OpenPGP and S/MIME mail client vulnerabilities

Overview

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

Description

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.

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Impact

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

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Solution

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

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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.

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Vendor Information

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9Folders, Inc. Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Airmail Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Apple Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Evolution Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Flipdog Solutions, LLC Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

GPGTools Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

GnuPG __ Affected

Updated: May 15, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

<https://lists.gnupg.org/pipermail/gnupg-users/2018-May/060334.html&gt;

Google Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

IBM Corporation Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

KMail Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

MailMate Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Microsoft Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Mozilla Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Postbox, Inc. Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

R2Mail2 Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Ritlabs, SRL Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Roundcube Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

The Enigmail Project Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

The Horde Project Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Trojita Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

eM Client Affected

Updated: May 14, 2018

Status

Affected

Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

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CVSS Metrics

Group	Score	Vector
Base	0	AV:–/AC:–/Au:–/C:–/I:–/A:–
Temporal	0	E:ND/RL:ND/RC:ND
Environmental	0	CDP:ND/TD:ND/CR:ND/IR:ND/AR:ND

References

• <https://efail.de/&gt;
• <https://efail.de/efail-attack-paper.pdf&gt;
• <https://tools.ietf.org/html/rfc4880&gt;

Acknowledgements

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

This document was written by Trent Novelly.

Other Information

CVE IDs:	[CVE-2017-17688 ](<http://web.nvd.nist.gov/vuln/detail/CVE-2017-17688 >), CVE-2017-17689
Date Public:	2018-05-14 Date First Published: