Software Engineering Institute

Email clients supporting the OpenPGP or S/MIME standards may be vulnerable to a CBC/CFB gadget attack which may allow an attacker to inject content into an encrypted email which would establish an exfiltration channel when decrypted by the victim's email client. For example, injecting an HTML image tag which, when rendered by the email client, sends the plaintext as part of an HTTP request.

CVE-2017-17688: OpenPGP CFB Attacks
CVE-2017-17689: S/MIME CBC Attacks

Additionally some email clients, which do not isolate multiple MIME parts, allow attackers to wrap an encrypted message into plaintext MIME parts, which when decrypted and rendered by the email client results in an HTML based back-channel, eliminating the need to perform the gadget attacks.

Additional details can be found in the paper describing the attacks.

A remote attack could recover plaintext from encrypted emails without access to the encryption keys.

The CERT/CC is currently unaware of a practical solution to this problem. However, there are some mitigations that may be taken:

Decrypt mail outside of mail client
Using a separate application outside of your mail client to decrypt incoming emails prevents exfiltration channels from being opened by the email client.

Disable HTML rendering
Preventing your email client from rendering HTML will prevent the predominant form of establishing exfiltration channels.

Disable Remote Content Loading
Preventing your email client from loading remote content without permission can also help against the predominate form of establishing exfiltration channels.