OpenPGP and S/MIME mail client vulnerabilities


Mail clients may leak plaintext messages while decrypting OpenPGP and S/MIME messages.


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.

Vendor Information (Learn More)

Vendor Status Date Notified Date Updated
9Folders, Inc. Affected 14 May 2018
Airmail Affected 14 May 2018
Apple Affected 14 May 2018
eM Client Affected 14 May 2018
Evolution Affected 14 May 2018
Flipdog Solutions, LLC Affected 14 May 2018
GnuPG Affected 14 May 2018
Google Affected 14 May 2018
GPGTools Affected 14 May 2018
IBM Corporation Affected 14 May 2018
KMail Affected 14 May 2018
MailMate Affected 14 May 2018
Microsoft Affected 14 May 2018
Mozilla Affected 14 May 2018
Postbox, Inc. Affected 14 May 2018

Other Reference :



Credit is attributed to Damian Poddebniak, Christian Dresen, Jens Muller, Fabian Ising, Sebastian Schinzel1, Simon Friedberger, Juraj Somorovsky, and Jorg Schwenk

Document BY : Trent Novelly.

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