Your organization has outward-facing systems, such as those focused on customer interaction or users’ desktops, that must trust a particular CA in order to perform day-to-day business. However, your organization also includes systems that don’t need to trust a particular CA but are, in fact, trusting it. For example, internal systems that communicate only with other internal systems don’t need to trust any CAs but the internal CA(s). In addition, partner-focused systems that communicate with a limited number of partners require just a handful of CAs.
Most organizations have no visibility into which root certificates they are trusting and where those root certificates are deployed; consequently, they cannot limit their exposure to certificate-based attacks. As a critical first step, organizations must gain visibility into which root certificates are being trusted within their environment. They must compile an inventory of their root certificates so they can reduce the risks caused by unnecessary trust. In the AcmeCA scenario, for example, you would see that AcmeCA is installed on multiple systems within your organization, determine that these systems don’t need to trust AcmeCA, and remove it. Thus, an attacker would be unable to use a fraudulent certificate from AcmeCA to successfully attack your organization.
By identifying CAs that should not be trusted on mission-critical systems, organizations have the intelligence and risk awareness to prevent attacks that leverage certificates from those CAs. One way to take action is through certificate whitelisting, which ensures that whitelisted certificates are included in trust stores and blacklisted certificates are excluded from trust stores. Certificate whitelisting limits the number of CAs that are trusted, allowing organizations to secure and protect the CAs they trust while flagging or disallowing untrusted SSL/TLS sessions. As a result, the attack surface is dramatically reduced.
Organizations can eliminate unnecessary risk from digital certificates signed by untrusted CAs by establishing and enforcing certificate whitelists and updating which CAs are trusted within the enterprise. They can enforce baseline requirements for which CAs should be trusted (whitelist) and not trusted (blacklist) on mission-critical systems and ensure whitelisted certificates are included in trust stores and blacklisted certificates are excluded, preventing attacks that leverage certificates from blacklisted CAs.
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