A recent vulnerability in the DomainKeys Identified Mail (DKIM) protocol allowed attackers to forge the authenticity of email messages. What are the practical implications? Can DKIM no longer be trusted for email authentication?
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DomainKeys Identified Mail is designed to cryptographically sign the From address in an email, giving users the option to only receive signed email from legitimate providers. The recently publicized vulnerability in DKIM affected large email providers and basically allowed attackers to forge DKIM-signed emails from the vulnerable providers. Email systems not using DKIM are vulnerable to the same issue of forged emails, but this vulnerability is particularly dangerous because of the element of trust involved with DKIM. Forged DKIM emails could be more effectively used as part of a phishing or social engineering attack than unsigned emails because they look more legitimate.
A classic problem in cryptography involves vulnerabilities arising in the implementation rather than in the cryptography itself. This vulnerability is found in the implementation of the protocol by the email providers, not in the DKIM protocol. These email providers used test or weak signing keys for the DKIM certificates in violation of the DKIM RFC. The weak signing keys used by these email providers stand in contrast with other areas of PKI that have stopped using certificates of less than 1,024-bit keys. Using test keys is like using a self-signed certificate on a website where a user is prompted with a pop-up asking the user if they want to trust the website. Email systems should never be configured to trust test certificates.
DKIM can still be used for email authentication, but enterprises should verify that the certificates in use for DKIM are 1,024 bits or greater, signed by a generally trusted certificate authority, and that their email systems are configured to not use or accept test certificates.
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