Two new vulnerabilities have been discovered impacting multiple email clients. The vulnerabilities impact customers that use OpenPGP or S/MIME to secure email communications. Successful exploitation may allow threat actors to decrypt sensitive email communications when the victim is using a vulnerable email client or plugin. Threat actors combine known weaknesses in the encryption algorithms with automatic rendering of active content to perform this new attack. Threat actors first need to obtain the encrypted messages, then send them to a user with a vulnerable email client.
What we’re doing about it
- The eSentire Threat Intelligence team is actively monitoring the situation for changes and additional details
What you should do about it
- Apply security patches to affected email clients after their release
- When possible, disable Remote Content Loading from email clients
- Decrypt emails outside of the mail client
For a full list of affected email clients, please see the official EFAIL whitepaper1.
There are two techniques for exploiting EFAIL. The first being direct exfiltration which affects both PGP and S/MIME. In this scenario, the adversary intercepts an encrypted email and places it into a new email with HTML tags around the encrypted section. This new email is then sent to the victim. When received by the victim’s mail client, the message is unencrypted, but the additional HTML tags cause a misinterpretation of data, sending the unencrypted message to a remote server where it can be read.
The second technique is the modification of ciphertext using known weaknesses in the Cipher Block Chaining. Since threat actors know the structure of S/MIME emails, they can perform a plaintext attack and embed HTML tags, similar to the first technique.
 Efail: Breaking S/MIME and OpenPGP Email Encryption